Xavier Marchal

Moustapha El

Bertrand Mathieu

Thibault Cholez

Guillaume Doyen

Wissam Mallouli

Olivier Festor

Leveraging NFV for the Deployment of NDN: Application to HTTP Traffic Transport

Keywords: Network Function Virtualization, Named Data Networking, experimental study, Web traffic

For a few years, Network-Function Virtualization (NFV) acts as the most promising solution for the flexible implementation and management of future network services. If most of current efforts in this area focus on IP-based Virtual Network Functions (VNF), the case of Information-Centric Networking (ICN) is interesting since it can demonstrate that NFV is a promising technology for ISP to deploy such new innovative network stacks. In this context, we propose to design and implement a NFV compliant architecture to easily deploy ICN islands. Especially, at the core of this architecture, we present an HTTP/NDN gateway, which enables our network to carry real HTTP traffic. Finally, we show early functional experimental results of an initial testbed deployment exhibiting the capability of our global infrastructure to retrieve the top-1000 of the most popular web sites.

I. INTRODUCTION

Internet pursues its fast paced evolution while more and more users and bandwidth-consuming services depend on it, putting a high pressure on the underlying infrastructure. To address this challenge, several disruptive technologies have emerged from the networking community. From one side, based on the observation that the main usage of today's Internet is related to content diffusion, Information-Centric Networking (ICN) architectures, and particularly the Named-Data Networking (NDN) initiative, propose a paradigm shift to optimize data delivery, where the Internet data plane shifts from host-based network mechanisms to content-based ones and include caching on all network devices. From the other side, NFV (Network Function Virtualization), as defined by the ETSI [START_REF] Festor | Network Functions Virtualisation (NFV); Management and Orchestration[END_REF], can reduce operating cost and foster the innovation in the network core by replacing specialized and expensive dedicated network equipment by commodity servers running Virtualized Network Functions (VNF).

In this context, we believe that NDN could only be deployed incrementally, step after step, at low cost for reducing network investment by network operators, and that running NDN nodes in an NFV infrastructure is the only reasonable path leading to a progressive deployment by ISPs of this new network protocol. Therefore, our research question encompasses the deployment of NDN-based VNF in an operational context. In this paper, we propose a HTTP/NDN gateway to interconnect ICN islands to the IP world, and an experimental architecture supporting this research direction and able to process the web traffic passing through a virtualized NDN network.

The rest of the paper is organized as follows. Section II presents the related work on NFV and ICN deployments. Section III describes our architecture with an emphasis on the design of the key component that is the HTTP/NDN gateway. Section IV validates our approach with a first analysis of web traffic delivered through our testbed. Finally, Section V concludes the paper and gives directions for future work.

II. RELATED WORK

A. NFV Deployment Initiatives

Currently, several efforts aim at providing implementation of the NFV reference architecture provided by the ETSI [START_REF] Festor | Network Functions Virtualisation (NFV); Management and Orchestration[END_REF]. The OPNFV consortium proposes to gather all existing components, which can be integrated in a modular architecture, to produce a complete NFV implementation. They rely on both OpenStack and OpenDaylight, and, to date, their fourth version of the architecture "Danube", is able to address many NFV scenarios. Concurrently, the Telco working-group, which takes part of the OpenStack consortium, is devoted to the evolution of OpenStack toward NFV support. The methodology of the working group consists in considering the ETSI NFV use-cases as primary requirements and address, according to established priorities, the development/adaptation of elements in the OpenStack reference architecture. We also note that academics take part of these efforts with implementation proposals such as OpenANFV [START_REF] Ge | OpenANFV: accelerating network function virtualization with a consolidated framework in OpenStack[END_REF] and Cloud4NFV [START_REF] Soares | Cloud4NFV: A platform for Virtual Network Functions[END_REF] that especially focus on automation for NFV management, and infrastructure elasticity. Only a few recent papers consider NFV to deploy ICN. In particular, Ueda et al. [START_REF] Ueda | Towards the NFVI-Assisted ICN: Integrating ICN Forwarding into the Virtualization Infrastructure[END_REF], from the ICN2020 project, focus on the performance aspects of deploying ICN routers as VNF.

B. Leveraging SDN for the deployment of ICN

Given the maturity of ICN solutions and the availability of early-implementations, several efforts are currently conducted to move ICN from a lab-restricted infrastructure to a fully deployable one, in an operated context. To that aim, SDN is often used to bring required control facilities needed to run a non-IP stack. However, such a coupling induces a set of challenges that we briefly provide here: (1) capability to carry and control both ICN and IP traffic; (2) the compatibility with current SDN standard [START_REF] Mckeown | OpenFlow: enabling innovation in campus networks[END_REF] (e.g. OpenFlow); (3) network protocols encapsulation strategy and (4) binding of the solution with a topological location (i.e. dedicated island, transit operator network, service provider, etc.). As an example of such architecture, CONET [START_REF] Salsano | Information centric networking over SDN and OpenFlow: Architectural aspects and experiments on the Ofelia testbed[END_REF] proposes to deploy ICN in Openflow 1.0 transit networks. The protocol encapsulation is based on the computing of a hash value of content names inserted into UDP headers. With the same idea, in [START_REF] Nguyen | Providing CCN functionalities over openflow switches[END_REF], the authors propose to use the IP header to carry a similar hash, thus making IP a protocol usable for both ICN and IP protocol stacks and avoiding any change in Openflow at the price of completely re-interpreting the meaning of IP header fields. By contrast, NDNFlow [START_REF] Van Adrichem | NDNFlow: Software-defined Named Data Networking[END_REF] proposes to enable the transit of both IP and ICN traffic by using a dedicated non-Openflow channel between the routers and the controller. Finally, in [START_REF] Vahlenkamp | Enabling information centric networking in IP networks using SDN[END_REF], the authors propose to use a full-overlay solution to avoid any cohabitation between IP related functions and ICN ones.

Whatever the solution proposed, the coupling of IP technologies with ICN ones requires a protocol adaptation (e.g. data-plane protocol encapsulation, dedicated use of OpenFlow as a control framework), which either takes it away from its standard use while also impacting the infrastructure already implemented. Another approach, proposed by Moiseenko et al. [START_REF] Moiseenko | TCP/ICN: Carrying tcp over content centric and named data networks[END_REF], is to work on TCP/ICN conversion through gateways. By contrast, we think the HTTP/ICN mapping is more adapted because HTTP objects share common properties with ICN content (URI-names, cachability). In addition, we leverage virtualization and especially NFV to enable the full decoupling of the infrastructure domain from the tenant one, which can host VNFs based on heterogeneous protocol stacks. As such, we consider NFV as the key-enabler for the ICN technology and we propose to design, implement and validate a dedicated framework enabling the deployment of ICN as VNF.

III. A VIRTUALIZED ICN CARRYING WEB TRAFFIC

A. Our Architecture

We propose a flexible network architecture based on NFV [START_REF] Festor | Network Functions Virtualisation (NFV); Management and Orchestration[END_REF] and SDN [START_REF] Mckeown | OpenFlow: enabling innovation in campus networks[END_REF] principles. The full architecture is described in [START_REF] Aguessy | Architecture of the DOCTOR Virtualized Node[END_REF]. We hereafter focus on the necessary blocks illustrating the HTTP/NDN use-case as illustrated in Figure 1 and was demonstrated in [START_REF] Marchal | A virtualized and monitored NDN infrastructure featuring a NDN/HTTP gateway[END_REF]. We focus on the NFV Infrastructure (NFVI), which enables the resource virtualization and management to host VNFs deploying an ICN protocol stack, ant more precisely on the Data plane, the control plane defining the management and orchestration (MANO) aspects being left for future work. As a computing virtualization framework, we have chosen Docker, which relies on a lightweight virtualization principle. Even if Docker offers lower isolation guarantees, this choice was driven by the better performance compared to other virtualization solutions. Indeed, our initial performance evaluation [START_REF] Aguessy | Architecture of the DOCTOR Virtualized Node[END_REF] showed that, in a NFV context, the throughput of Openstack/KVM VNFs is half of the throughput of Docker. A virtual network based on OpenVSwitch is also deployed to ensure end-to-end network connectivity between the virtualized network services and enable a software control of the networking infrastructure. Then, as an ICN protocol stack, we have selected NDN [START_REF] Zhang | Named Data networking[END_REF] due to the maturity of its specification and implementation. We "dockerized" the NDNx software to make it a VNF, deployable in our architecture.

Please note that we use NDN over IP, and not directly over Ethernet because most of NFV tools are still IP-dependent. Finally, we consider the Web as our main application-layer service due to its high-popularity and predominance in the global network shares. Moreover, the benefits of transporting web traffic through an ICN have been investigated at the origin of NDN, which especially focuses on the gain that native multicasting of communications and caching can bring to massive content diffusion. However, since current web clients and servers do not yet implement NDN, and because telco operators are still reluctant to widely deploy it, we have designed and implemented dedicated gateways to perform an HTTP/NDN conversion. Those gateways are conceived as VNFs and can be deployable where and when required, thanks to NFV principles. Two kinds of gateways are defined: (1) an ingress gateway (iGW), aiming at converting HTTP users' requests into NDN Interest messages to find the content in the NDN network, and converting NDN Data messages into HTTP replies sent to the end-users; and (2) an egress gateway (eGW), the counterpart of the first one, aiming at converting NDN messages into HTTP requests towards IP web sites if the content is not available in the ICN network, and converting HTTP replies into NDN Data messages to the iGW.

