text:
Aircraft carrier

An aircraft carrier is a warship that serves as a seagoing airbase, equipped with a full-length flight deck and facilities for carrying, arming, deploying, and recovering aircraft. Typically, it is the capital ship of a fleet, as it allows a naval force to project air power worldwide without depending on local bases for staging aircraft operations. Carriers have evolved since their inception in the early twentieth century from wooden vessels used to deploy balloons to nuclear-powered warships that carry numerous fighters, strike aircraft, helicopters, and other types of aircraft. While heavier aircraft such as fixed-wing gunships and bombers have been launched from aircraft carriers, these aircraft have not landed on a carrier. By its diplomatic and tactical power, its mobility, its autonomy and the variety of its means, the aircraft carrier is often the centerpiece of modern combat fleets. Tactically or even strategically, it replaced the battleship in the role of flagship of a fleet. One of its great advantages is that, by sailing in international waters, it does not interfere with any territorial sovereignty and thus obviates the need for overflight authorizations from third-party countries, reduces the times and transit distances of aircraft and therefore significantly increases the time of availability on the combat zone.

There is no single definition of an "aircraft carrier", and modern navies use several variants of the type. These variants are sometimes categorized as sub-types of aircraft carriers, and sometimes as distinct types of naval aviation-capable ships. Aircraft carriers may be classified according to the type of aircraft they carry and their operational assignments. Admiral Sir Mark Stanhope, RN, former First Sea Lord (head) of the Royal Navy, has said, "To put it simply, countries that aspire to strategic international influence have aircraft carriers." Henry Kissinger, while United States Secretary of State, also said: "An aircraft carrier is 100,000 tons of diplomacy."

As of, there are 47 active aircraft carriers in the world operated by fourteen navies. The United States Navy has 11 large nuclear-powered fleet carriers—carrying around 80 fighters each—the largest carriers in the world; the total combined deck space is over twice that of all other nations combined. As well as the aircraft carrier fleet, the US Navy has nine amphibious assault ships used primarily for helicopters, although these also each carry up to 20 vertical or short take-off and landing (V/STOL) fighter jets and are similar in size to medium-sized fleet carriers. India, the United Kingdom and China each operate two aircraft carriers. France and Russia each operate a single aircraft carrier with a capacity of 30 to 60 fighters. Italy operates two light V/STOL carriers and Spain operates one V/STOL aircraft-carrying assault ship. Helicopter carriers are operated by Japan (4, two of which are being converted to operate V/STOL fighters), France (3), Australia (2), Egypt (2), South Korea (2), China (3), Thailand (1) and Brazil (1). Future aircraft carriers are under construction or in planning by China, France, India, Russia, South Korea, Turkey, and the US.

General features

 * Speed is a crucial attribute for aircraft carriers, as they need to be able to be deployed quickly anywhere in the world and have to be fast enough to evade detection and targeting from enemy forces. A high speed also increases the "wind over the deck", boosting the lift available for fixed-wing aircraft to carry fuel and ammunition. In order to evade nuclear submarines, the carriers should have a speed of more than 30 kn.
 * Aircraft carriers are among the largest types of warships due to their need for ample deck space.
 * An aircraft carrier must be able to perform increasingly diverse mission sets. Diplomacy, power projection, quick crisis response force, land attack from the sea, sea base for helicopter and amphibious assault forces, anti-surface warfare (ASUW), defensive counter air (DCA), and humanitarian aid disaster relief (HADR) are some of the missions the aircraft carrier is expected to accomplish. Traditionally an aircraft carrier is supposed to be one ship that can perform at least power projection and sea control missions.
 * An aircraft carrier must be able to efficiently operate an air combat group. This means it should handle fixed-wing jets as well as helicopters. This includes ships designed to support operations of short-takeoff/vertical-landing (STOVL) jets.

Basic types

 * Aircraft cruiser
 * Amphibious assault ship and sub-types
 * Anti-submarine warfare carrier
 * Balloon carrier and balloon tenders
 * Escort carrier
 * Fleet carrier
 * Flight deck cruiser
 * Helicopter carrier
 * Light aircraft carrier
 * Seaplane tender and seaplane carriers
 * Utility carrier: This type was mainly used in the US Navy, in the decade after World War 2 to ferry aircraft.

Some of the types listed here are not strictly defined as aircraft carriers by some sources.

By role

A fleet carrier is intended to operate with the main fleet and usually provides an offensive capability. These are the largest carriers capable of fast speeds. By comparison, escort carriers were developed to provide defense for convoys of ships. They were smaller and slower with lower numbers of aircraft carried. Most were built from mercantile hulls or, in the case of merchant aircraft carriers, were bulk cargo ships with a flight deck added on top. Light aircraft carriers were fast enough to operate with the main fleet but of smaller size with reduced aircraft capacity.