B. HTTP to NDN translation

We now detail the two gateways we have developed, being the main VNFs running in our NFV-based testbed. First, we had to design a translation scheme to transport over NDN the main HTTP protocol header information, as described below:

1) Naming scheme. In order to communicate with each other, the gateways follow a naming pattern based on the official naming proposition to convert URL to ICN names 1 . Splitting the domain per sub-level and appending them in reverse order gives us a better NDN routing capability with route aggregation compared to a "monolithic" domain name like we saw in another NDN gateway [START_REF] Aguessy | Architecture of the DOCTOR Virtualized Node[END_REF]. Also, by adding to the name the prefix "/http", an egress gateway can register only this prefix in order to be the default producer for all the traffic or, for example, "/http/com" prefix to be the default producer for all .com domains. Furthermore, beginning an ICN name by the protocol name "http" can later enable per protocol routing and traffic management with different strategies or routes applied to each of them.

2) Request a web content in the NDN network. Since NDN Interest packet can't carry data while HTTP request's header does, the Ingress gateway (or a NDN client) must exchange different messages in 3 steps to retrieve HTTP content. First, iGW sends an Interest which name components contain, as illustrated in Table 1.a:

 the requested domain splitted by sub-domains and in reverse order (for example: www.google.com becomes /com/google/www),  the path of the content on the web server,  a hash of the request's header (a SHA1 of the HTTP header and up to 1024 bytes of the request body),  the full route to the sender as a single name component in a binary TLV format.

This Interest packet is sent in the NDN network to ask someone to handle the request. So, the Egress gateway (or a NDN server) knows uppon reception that someone has an HTTP request to be satisfied, but also the netwok name to reach it. Please note that the SHA1 of the HTTP header is necessary to be sure that we respect the way the HTTP request was made to match users' properties (user agent, etc.). Choosing carefully a subset of fields to be considered to compute the hash can improve ICN caching while giving consistent results to users.

3) Retreive the HTTP request. Then, the Egress gateway extracts information from the first Interest sent by the Ingress gateway, more precisely the two last components: the sender route and the hash, and send an Interest in order to retrieve the full HTTP request (Table 1.b). Once the full HTTP request is received by the Egress gateway from the NDN network, it can now ask the HTTP server in the IP network for the actual web content.

4) Publish the HTTP response. After receiving the HTTP response form the IP web server, the Egress gateway splits it into Data packets with a NDN name like the first Interest, but without the sender route (Table 1.c). Following NDN principle, it is up to the NDN client (the original one or any other) to send Interest packets to retrieve each chunk of the HTTP response. In Table 2 is given an example of our naming pattern for the web content: "http://detectportal.firefox.com/success.txt".

IV. IMPLEMENTATION AND FUNCTIONNAL VALIDATION

In order to validate the capability of our solution, we have conducted a set of functional tests which consist in determining the set of HTTP objects able to successfully cross our NFVbased NDN domain. In this section, we first present the architecture we have implemented. Then, we describe the scenario, tools and the dataset we have used to perform the tests. Finally, we provide the set of results we obtained as well as a brief related analysis.

A. Implementation and Testbed Deployment

Our testbed, depicted in Figure 1, is bi-located in two distant cities. Each site operates a set of servers and a VPN tunnel interconnects both. Following the NFV philosophy, the infrastructure is composed of standard x86 servers with the following configuration: dual Xeon CPU (E5-2630v3, 8c@2.4GHz), 128GB of RAM (DDR4) and 2x10Gbps (Intel X540) + 4x1Gbps (BCM 5720) for network connectivity. The VNF composing the logical architecture are instantiated by Docker (v1.6.0) containers based on Ubuntu server (v15.04). We can distinguish three types of components: NDN nodes executing a NDN Forwarding Daemon (NFD v0.5.1), the Ingress gateway and the Egress gateway. These two last components rely on the ndn-cxx (0.5.1) library. Finally, OpenVSwitch (v2.3.1) interconnects the different containers.

B. Dataset and Scenario

The first validation test our architecture has to pass, in order to demonstrate his capability to carry real end-users traffic accessing web content, consists in determining to what extent it is able to (1) successfully map HTTP requests and responses to respectively NDN Interest and Data packets and (2) transport this traffic across the testbed in a reliable way (i.e. without any loss of packet or excessive transit duration leading to the expiration of timers in HTTP user-agents). To that aim, we have challenged our architecture against the request of the top 1000 most popular HTTP web sites2 . We deliberately do not consider HTTPS in this first version since HTTPS contents are end-to-end encrypted and thus cannot be converted by intermediaries (to benefit from ICN features like caching and multicast) if no agreement between the gateways and the endpoints to delegate keys are established. Each entry in this top-1000 consists in a web page standing for the entry point of the site and gathers a set of HTTP objects (e.g. images, audio, JavaScript, CSS sheets, etc.) which leads to several requests and answers to retrieve the whole page content. All websites were tested reachable but may still contain dead links. As a source request injector as well as response collector and parser, we have implemented a Web Scrapper based on the Jaunt implementation.

The latter especially provides insights about (1) connection errors (e.g. timeout) and ( 2) HTTP-related errors, which allows us to understand when and why our testbed may fail to carry some HTTP traffic. For each of these two cases, we relate the feedback collected and analyzed from the scrapper to a particular HTTP objet to eventually determine the capability of our architecture to carry such type of content.

C. Results

We have collected 73,236 web objects in total, whose distribution per site is provided in Figure 2, each retrieved with a dedicated HTTP request. Apart this set, we note that still 5,874 objects were not retrieved though the NDN island due to the use of HTTPS as a transport protocol. In this case, they are retrieved directly over IP (HTTPS traffic does not use the NDN network as web proxy) and we remove them from the set of results considered below.

As a first result, it appears that the testbed infrastructure has been able to retrieve 98.24% of all HTTP objects. Among these objects, 73.96% stands for images, 1.51% are scripts and 12.20% are links toward external resources (e.g. CSS sheets) while the last 2.33% are heterogeneous external resources embedded in tags such as iframe, area, span, etc. In order to be well understood, this global functional result must be put in front of the set of errors. We have shown in Figure 3 (plot in log-log scale for a better readability) that only a few web sites, hosting a significant set of image objects (from 10 to 100) induce most of the errors in this category while the majority of web sites gets a perfect or almost retrieval, thus assessing the global good performance of the gateways. Finally, we plot in Figure 4 the frequency distribution of the percentage of a web page content, given by the number of HTTP objects retrieved over the set of all objects that the infrastructure requests. The result (plot in semi-log scale) shows that most of top-1000 HTTP web sites can be retrieved entirely or almost while only a few (i.e. 17 sites over the 1000 considered in this study) bring poor results (i.e. less than 80% of content retrieval). This last category of sites is mainly composed of distant websites (Chinese, Korean and Russian), leading us to the conclusion that the problem comes from our eGW timeout value which is too restrictive for those particular websites. Concerning the overhead, the gateway adds approximately 30ms (only for the 1st packet, the rest of the content is streamed). The achieved throughput of 180Mbit/s is also good and in fact limited by the NDN routing function.

V. CONCLUSION AND FUTURE WORK

NDN is a promising technology but its clean slate approach makes a large-scale deployment complex and unlikely. However, coupling NDN with a NFV approach can provide network operators with a progressive and affordable deployment solution, but it must be assessed before. As a first step, we built a testbed with the objective to process unencrypted web traffic in a virtualized NDN architecture based on Docker and OpenVSwitch. At the core of this infrastructure, we designed and implemented an HTTP/NDN gateway, that can map the HTTP protocol with NDN messages and properly deliver most of web content (>98%), according to our measurements. This allows ISP to progressively deploy ICN islands to benefit from this network stack features for content delivery in their core network, with the limited investment costs offered by NFV.

In our future work, we will improve the caching strategy of static web objects by giving them an identifier that can be shared between users and we will evaluate the gain of NDN when carrying web traffic. In parallel, we will develop the MANO part of our architecture by leveraging the TOSCA language to describe the network topology and to orchestrate the NDN-based VNFs. Our orchestrator will address in particular the scalability and the security of the NDN island, for instance by defining reaction policies to well-known NDN attacks like the Content Poisoning Attack, or to scale up the infrastructure when a gateway is overloaded.

Figure 1 :

 1 Figure 1: Overview of our virtualized NDN infrastructure

Figure 2 :Figure 3 :Figure 4 :

 234 Figure 2: Distribution of content size of web pages, given in #web objects (semi-log scale) Figure 3: Frequency distribution of image errors (log-log scale) Figure 4: Frequency distribution of the percentage of successful web site retrieval (semi-log scale)

Table 1 :

 1 Naming pattern of the protocol

	a	/http/reverse_splitted_domain_name/URI/sender_route/sha1
	b	/sender_route/sha1(/segment)
	c /http/reverse_splitted_domain_name/URI/sha1(/version/segment)

1 http://www.icn-names.net/

Table 2 :

 2 Example of HTTP request translation

	a	/http/com/firefox/detectportal/success.txt
		/%07%0B%08%04http%08%03iGW/1E69...
	b	/http/iGW/1E69...(/segment)
	c /http/com/firefox/detectportal/success.txt/1E69...(/version/segment)

https://gtmetrix.com/top1000.html


H Rouzé

M Leray

Hélène Magalon

Lucie Penin

Pauline Gélin

N Knowlton

C Fauvelot

Molecular characterization reveals the complexity of previously overlooked coral-exosymbiont interactions and the implications for coral-guild ecology

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Coral reefs are also known for the prevalence of cryptic species [START_REF] Knowlton | New taxonomy and niche partitioning on coral reefs: Jack of all trades or master of some?[END_REF] , and corals with their mutualistic partners, both endosymbionts and exosymbionts, are no exception in this regard. In the case of the crustacean symbionts of pocilloporid corals, all members of the association include complexes of cryptic species. The best understood of these are the 22 described species of Trapezia crabs, whose distinctive color patterns and tendency to live in male female pairs can make identification of species less challenging. Identifications are mainly based on color and/ or color patterns of the carapace, as well as, in a few cases, behavior [START_REF] Odinetz | L' éthologie au service de la systématique: l' exemple des Trapezia (crustacés décapodes, brachyoures)[END_REF] , but some similar species can be difficult to distinguish because of possible phenotypic plasticity [START_REF] Sin | Distribution and host specialization in Tetralia crabs (Crustacea: Brachyura) symbiotic with corals in the Great Barrier Reef, Australia[END_REF][START_REF] Castro | Trapeziid crabs (Brachyura: Xanthoidea: Trapeziidae) of New Caledonia, eastern Australia, and the Coral Sea. Les Fonds meubles des lagons de Nouvelle Caledonie (Sédimentologie, Benthos)[END_REF] or because of more ambiguous distinguishing features at the juvenile stage [START_REF] Stella | Variation in the structure of epifaunal invertebrate assemblages among coral hosts[END_REF][START_REF] Rouzé | Juvenile Trapezia spp. crabs can increase juvenile host coral survival by protection from predation[END_REF] . Similarly, cryptic species of A. lottini are likely, based on correlated genetic and color pattern differences [START_REF] Williams | The marine Indo-West Pacific break: contrasting the resolving power of mitochondrial and nuclear genes[END_REF][START_REF] Knowlton | Species of marine invertebrates: a comparison of the biological and phylogenetic species concepts[END_REF] and male-female pairing on corals, but because the clades have not been formally described, they are not routinely distinguished in ecological studies [START_REF] Stella | Variation in the structure of epifaunal invertebrate assemblages among coral hosts[END_REF][START_REF] Stier | Housekeeping mutualisms: do more symbionts facilitate host performance?[END_REF][START_REF] Lassig | Communication and coexistence in a coral community[END_REF] . The most challenging are the coral hosts themselves, because of widespread phenotypic plasticity in response to environmental conditions [START_REF] Paz-García | Switch between morphospecies of Pocillopora corals[END_REF] , the slow evolution of mitochondrial DNA (e.g. [START_REF] Huang | Slow mitochondrial COI sequence evolution at the base of the metazoan tree and its implications for DNA barcoding[END_REF] ), and the possibility of interspecific hybridization [START_REF] Combosch | Interspecific hybridization and restricted trans-Pacific gene flow in the Tropical Eastern Pacific Pocillopora[END_REF] in corals generally. This is true in particular for members of the Pocilloporidae, despite the fact that some species have been used as a coral version of a 'laboratory-rat' for many years. However, progress in defining evolutionary lineages is being made [START_REF] Schmidt-Roach | With eyes wide open: a revision of species within and closely related to the Pocillopora damicornis species complex (Scleractinia; Pocilloporidae) using morphology and genetics[END_REF] . For example, the so-called P. damicornis species, long recognized to be phenotypically plastic [START_REF] Veron | Scleractinia of Eastern Australia. Part I Families: Thamnasteriidae, Astrocoeniidae, Pocilloporidae[END_REF] , has recently been found to consist of at least three evolutionary lineages, including types α (P. damicornis sensu [START_REF] Schmidt-Roach | With eyes wide open: a revision of species within and closely related to the Pocillopora damicornis species complex (Scleractinia; Pocilloporidae) using morphology and genetics[END_REF] ) and β (P. acuta sensu [START_REF] Schmidt-Roach | With eyes wide open: a revision of species within and closely related to the Pocillopora damicornis species complex (Scleractinia; Pocilloporidae) using morphology and genetics[END_REF].

Recognizing this cryptic diversity is essential for understanding how these mutualisms work, and how evolutionary events, environmental factors and anthropogenic pressures may shape or threaten the ecological persistence of these partnerships [START_REF] Bickford | Cryptic species as a window on diversity and conservation[END_REF][START_REF] Der Heyden | The application of genetics to marine management and conservation: Examples from the Indo-Pacific[END_REF] . Several species may co-occur and interact within a single host, leading to preferential patterns of associations with potential inhibitory or synergistic effects on the fitness of the host [START_REF] Stachowicz | Multiple mutualists provide complementary benefits to their seaweed host[END_REF] . Patterns of co-occurrence of exosymbiont associations may in turn be contingent upon the host phenotype or genotype [START_REF] Faucci | Host shift and speciation in a coral-feeding nudibranch[END_REF][START_REF] Tsang | Host-associated speciation in the coral barnacle Wanella milleporae (Cirripedia: Pyrgomatidae) inhabiting the Millepora coral[END_REF][START_REF] Best | Phenotypic and phylogenetic evidence for the role of food and habitat in the assembly of communities of marine amphipods[END_REF] . Recent evidence suggests that the exosymbiont species are not functionally equivalent in terms of the benefits they provide [START_REF] Mckeon | Species and size diversity in protective services offered by coral guard-crabs[END_REF] .

In this context, the primary objective of the present study was to describe patterns of association among P. damicornis types α and β (sensu [START_REF] Schmidt-Roach | With eyes wide open: a revision of species within and closely related to the Pocillopora damicornis species complex (Scleractinia; Pocilloporidae) using morphology and genetics[END_REF] ; note that we used this designation because it better reflects the phylogenetic proximity between the two clades) and their decapod exosymbionts in two distinct biogeographic regions, the understudied South Western Indian Ocean (Reunion Island, RI), described as an evolutionary hotspot [START_REF] Hoareau | The Southwestern Indian Ocean as a potential marine evolutionary hotspot: perspectives from comparative phylogeography of reef brittle-stars[END_REF] , and the South Western Pacific Ocean (New Caledonia, NC), a region of interest for its high biodiversity [START_REF] Myers | Biodiversity hotspots for conservation priorities[END_REF] . At each location, surveys took place at a site with high impacts (HI) and low impacts (LI) from human populations. We used DNA barcoding to identify coral host species and delineate cryptic lineages of exosymbionts. We then analyzed patterns of co-occurrence among species involved in this mutualistic network in order to evaluate key exosymbiotic assemblages for the coral host.

Results

Species characterization. Coral host identification and colony sizes. We collected a total of 234 colonies, among which 88 and 146 were identified molecularly as P. damicornis types α and β , respectively (hereafter referred to α and β colonies; Table 1). In New Caledonia, higher proportions of P. damicornis type α were observed among both adults (81%) and juveniles (69%), and almost all type β colonies came from high impact sites (NC-HI, Table 1). In Reunion Island, only P. damicornis type β was found. Within type β , based on open reading frame (ORF) sequences, we observed no genetic differentiation between Reunion Island and New Caledonia or between the two sites at Reunion Island (Fig. S1).

Among the 234 colonies, 114 were juveniles (RI: N = 56; NC: N = 58) and 120 were adults (RI: N = 61; NC: N = 59). The planar areas calculated for selected juvenile colonies (types α and β ) were not significantly different among the four sites (one-way ANOVA: F(3,100) = 2.22, P = 0.09; NC: mean ± standard error: 20 ± 1.4 cm 2 at NC-LI and 25 ± 2.5 cm 2 at NC-HI; RI: mean ± standard error: 23 ± 2.5 cm 2 at RI-LI and 17 ± 1.9 cm 2 at RI-HI). Furthermore, planar areas of juvenile colonies α and β , simultaneously present at NC-HI, were not significantly different (Kruskal-Wallis: df = 1, χ 2 = 3.389, P = 0.07). In contrast, planar areas of selected adult colonies were significantly different among sites (one-way ANOVA: F(3,102) = 165.7, P < 0.001), with colonies from New Caledonia being larger (mean ± standard error: 601 ± 22.6 cm 2 at NC-LI and 416 ± 11.5 cm 2 at NC-HI) than those from Reunion Island (median ± standard deviation: 297 ± 25.8 cm 2 at RI-LI and 100 ± 5.9 cm 2 at RI-HI). At NC-HI, adult coral colonies α versus β exhibited no significant differences in their planar areas (Kruskal-Wallis: df = 1, χ 2 = 3.39, P = 0.96). Exosymbiont identifications. In total, 162 specimens of the snapping shrimp A. lottini were collected. Phylogenetic analyses of both 16 S and COI sequences identified two highly supported divergent lineages (PP > 97%, BS > 85%; Fig. 1; Table 2). Net nucleotide divergence between these lineages was 15.2% for the COI gene, and 7% for the 16S gene. Lineage L1, characterized by a complex genetic structure (Fig. 1), occurred in both New Caledonia and Reunion Island, while lineage L2 was exclusively found in New Caledonia. Looking a posteriori at the color patterns of the two lineages, we observed that, while both exhibited a general orange color, the midline of the cephalothorax and abdomen of the L1 specimens presented a solid black band whereas lineage L2 displayed red dots down the midline (Fig. 1).

Region

Based on all these results, snapping shrimps A. lottini are considered in all subsequent analyses as two genetically distinct lineages, called A. lottini L1 and L2, which based on studies of Alpheus species separated by the Isthmus of Panama [START_REF] Knowlton | Divergence in proteins, mitochondrial-DNA, and reproductive compatibility across the Isthmus of Panama[END_REF] are almost certainly distinct species that diverged millions of years ago.

We collected a total of 332 Trapezia crabs which, based solely on morphological traits [START_REF] Castro | Phylogeny and systematics of the Trapeziidae Miers, 1886 (Crustacea: Brachyura), with the description of a new family[END_REF] , were identified as belonging to eight morphospecies (Fig. S2): T. guttata (Rüppel 1830), T. serenei (Odinetz 1984), T. bella (Dana 1852), T. speciosa (Dana 1852), T. cymodoce (Herbst 1801), T. lutea (Castro 1997), T. bidentata (Forskal 1775) and T. septata (Dana 1852). However, the phylogenetic analysis of a subset of individuals (at least one specimen per heterosexual pair and all juveniles; Table 2) revealed nine distinct supported clades based on 16S (N = 183, Fig. 2a) and ten based on COI (N = 163, Fig. 2b) (outgroup and GenBank/BOLD references excluded), which were also identified as distinct with bPTP models (Fig. 2).

Phylogenetic analyses of both COI and 16S genes revealed two highly supported monophyletic groups within the T. lutea clade (L1, L2), as well as two others within the T. bidentata clade (L1, L2); in both cases, these were considered as two distinct putative species with the bPTP models (Fig. 2). Also in both cases, the lineages were geographically partitioned: T. lutea L2 and T. bidentata L2 occurred in New Caledonia while T. lutea L1 and T. bidentata L1 occurred in Reunion Island (Fig. 2). Net nucleotide divergences (T. bidentata lineages: 2.1 and 2.5% for 16S and COI, respectively; T. lutea lineages: 2.6 and 4.4% for 16S and COI, respectively; Table S1) were comparable to what we observed between morphologically defined species both in allopatry or in sympatry (Table S1). However, no diagnostic color patterns specific to the genetic lineages within T. lutea and T. bidentata could be found (Fig. S2).

The discrepancy between the two markers in numbers of distinct putative species was due to inconsistent differentiation between two sympatric described species, T. serenei (RI and NC) and T. bella (RI) that mate assortatively. Although these taxa can be clearly distinguished by color patterns (Fig. S2), the 16S gene showed a very low level of genetic divergence (0.4%, Table S1) and branch support (PP: 84%; BS: 66%, respectively), in contrast to the COI gene (genetic difference: 1.4%; PP: 100%; BS: 97%). This inconsistency was also obtained with bPTP models, which delimited two distinct putative species based on the COI phylogeny, but only one putative species based on the 16S phylogeny (Fig. 2).

Based on these results of genetic distinction and/or color patterns, ten distinct species/lineages of Trapezia crabs are considered in all subsequent analyses: T. guttata, T. speciosa, T. serenei, T. bella, T. cymodoce, T. lutea L1 and L2, T. bidentata L1 and L2 and T. septata.

Community composition of exosymbiont lineages.

Factorial analysis (Fig. 3) showed compositional differences among communities of exosymbionts collected in P. damicornis α and β colonies according to biogeographic region (Indian versus Pacific Ocean) and sites (RI-LI, RI-HI, NC-LI and NC-HI). Three trapeziid crabs species (T. speciosa, T. septata or T. guttata) had a strong influence on the structure of exosymbiont communities, especially in adult corals. Differences in community composition between adult coral hosts α versus β were significant when all colonies from New Caledonia and Reunion Island were considered (ANOSIM: R = 0.3814, P = 0.001) (Fig. 3). Statistical tests also indicated significant differences in composition between β hosts from New Caledonia and Reunion Island (ANOSIM: R = 0.3999, P = 0.001), but not between α versus β hosts from New Caledonia (ANOSIM: N α = 48 and N β = 11, R = -0.032 P = 0.63). Exosymbiotic communities were not significantly different between the two reef sites of New Caledonia (ANOSIM: R = 0.014, P = 0.28), but they differed significantly between the two sites at Reunion Island (ANOSIM: R = 0.688, P = 0.001). Five key species among the ten identified (A. lottini L1, A. lottini L2, T. guttata, T. septata, and T. speciosa; Fig. 3) drove these differences (Table S2). The first axis of the FCA, accounting for 28.4% of the total variance, discriminated communities characterized by T. guttata and A. lottini L1 (mainly observed in α colonies from NC-HI and β colonies from RI-LI) from communities characterized by T. septata and A. lottini L2 specific to New Caledonian sites (NC-LI and NC-HI mainly composed of α colonies). Trapezia septata was the most prevalent species in α colonies (67%, Table 3) in New Caledonia, either in association with one pair of A. lottini L2 (23%) or with one pair of A. lottini L1 (21%), or as a unique species pair of exosymbionts (6.3%). Moreover, T. septata appeared to be the most gregarious species as it co-occurred with several other trapeziid species (T. guttata, T. cymodoce, T. serenei or T. lutea L2; Table 3). In contrast, T. guttata observed in both studied regions was found preferentially associated with different exosymbiotic species depending on the island: T. guttata (Table 3) mainly occurred with a pair of A. lottini L1 (68% of colonies harboring T. guttata), or living as a solitary pair (32% of colonies harboring T. guttata) in colonies from Reunion Island, while in New Caledonia it co-occurred with A. lottini L1 as often as with A. lottini L2 in β colonies. The second axis, representing 26.1% of the total variance, differentiated communities characterized by the presence of T. speciosa, which was typically found in colonies from RI-HI at Reunion Island (Table 3). This crab species was mostly observed by itself (85% of total communities including T. speciosa; Table 3) and occasionally in association with A. lottini L1 (N = 3). About half of P. damicornis juvenile colonies did not host any exosymbiont, regardless of location or host species type (α : 40% and β : 49%; Figs 3 and4, Table 3). However, when present (RI: N RI-LI = 18, N RI-HI = 19 and NC: N NC-LI = 23 and N NC-HI = 2), exosymbiont communities were significantly different among the three reef sites where numbers were large enough to be analyzed (RI-HI, RI-LI and NC-LI; ANOSIM: R = 0.2782, P = 0.001), as well as between groups of juvenile β colonies from both sites of Reunion Island (RI-LI versus RI-HI; ANOSIM: R = 0.1287, P = 0.02). Three trapeziid crab species (T. speciosa, T. guttata and T. septata) played a key role in the community composition of juvenile corals (Table S2), and mainly occurred by themselves (Figs 3 and4; Table 3). The first axis of the FCA, explaining 47.5% of the total variance, discriminated communities characterized by the two most prevalent species (T. speciosa and T. guttata) (Table 3). The communities shaped by the presence of T. speciosa were prevalent in juvenile hosts of type β from Reunion Island (64%; N RI-HI = 16 and N RI-LI = 8; Table 3), while those characterized by the presence of T. guttata were widely distributed on reefs of both Reunion and New Caledonia and in both host types: α (63%) and β (RI: 30% and NC: N = 1 colony; Table 3). The second axis of the FCA, explaining 28.3% of the total variance, differentiated exosymbiotic communities composed of crabs T. septata, the second most abundant species found in association with juvenile P. damicornis type α in New Caledonia (N NC-LI = 6 and N NC-HI = 0). Trapezia speciosa, T. guttata and T. septata co-occurred in rare cases but they were never recorded with T. bella, T. serenei, T. lutea L1 and L2 or T. bidentata L1 and L2.

S Shrimp K2 + G Alpheus A. lottini L1 N = 38 KY746726-KY746763 NC, RI - - - - G = 0.21 Alpheus A. lottini L2 N = 31 KY746764-KY746794 NC KP725488.1 Genbank Mayotte [1] Alpheus A. angulosus N = 1 - - FJ528454 Genbank [2] Crab HKY + G Quadrella Q. maculosa N = 1 - - FJ548954 Genbank [3] G = 0.16 Trapezia T. guttata N = 50 KY747168-KY747217 RI, NC Trapezia T. serenei N = 5 KY747063-KY747067 RI, NC Trapezia T. bella N = 5 KY747058-KY747062 RI Trapezia T. speciosa N = 57 KY747068-KY747124 RI Trapezia T. cymodoce N = 8 KY747050-KY747057 NC Trapezia T. lutea L1 N = 5 KY747039-KY747043 RI Trapezia T. lutea L2 N = 6 KY747044-KY747049 NC Trapezia T. bidentata L1 N = 2 KY747036-KY747037 RI Trapezia T. bidentata L2 N = 1 KY747038 NC Trapezia T. septata N = 43 KY747125-KY747167 NC COI Shrimps K2 + G + I Alpheus A. lottini L1 N = 46 KY746795-KY746840 NC, RI G = 2.28 Alpheus A. lottini L2 N = 32 KY746841-KY746872 NC, NC MBMIA477-06 BOLD FP I = 0.68 Alpheus A. idiocheles N = 1 MBMIA293-06 BOLD FP Crab GTR + G Lybia L. sp. LP-2009 N = 1 - - GQ260943 Genbank NLI [4] G = 0.25 Trapezia T. guttata N = 52 KY746954-KY747005 RI GBCMD9223-13 BOLD Australia [5] Trapezia T. serenei N = 5 KY747031-KY747035 RI, NC MBMIA599-06 BOLD FP Trapezia T. bella N = 5 KY747026-KY747030 RI Trapezia T. speciosa N = 50 KY746873-KY746922 RI Trapezia T. cymodoce N = 4 KY747018-KY747021 NC HM751069 Genbank [6] Trapezia T. lutea L1 N = 5 KY747006-KY747010 RI Trapezia T. lutea L2 N = 7 KY747011-KY747017 NC Trapezia T. bidentata L1 N = 2 KY747022-KY747023 RI Trapezia T. bidentata L2 N = 2 KY747024-KY747025 NC Trapezia T. bidentata L3 N = 1 - - GQ260917.1 Genbank Line Islands DS Trapezia T. septata N = 31 KY746923-KY746953 NC GBCMD9204-13 BOLD Australia [5] Trapezia T. tigrina N = 1 MBMIA398-06 BOLD FP DS Trapezia T. punctimanus N = 1 MBMIA186-06 BOLD FP DS Trapezia T. rufopunctata N = 1 MBMIA534-06 BOLD FP DS

Species interactions.

Observed patterns of association among the five key species revealed by the FCA were compared to their null distributions. These were generated with an unconstrained model (M1, which assumes that all species are able to colonize equally any coral host), a 'frequency' constrained model (M2, which assumes that some species are naturally able to colonize more corals than others), and a 'richness' constrained model (M3, only used in the analysis of species co-occurrence, which assumes that some corals are naturally more suitable to host different numbers of species). Exosymbionts species are denoted as follows: Trapezia speciosa (Tspe), T. guttata (Tgut), T. septata (Tsept), T. cymodoce (Tcymo), T. bidentata L1 (TbidL1) and L2 (TbidL2), T. bella (Tbel), T. serenei (Tser), T. lutea L1 (TlutL1) and L2 (TlutL2) for crabs, and Alpheus lottini L1 (AlotL1), A. lottini L2 (AlotL2) or non-identified (Alot) for shrimps. Species codes in bold represent the significant variables on axes 1 and/or 2 of the FCA analysis.

More than half (61%) of adult colonies from Reunion Island (all type β ) harbored only one of the five key species (Fig. 4a), a proportion significantly higher than expected by chance (observed: 37 colonies; 95% confidence interval (CI): 19-34 colonies for M1 and 1-6 colonies for M2). The frequency of corals occupied by a single species was particularly high at the RI-HI site (Table 3). In New Caledonia, the diversity of exosymbionts found in type β adult colonies did not differ from the null expectations (Fig. 4a; M1 and M2). On the other hand, type α adult colonies displayed a combination of two of the five key species in 34 out of 48 adult colonies, a proportion much higher than expected by chance (95% CI: 13-26 colonies for M1 and 7-17 colonies for M2; Fig. 4a). The number of adult colonies inhabited by a single species (observed: 12 colonies) overlapped with the null distributions obtained with the unconstrained model (95% CI: 10-20 colonies for M1), but not with the frequency constrained model (95% CI: 0-4 colonies for M2). Only 4% of adult colonies, all type α , contained three of the five key species, and no coral harbored four or five key species (Fig. 4a). Juvenile corals belonging to P. damicornis type β from Reunion and type α from New Caledonia, in contrast, did not harbor more than one exosymbiont species T. serenei 1

T. serenei + A. lottini (N = 3 juveniles) 1 T. lutea L2 1 A. lottini L2* - 6 5 2
T. cymodoce 2

T. guttata* 2 T. serenei 1 - - 5 3
T. guttata* A. lottini L1* - 1 3
A. lottini L2* - 1 2 - - 14 2 1 1 T. cymodoce A. lottini L2* - 2 T. lutea L2 1 A. lottini L1* (N = 1 juvenile) 1 A. lottini L1* T. lutea L2 1 - - 2
T. lutea L2 A. lottini L1* - 1 1
A. lottini L2* - 1 - - 1
T. bidentata L2 A. lottini L1* - 1 A. lottini L2* - 1 T. serenei A. lottini L2* - 1 - A. lottini L2* - 1 - - - 7 9 17 RI T. guttata* - - 7 7 2
A. lottini L1* -13 1 1

T. bella 1 (except one juvenile α from NC and three juveniles β from RI, Table 3), an observed distribution overlapping the null distributions (Fig. 4b) of the unconstrained model M1 but not the null distributions of the constrained model M2 (Fig. 4b). Indeed, when constraining species frequency in randomizations (M2), the observed proportions of juvenile corals inhabited by a single species was significantly higher than expected by chance in New Caledonia (observed: 21 colonies; 95% CI: 7-14 colonies) and in Reunion Island (observed: 31 colonies; 95% CI: 12-22 colonies). Only one out of the 18 juvenile β colonies from New Caledonia was inhabited by exosymbionts (one T. guttata), preventing any comparison with null distributions. Observed patterns of species co-occurrence in adult colonies compared to their null distributions generated with unconstrained (M1) and constrained (M2 and M3) models showed contrasting results between host species and locations (Fig. 5). Type β colonies inhabited by two species of exosymbionts at Reunion Island (30% of the coral population) were characterized by the presence of two shrimp-crab combinations, both deviating from null models of random association (Fig. 5). The first crab-shrimp association, T. speciosa with A. lottini L1, was significantly under-represented in the three null models (observed: 3 colonies; 95% CI: 7-14 colonies for M1 and M2 and 8-16 colonies for M3), while the second, T. guttata with A. lottini L1, was significantly over-represented in comparison to the null distributions obtained with models M1 and M2 only (observed: 15 colonies; 95% CI: 5-12 colonies for M1 and M2 and 8-15 colonies for M3; Fig. 5). Moreover, not a single colony hosted T. guttata and T. speciosa together, the two most common Trapezia species in the Reunion Island survey, which is far below what (a) adults of Pocillopora damicornis type β from Reunion Island (RI) and from New Caledonia (NC), and of P. damicornis type α from New Caledonia (NC), and (b) juveniles of P. damicornis type β from Reunion Island (RI) and type α from New Caledonia (NC). Black circles linked with a black line represent the observed data, while grey rectangles represent the 95% confidence intervals estimated from 10,000 randomly simulated communities generated with the unconstrained null model M1 and the 'frequency' constrained model M2 (see methods for a description of the models). would be expected by chance based on all three null models (95% CI null distributions: 6-13 colonies for M1 and M2, and 2-9 colonies for M3). Among the five key species, only T. speciosa mostly occurs by itself in a proportion significantly higher than distributions of the three null models (observed: 24 colonies; 95% CI: 6-14 colonies for M1, 7-15 colonies for M2 and 7-18 colonies for M3; Fig. S3). Type β colonies from New Caledonia (Fig. 5) also harbored shrimp-crab combinations. However, unlike corals from Reunion Island, T. guttata never co-occurred with A. lottini L1 or did not show any preferential association with A. lottini L2 (observed distributions overlap with the null distributions of the three null models, Fig. 5). The most prevalent association observed was T. septata with A. lottini L1 (observed: 4 colonies, Fig. 5), a proportion slightly higher than the null distributions of models M1 and M3 (95% CI: 0-3 colonies for both models) but overlapping with null distributions of model M2 (95% CI: 1-4 colonies). The same preferential association between T. septata and A. lottini L1 was recorded in type α colonies based on models M1 and M3 (observed: 13 colonies; 95% CI null distributions: 4-11 colonies for M1 and 2-11 colonies for M3; Fig. 5). However, the co-occurrence of T. septata with A. lottini L2, the most frequently observed combination, did not deviate from predictions of the null models M1 and M2 (observed: 16 colonies; 95% CI: 11-18 and 13-19 colonies for M1 and M2, respectively) but deviate from those obtained with the null model M3 (95% CI: 2-11 colonies). In addition, type α colonies often hosted T. guttata with one of the two lineages of A. lottini but their co-occurrence did not deviate from the three null models (Fig. 5). The co-occurrences of A. lottini L2 with A. lottini L1 (observed: 1 colony; 95% CI null distributions: 8-14 colonies for M1 and M2, and 2-11 colonies for M3) and T. guttata with T. septata (observed: 2 colonies; 95% CI null distributions: 3-8 colonies for M1 and M2, and 3-12 colonies for M3) were observed less frequently than expected by chance in corals for all the null models (Fig. 5).

T. speciosa* A. lottini L1* - 2 1 1 - T. guttata* 1 - 7 23 15
T. bidentata L1 + T. serenei 1 T. bella A. lottini L1* - 2 - - 1 2
T. bidentata L1 A. lottini L1* - 1 T. lutea L1 A. lottini L1* - 1 1 - - 2 1 1 - A. lottini L1* - 1 - - - 1 13 1 6

Table 3. Composition of exosymbiotic communities and their frequencies in colonies of

Discussion

Genetic data confirmed the ubiquity of sibling species among crustaceans (e.g. [START_REF] Knowlton | Sibling species in the sea[END_REF][START_REF] Lee | Global phylogeography of a cryptic copepod species complex and reproductive isolation between genetically proximate 'populations[END_REF][START_REF] Edmands | Phylogeography of the intertidal copepod Tigriopus californicus reveals substantially reduced population differentiation at northern latitudes[END_REF][START_REF] Williams | Evidence for three major clades within the snapping shrimp genus Alpheus inferred from nuclear and mitochondrial gene sequence data[END_REF][START_REF] Mathews | Genetic, ecological, and behavioural divergence between two sibling snapping shrimp species (Crustacea: Decapoda: Alpheus)[END_REF][START_REF] Daniels | Molecular and morphometric data demonstrate the presence of cryptic lineages among freshwater crabs (Decapoda: Potamonautidae: Potamonautes) from the Drakensberg Mountains, South Africa[END_REF][START_REF] De Bruyn | Huxley's line demarcates extensive genetic divergence between eastern and western forms of the giant freshwater prawn, Macrobrachium rosenbergii[END_REF] ). Alpheus lottini, currently considered in ecological studies as a single species, constituted two highly divergent genetic lineages that mate assortatively and exhibited corresponding differences in color pattern, indicating that they are almost certainly separate species. These two lineages, previously detected in allopatry and sympatry in other studies [START_REF] Williams | The marine Indo-West Pacific break: contrasting the resolving power of mitochondrial and nuclear genes[END_REF][START_REF] Knowlton | Species of marine invertebrates: a comparison of the biological and phylogenetic species concepts[END_REF] , were also found in sympatry in New Caledonia. However, the two distinct lineages were rarely found within the same coral host, indicating habitat partitioning typical of a micro-allopatric distribution. Similarly, molecular analyses revealed the presence of previously unrecognized, allopatric cryptic taxa within two morphospecies of Trapezia (T. lutea and T. bidentata) that have genetic divergences comparable to those between closely related described sympatric and/or allopatric species (Table S1) and that mate assortatively. While we did not observe any conspicuous color pattern differences (Fig. S2), differentiation of these potentially cryptic species could be explored in the future using morphometric measurements or more detailed color pattern analysis. With all these observations it seems unlikely, but not excluded, that these divergent clades may highlight strong geographically based genetic structure within both species. On the other hand, analyses based on 16S sequences did not support a genetic distinction between T. bella and T. serenei, despite a clear distinction of two monophyletic clades based on the COI marker, color patterns (Fig. S2 39 ) and mating pattern. Given the slower evolutionary rate of the 16S gene compared to COI gene (e.g. [START_REF] Lefébure | Relationship between morphological taxonomy and molecular divergence within Crustacea: proposal of a molecular threshold to help species delimitation[END_REF] ), together with the low pairwise genetic difference in COI (1.4%), our findings suggest a recent speciation event between T. bella and T. serenei.

Molecular identification of surveyed hosts and exosymbionts enabled the detection of preferential associations that would otherwise have been missed. In particular, it revealed that each of the two genetically revealed lineages of A. lottini had different affinities with trapeziid crabs. For example in New Caledonia, while both A. lottini lineages were commonly found in association with T. septata, only the partnership with A. lottini L1 occurred more often than by chance (models M1 and M3, Fig. 5). These findings suggest that the lineages of A. lottini not only differ genetically but also in ecological traits that affect the strength of interactions with co-occurring trapeziid crabs.

Shrimp-crab associations were predominant in adult P. damicornis α and β colonies in both biogeographic provinces. This pattern is in accordance with behavioral observations that described resident crabs and snapping shrimps occupying slightly different portions of the coral host [START_REF] Patton | Distribution and ecology of animals associated with branching corals (Acropora spp.) from the Great Barrier reefs, Australia[END_REF] , therefore minimizing competitive interactions [START_REF] Lassig | Communication and coexistence in a coral community[END_REF][START_REF] Glynn | Some physical and biological determinants of coral community structure in the eastern Pacific[END_REF] . Some authors have even reported positive interactions, with A. lottini physically cleaning the chelipeds of T. cymodoce [START_REF] Lassig | Communication and coexistence in a coral community[END_REF] . On the other hand, both are highly territorial and exclude conspecific intruders that share similar ecological requirements [START_REF] Lassig | Communication and coexistence in a coral community[END_REF] . Mechanical or chemical communication by direct contact with crabs might vary according to the exosymbiotic species involved. Furthermore, our survey showed that the species involved in crab-shrimp associations differed greatly between geographic provinces, mostly because several trapeziid species were restricted to only one locality (e.g., T. speciosa at Reunion Island and T. septata in New Caledonia) and because A. lottini L2 was absent from Reunion Island. Moreover, we found that patterns of association for a given species pair differed between New Caledonia and Reunion Island, highlighting differences in the strength of interactions at the regional scale. For example, T. guttata preferentially associated with A. lottini L1 in Reunion Island (null models M1 and M2; Fig. 5), while these two species co-occurred randomly (no deviation from the three null models) in New Caledonia.

Marked differences in community composition at the local scale, i.e., between sites within a region, were also detected. One of the most striking differences occurred at Reunion Island, where adult corals of the high impacted site were mostly inhabited by a single crab species (T. speciosa) in proportions higher than expected by chance (i.e., three null models; Fig. S3). The establishment of dominant multi-specific or mono-specific assemblages of mutualists can be generated by several ecological and/or evolutionary factors (reviewed in ref. 1). At this site, characterized by high sedimentation and eutrophication [START_REF] Cuet | Phase pilote d ' extension du RNO à La Réunion[END_REF] , corals were smaller and with thicker branches than at the less impacted site. Therefore, one possible mechanism explaining the preponderance of monospecific assemblages may simply be the limited habitat space [START_REF] Lassig | Communication and coexistence in a coral community[END_REF] . However, observed patterns of species interactions indicate that simple space limitation is not the main ecological mechanism involved. For example, the occurrence of T. speciosa with other key common species was significantly lower than expected (i.e. A. lottini L1 or T. guttata, Fig. 5) under the three null models whereas other sedentary coral-obligate species (e.g., Paragobiodon fishes, data not shown) sometimes co-occurred with T. speciosa in the same coral colony. However, the territorial behavior of T. speciosa against close competitors may lead to the exclusion of conspecifics and A. lottini, as suggested by studies highlighting differences in competitive and territoriality abilities [START_REF] Odinetz | L' éthologie au service de la systématique: l' exemple des Trapezia (crustacés décapodes, brachyoures)[END_REF][START_REF] Mckeon | Species and size diversity in protective services offered by coral guard-crabs[END_REF] among exosymbionts. The decrease of diversity within guilds of mutualists in response to interspecific competition is a general ecological process, particularly well documented in terrestrial ecosystems (e.g. plant-mycorrhizal mutualisms [START_REF] Jansa | Are there benefits of simultaneous root colonization by different arbuscular mycorrhizal fungi?[END_REF] ). The chronic exposure to environmental stressors at this site may also affect the dynamics of interactions among exosymbionts, from mutualism to competition, as was previously observed during prolonged exposure to thermal stress [START_REF] Stella | From cooperation to combat: adverse effect of thermal stress in a symbiotic coral-crustacean community[END_REF] . Finally, ecological specialization to host morphology could also be part of the explanation as it has been observed in other systems, such as cryptic species of barnacles which preferentially associate with different Millepora morphospecies [START_REF] Tsang | Host-associated speciation in the coral barnacle Wanella milleporae (Cirripedia: Pyrgomatidae) inhabiting the Millepora coral[END_REF] . Regardless of the mechanisms involved, this represents a rare case where Trapezia species do not display any preferential partnership with a heterosexual pair of A. lottini shrimps [START_REF] Stier | Housekeeping mutualisms: do more symbionts facilitate host performance?[END_REF][START_REF] Mckeon | Multiple defender effects: synergistic coral defense by mutualist crustaceans[END_REF][START_REF] Castro | Movements between coral colonies in Trapezia ferruginea (Crustacea: Brachyura), an obligate symbiont of scleractinian corals[END_REF] or even with other congeners in adult coral hosts.

At the juvenile stage, about half of monitored coral colonies lacked exosymbionts, while the others were associated with a single juvenile trapeziid crab. Interestingly, T. speciosa crabs were dominant among juvenile coral colonies at the impacted reef site in Reunion Island, similarly to what was observed for adult corals. This suggests either high levels of self-recruitment in these crab populations (e.g. [START_REF] Ritson-Williams | Host specificity of four corallivorous Phestilla nudibranchs (Gastropoda: Opisthobranchia)[END_REF] ) or recruitment failure of other crab species (e.g., T. guttata), most likely linked with unfavorable environmental conditions. Conversely, there was no relationship between community composition of exosymbionts in juvenile versus adult corals at the low impacted site in Reunion Island (RI-LI) or at either site in New Caledonia, all characterized by multispecific exosymbiotic communities in adult corals. One hypothesis for this result is that patterns of recruitment are highly variable through time, which translates into changes in species associations in adult corals. Alternatively, the establishment of multispecific partnerships may occur later during the ontogeny of the coral colony through migrations of adult shrimps and crabs [START_REF] Castro | Movements between coral colonies in Trapezia ferruginea (Crustacea: Brachyura), an obligate symbiont of scleractinian corals[END_REF] or even through effective recruitment of other exosymbiotic species when living space becomes sufficient.

The absence of differences in exosymbiont community structure between sympatric P. damicornis types α and β in New Caledonia is another interesting ecological feature of mutualistic coral-exosymbionts partnerships that emerged from our results. It suggests that associated exosymbiotic communities are not strictly host specific, and that similar exosymbiont communities can be found in closely related coral species, which despite their different reproductive traits [START_REF] Schmidt-Roach | Assessing hidden species diversity in the coral Pocillopora damicornis from Eastern Australia[END_REF] , may not be completely reproductively isolated [START_REF] Combosch | Interspecific hybridization and restricted trans-Pacific gene flow in the Tropical Eastern Pacific Pocillopora[END_REF] . This preliminary assumption will need further investigation to be confirmed by obtaining more balanced sample sizes of types α and β from the same site. Furthermore, an effect of the host genet as a driver of exosymbiont community composition (e.g. [START_REF] Baums | New insights into the dynamics between reef corals and their associated dinoflagellate endosymbionts from population genetic studies[END_REF][START_REF] Parkinson | The extended phenotypes of marine symbioses: ecological and evolutionary consequences of intraspecific genetic diversity in coral-algal associations[END_REF] ) among coral colonies type α or β cannot be assumed, since some coral clones (i.e., colonies with similar multi-locus genotypes) had different exosymbiotic communities (data not shown).

Variation in exosymbiotic community composition between regions and habitats likely translates into fitness differences among host populations, depending on ecological traits of the exosymbiont species involved. The ability of one heterosexual pair of crabs and shrimps to co-occur, while they both exclude congeners, represents a selective advantage to the coral host because the benefits provided to the host facing acute environmental stressors increase with the number and diversity of exosymbionts (e.g., sedimentation load [START_REF] Stier | Housekeeping mutualisms: do more symbionts facilitate host performance?[END_REF] , predation [START_REF] Mckeon | Multiple defender effects: synergistic coral defense by mutualist crustaceans[END_REF][START_REF] Glynn | Some physical and biological determinants of coral community structure in the eastern Pacific[END_REF] ). Therefore, coral hosts associated with a single pair of T. speciosa in the high impacted site at Reunion Island may prove less resilient to future episodes of sedimentation or predator outbreak than hosts harboring multi-specific crab-shrimp communities. Alternatively, the association between the coral host and a single species of Trapezia could be more stable in a permanently disturbed environment than fragile crab-shrimp communities [START_REF] Stella | From cooperation to combat: adverse effect of thermal stress in a symbiotic coral-crustacean community[END_REF] in which both species depend on each other [START_REF] Lassig | Communication and coexistence in a coral community[END_REF] . Moreover, services provided to the host are dependent upon the species involved as well as the nature of the environmental stressor. For example, it was shown that the co-occurrence of T. serenei crabs and A. lottini shrimps had an additive effect on the removal of sediments, but they combined synergistically to better defend their coral host from corallivorous predators [START_REF] Mckeon | Multiple defender effects: synergistic coral defense by mutualist crustaceans[END_REF] . Because A. lottini has been considered as a single species in ecological studies to date, we do not know how the newly discriminated species might differ in their functional roles.

Our findings highlighted the complexity of coral-guild networks with preferential exosymbiotic associations that vary between sites and biogeographic regions. Ecological trade-offs for coral hosts that associate with monoversus multi-specific exosymbiotic communities, as well as functional benefits of various species pairs including the cryptic taxa identified herein, remain poorly understood. Therefore, further research focusing on functional traits of key exosymbiont species, as well as on the nature of interactions (synergy versus antagonism) among them in the context of their sensitivity to various environmental conditions is critical. Understanding the geographical mosaic of exosymbiont species associations will shed light on how these communities affect the resistance and resilience of coral populations to ongoing environmental changes.

Material and Methods

Specimen collection and measurement. Colonies presenting P. damicornis-like morphology (identified a posteriori as types α and β sensu Schmidt-Roach et al. [START_REF] Schmidt-Roach | With eyes wide open: a revision of species within and closely related to the Pocillopora damicornis species complex (Scleractinia; Pocilloporidae) using morphology and genetics[END_REF] ) were collected by SCUBA or snorkeling in two distinct biogeographic regions: Reunion Island (RI) in the South Western Indian Ocean and New Caledonia (NC) in the South Western Pacific Ocean (Table 1). At each island, we selected two fringing reefs where juveniles and adults were abundant and that appeared to differ in the intensity of anthropogenic environmental stressors based on reports of sedimentation rates and/or nutrient concentrations (NC [START_REF] Jacquet | Impact des apports en nutriments sur le réseau trophique planctonique du lagon sud-ouest de Nouvelle-Calédonie[END_REF][START_REF] Jacquet | Response of phytoplankton communities to increased anthropogenic influences (southwestern lagoon, New Caledonia)[END_REF][START_REF] Torré Ton | Correspondence between the distribution of hydrodynamic time parameters and the distribution of biological and chemical variables in a semi-enclosed coral reef lagoon[END_REF] and and RI [START_REF] Cuet | Phase pilote d ' extension du RNO à La Réunion[END_REF] ). We chose 'RI-HI' in RI (21°20'S, 55°28'E) and 'NC-HI' in NC (near S te Marie Bay, 22°18'S, 166°28'E) as high impacted sites, and 'RI-LI' in RI (lagoon channel, 21°5'S, 55°13'E) and 'NC-LI' in NC (22°15'S, 166°19'E) as low impacted sites. Since differences in coral size are known to affect the composition of exosymbiont communities [START_REF] Rouzé | Juvenile Trapezia spp. crabs can increase juvenile host coral survival by protection from predation[END_REF][START_REF] Abele | Comparative species richness in fluctuating and constant environments: coral-associated decapod crustaceans[END_REF][START_REF] Caley | Separating ecological effects of habitat fragmentation, degredation, and loss on coral commensals[END_REF] , we focused our sampling on two size classes of corals: juveniles colonies with diameter ≤ 5 cm [START_REF] Penin | Early post-settlement mortality and the structure of coral assemblages[END_REF] and large adult colonies with diameter > 10 cm. Numbers of adult and juvenile corals sampled at each site are summarized in Table 1. We measured the maximum diameter (L) and the maximum perpendicular diameter (l) of each coral colony to estimate a posteriori the planar area using the formula for an ellipse (S = π × L/2 × l/2). A plastic bag was placed over each colony to minimize the loss of exosymbionts prior to detachment from the substrate using a hammer and chisel. Individual colonies were then transported back to the laboratory. All Trapezia crabs and A. lottini shrimps were extracted by hand by fragmenting the colonies, identified to the species level using available morphological identification keys [START_REF] Castro | Phylogeny and systematics of the Trapeziidae Miers, 1886 (Crustacea: Brachyura), with the description of a new family[END_REF] , photographed alive and individually preserved in 80% ethanol. A small piece of the coral host sampled directly underwater was also preserved in 80% ethanol for subsequent molecular identification. All experimental protocols carried out on live material (corals and exosymbionts) were performed according to collection permits approved and obtained by the 'Direction de l'Environnement, de l' Aménagement et du Logement de La Réunion' (order n° 2013-23) and the 'National Marine Reserve of Reunion Island' as well as the Province Sud of New Caledonia (order n° 1315-2014/ARR/DENV).

Molecular analyses.

Host species identity was checked for all sampled colonies by amplifying the mitochondrial open reading frame (ORF) region using the FATP6.1 and the RORF primers [START_REF] Flot | Patterns of genetic structure among Hawaiian corals of the genus Pocillopora yield clusters of individuals that are compatible with morphology[END_REF] . Coral DNA (polyps + microorganisms) was extracted using the Gentra Puregene Tissue Kit (Qiagen, Germany). For crustaceans, a small piece of muscle tissue was sampled from each individual crab and shrimp for total genomic DNA extraction using the DNeasy Blood and Tissue kit (Qiagen, Germany). A portion of the cytochrome c oxidase subunit I (COI) gene and the 16S rRNA gene were amplified for each exosymbiont using universal primers (COI: LCO1490/HCO2198 [START_REF] Folmer | DNA primers for amplification of mitochondrial cytochrome c oxidase subunit I from diverse metazoan invertebrates[END_REF] and 16S: 16Sar/16Sbr 66 ).

All PCR reactions were performed in a final volume of 20 μ L using the Type-It Microsat kit (Qiagen, Germany), following manufacturer recommendations and 1-10 ng of template DNA. Amplification conditions were as described in Schmidt-Roach et al. [START_REF] Schmidt-Roach | Assessing hidden species diversity in the coral Pocillopora damicornis from Eastern Australia[END_REF] for ORF and Leray et al. [START_REF] Leray | Moorea BIOCODE barcode library as a tool for understanding predator-prey interactions: insights into the diet of common predatory coral reef fishes[END_REF] for COI. The following temperature profile was used for the amplification of 16S: 95 • C for 5 min, followed by 35 cycles of 30 sec at 95 • C, 30 sec at 50 • C and 45 sec at 72 • C, followed by a final extension at 72 • C for 5 min. Both 16S and ORF amplicons were sent for sequencing in one direction to Genoscreen (Lille, France) and purified COI products to the Laboratory of Analytical of Biology of the Smithsonian National Museum of Natural History for sequencing in both directions.

Chromatograms were edited and aligned using MUSCLE implemented in Mega 6.06 and adjusted by eye in a single alignment per group and per marker. Publicly accessible databases (i.e., GenBank, BOLD) were searched for homologous sequences of Trapezia and Alpheus, as well as outgroup sequences, and these were added to previous alignments. For the hosts, the ORF sequences were aligned with those generated by Schmidt-Roach et al. [START_REF] Schmidt-Roach | Assessing hidden species diversity in the coral Pocillopora damicornis from Eastern Australia[END_REF] as the reference (PopSet accession number: 413910611).

Data analyses. Phylogenetic relationships among species were estimated independently per group and per marker using Bayesian Markov Chain Monte Carlo (MCMC) analysis (MrBayes 3.2.4). The best substitution model was determined using the Bayesian Information Criterion, as implemented in MEGA 6.06 (Table 2), across sites for 1 to 5 million generations. Bootstrap support values were also assessed from 2000 Maximum Likelihood (ML) replicate trees in MEGA 6.06. Phylogenetic trees were represented with both Bayesian posterior probabilities (PP) and ML bootstrap support (BS) values with FigTree 1.4.2. In addition, as a method for delimiting exosymbiont species, we used the Maximum Likelihood implementation of the Poisson Tree Processes model based on Bayesian phylogeny gene trees (bPTP [START_REF] Zhang | A general species delimitation method with applications to phylogenetic placements[END_REF] ). The bPTP model uses the number of substitutions to simulate speciation and coalescent events, assuming that the number of substitutions is significantly higher between individuals belonging to different species than between conspecifics. The NEXUS trees were used as input data and submitted to the bPTP webserver (http://species.h-its.org/ptp/), but with reduced number of sequences in the Trapezia dataset, because PTP tends to overestimate the number of recognized species when there is uneven sampling of individuals per species [START_REF] Zhang | A general species delimitation method with applications to phylogenetic placements[END_REF] . In our Trapezia matrix, a large number of sequences were assigned to T. guttata, T. speciosa and T. septata, so we first removed identical sequences and pruned additional sequences that showed little variation in our dataset. The bPTP analyses were run without the outgroup (improving species delimitation) with 5 million MCMC generations with a burn-in of 25%, and other parameters with default values. All sequences were deposited in GenBank (accession numbers: for exosymbionts detailed in Table 2, and KY747218-KY747245 for Pocillopora).

Following identification of molecular lineages for both coral hosts and exosymbionts, similarities between communities of exosymbionts were visualized in two dimensions using a Factorial Correspondence Analysis (FCA) performed using the ade4 package in R 3.1.2. FCA uses a chi-square distance to measure similarities between communities without taking into account coral colonies with no observations. Compositional differences were tested between coral types (α versus β ) and between locations (New Caledonia versus Reunion Island) using an analysis of similarity (ANOSIM) performed in R with the Vegan package [START_REF] Dixon | a package of R functions for community ecology[END_REF] .

To further explore patterns of association for the five most common exosymbiont species, we compared observed patterns of species richness and co-occurrence in our survey to patterns expected by chance in randomly simulated communities. To mimic ecological patterns under various scenarios, we generated a set of three null models: (1) an 'unconstrained' model (M1) in which each of the five species was randomly assigned to corals;

(2) a 'frequency' model (M2) in which the total number of occurrences of each species was fixed to account for the fact that some species are more common than others in communities; and (3) a 'richness' model (M3) (simulated for comparison to observed patterns of co-occurrence only) in which the total number of species per coral was fixed to consider the fact that some corals are naturally more suitable to host different numbers of species (e.g. larger corals can host more species of crab). The two constrained models were simulated in R using the Picante Package [START_REF] Kembel | Picante: R tools for integrating phylogenies and ecology[END_REF] . Each null model was generated separately depending on the biogeographic region (RI or NC) and/or the coral types (α or β ) to test their effect on coral community structure and because they likely comprise distinct species pools. For each null model, the total number of coral hosts was fixed at 61 (RI all type β ), 48 (NC, type α ) and 11 (NC, type β ) for adults and 56 (RI all type β ) and 40 (NC all type α ) for juveniles (Table 1), because these values corresponded to numbers of corals inhabited by at least one of these exosymbiont species. Ten thousand random communities were simulated for each null model, and the 95% confidence interval (CI) of the null distribution was estimated and then compared to our observed distribution. The hypothesis of random species association was rejected if observed values fell outside the 95% confidence interval. We tested for differences in planar areas of coral colonies between sites using analyses of variance (ANOVA), in case of homogeneity of variance, or with non-parametric Kruskal-Wallis tests on ranks.

  Adult α : N = 19, β : N = 10 Juvenile α : N = 10, β : N = 18

Figure 1 .

 1 Figure 1. Phylogenetic tree of Alpheus lottini sequences derived from Bayesian approaches using 16S (a) and COI (b) partial genes. Bayesian posterior probabilities (PP; first values) and Maximum Likelihood bootstrap support values (BS; in brackets) are presented in black. PP and BS values corresponding to single putative species recognized by the PTP analyses are presented in grey.

Figure 2 .

 2 Figure 2. Phylogenetic tree of Trapezia sequences derived from Bayesian approaches using 16S (a) and COI (b) partial genes. Bayesian posterior probabilities (PP; first values) and Maximum Likelihood bootstrap support values (BS; in brackets) are presented in black. PP and BS values corresponding to single putative species recognized by the PTP analyses are presented in grey. Locations are coded as follows: RI for Reunion Island, NC for New Caledonia, FP for French Polynesia and NLI for Line Islands.

Figure 3 .

 3 Figure 3. Factorial Correspondence Analysis (FCA) based on abundance of different Trapezia and Alpheus lottini species with Pocillopora damicornis types α versus β from Reunion Island and New Caledonia at adult (top) versus juvenile (bottom) stages.Exosymbionts species are denoted as follows: Trapezia speciosa (Tspe), T. guttata (Tgut), T. septata (Tsept), T. cymodoce (Tcymo), T. bidentata L1 (TbidL1) and L2 (TbidL2), T. bella (Tbel), T. serenei (Tser), T. lutea L1 (TlutL1) and L2 (TlutL2) for crabs, and Alpheus lottini L1 (AlotL1), A. lottini L2 (AlotL2) or non-identified (Alot) for shrimps. Species codes in bold represent the significant variables on axes 1 and/or 2 of the FCA analysis.

  Pocillopora damicornis type α and β (sensu Schmidt-Roach et al. 2014) at the adult and juvenile stages and in different sites [low impacted (LI) versus high impacted (HI)] from Reunion Island (RI) and New Caledonia (NC).* exosymbiotic key species.

Figure 4 .

 4 Figure 4. Number of exosymbiotic species among the five key species co-occurring within coral colonies:(a) adults of Pocillopora damicornis type β from Reunion Island (RI) and from New Caledonia (NC), and of P. damicornis type α from New Caledonia (NC), and (b) juveniles of P. damicornis type β from Reunion Island (RI) and type α from New Caledonia (NC). Black circles linked with a black line represent the observed data, while grey rectangles represent the 95% confidence intervals estimated from 10,000 randomly simulated communities generated with the unconstrained null model M1 and the 'frequency' constrained model M2 (see methods for a description of the models).

Figure 5 .

 5 Figure 5. Interspecific pairwise co-occurrence of five exosymbiotic key species in adult Pocillopora damicornis type β from Reunion Island (RI) or from New Caledonia (NC), and type α from New Caledonia (NC). Black circles represent the observed data. Colored rectangles represent the 95% confidence intervals from 10,000 randomly simulated interactions between shrimp-shrimp (red), crab-shrimp (purple) or crab-crab (blue) assemblages generated with the unconstrained null model M1, the 'frequency' constrained model M2 and the 'richness' constrained model M3 (see methods for a description of the models).

  , M. Leray, H. Magalon, L. Penin, P. Gélin, N. Knowlton & C. Fauvelot Scientific Reports 7:44923; doi: 10.1038/srep44923; published online 30 March 2017; updated on 16 May 2017 This Article contains typographical errors in Table 2. For Gene 'COI' , species 'T. guttata' under 'Present study' , the location 'RI' should read 'RI, NC' . This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ © The Author(s) 2017 OPEN

Table 1 . Number of colonies of Pocillopora damicornis type α and β (sensu Schmidt-Roach et al. 2014 27 ) monitored for their associated exosymbiotic communities at the adult and juvenile stages and from different sites at Reunion Island and New Caledonia.

 1

Table 2 . Sequence sets used for each crab and shrimp group with mtDNA 16 S and COI partial sequences. The

 2

nucleotide substitution models used are indicated as follows: K2 for Kimura-2-parameter

[START_REF] Kimura | A simple method for estimating evolutionary rate of base substitutions through comparative studies of nucleotide sequences[END_REF]

, HKY for Hasegawa-Kishino-Yano

[START_REF] Hasegawa | Dating of the human-ape splitting by a molecular clock of mitochondrial DNA[END_REF]

and GTR for General Time Reversible

[START_REF] Tavaré | Some probabilistic and statistical problems in the analysis of DNA sequences[END_REF]

, with Gamma correction (G) and/or with a certain fraction of sites evolutionary invariable (+ I). Locations are coded as follows: RI for Reunion Island, NC for New Caledonia, FP for French Polynesia and LNI for northern Line Islands. [1] 74 ; [2] 75 ; [3] 76 ; [4] 77 ; [5] 9 ; [6] 78 ; DS direct submission.

Scientific RepoRts | 7:44923 | DOI: 10.1038/srep44923

Scientific RepoRts | 7:46818 | DOI: 10.1038/srep46818 www.nature.com/scientificreports

Acknowledgements

We thank J. Poupin for identification assistance of exosymbionts. We also thank G. Mou-Tham, A. Pirog and J. Gadenne-Viscontini for field assistance as well as the staff of IRD from Noumea for logistical assistance in the field (M. Clarque, P. Naudin, and S. Tereua) and in laboratory (L. Millet, and D. Grulois). We are grateful to M. Gil, A. Stier and G. Lecellier for helping in the null model analyses. This work was funded by the Laboratoire d'Excellence CORAIL projects EXCESS and EXECO. Portions of the laboratory and computer work were conducted with the support of the L.A.B. facilities of the Smithsonian National Museum of Natural History (Washington DC). This is ENTROPIE contribution #202.

Author Contributions

H.R., H.M., L.P. and C.F. conceived the study and designed experiments; H.R., H.M., L.P., C.F. and N.K. contributed reagents and/or material; H.R. performed the fieldwork; H.R., M.L. and P.G. performed molecular analysis; H.R. analyzed the data and prepared figures and tables; H.R., M.L. and C.F. wrote the paper; H.R., M.L., C.F., H.M., L.P. and N.K. reviewed the paper. Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

Additional Information

Supplementary information accompanies this