The Soviet aircraft carrier Admiral Kusnetsov was termed a "heavy aircraft-carrying cruiser". This was primarily a legal construct to avoid the limitations of the Montreux Convention preventing 'aircraft carriers' transiting the Turkish Straits between the Soviet Black Sea bases and the Mediterranean Sea. These ships, while sized in the range of large fleet carriers, were designed to deploy alone or with escorts. In addition to supporting fighter aircraft and helicopters, they provide both strong defensive weaponry and heavy offensive missiles equivalent to a guided-missile cruiser.

By configuration

Aircraft carriers today are usually divided into the following four categories based on the way that aircraft take off and land:
 * Catapult-assisted take-off barrier-arrested recovery (CATOBAR): these carriers generally carry the largest, heaviest, and most heavily armed aircraft, although smaller CATOBAR carriers may have other limitations (weight capacity of aircraft elevator, etc.). All CATOBAR carriers in service today are nuclear-powered. Twelve are in service: ten Nimitz and one Gerald R. Ford-class aircraft carrier fleet carriers in the United States; and the Charles de Gaulle in France.
 * Short take-off barrier-arrested recovery (STOBAR): these carriers are generally limited to carrying lighter fixed-wing aircraft with more limited payloads. STOBAR carrier air wings, such as the Sukhoi Su-33 and future Mikoyan MiG-29K wings of RUSSIAN AIRCRAFT CARRIER Admiral Kuznetsov are often geared primarily towards air superiority and fleet defense roles rather than strike/power projection tasks, which require heavier payloads (bombs and air-to-ground missiles). Five are in service: two in China, two in India, and one in Russia.
 * Short take-off vertical-landing (STOVL): limited to carrying STOVL aircraft. STOVL aircraft, such as the Harrier family and Yakovlev Yak-38 generally have limited payloads, lower performance, and high fuel consumption when compared with conventional fixed-wing aircraft; however, a new generation of STOVL aircraft, currently consisting of the Lockheed Martin F-35B Lightning II, has much improved performance. Fourteen are in service; nine STOVL amphibious assault ships in the US; two carriers each in Italy and the UK; and one STOVL amphibious assault ship in Spain.
 * Helicopter carrier: Helicopter carriers have a similar appearance to other aircraft carriers but operate only helicopters – those that mainly operate helicopters but can also operate fixed-wing aircraft are known as STOVL carriers (see above). Seventeen are in service: four in Japan; three in France; two each in Australia, China, Egypt and South Korea; and one each in Brazil and Thailand. In the past, some conventional carriers were converted and these were called "commando carriers" by the Royal Navy. Some helicopter carriers, but not all, are classified as amphibious assault ships, tasked with landing and supporting ground forces on enemy territory.

By size

 * Fleet carrier
 * Light aircraft carrier
 * Escort carrier

Supercarrier

The appellation "supercarrier" is not an official designation with any national navy, but a term used predominantly by the media and typically when reporting on larger and more advanced carrier types. It is also used when comparing carriers of various sizes and capabilities, both current and past. It was first used by The New York Times in 1938, in an article about the Royal Navy's HMS Ark Royal (91), that had a length of 800 ft, a displacement of 22,000 ton and was designed to carry 72 aircraft. Since then, aircraft carriers have consistently grown in size, both in length and displacement, as well as improved capabilities; in defense, sensors, electronic warfare, propulsion, range, launch and recovery systems, number and types of aircraft carried and number of sorties flown per day.

China and the United Kingdom both have carriers in service or under construction with displacements ranging from 65,000 to 85,000 tons and lengths from 280 to 320 m which have been described as "supercarriers". The largest "supercarriers" in service as of 2022, however, are with the US Navy, with displacements exceeding 100,000 tons, lengths of over 337 m, and capabilities that match or exceed that of any other class.

Hull type identification symbols
Several systems of identification symbol for aircraft carriers and related types of ship have been used. These include the pennant numbers used by the Royal Navy, Commonwealth countries, and Europe, along with the hull classification symbols used by the US and Canada.

Origins

The 1903 advent of the heavier-than-air fixed-wing airplane with the Wright brothers' first flight at Kitty Hawk, North Carolina, was closely followed on 14 November 1910, by Eugene Burton Ely's first experimental take-off of a Curtiss Pusher airplane from the deck of a United States Navy ship, the cruiser USS Birmingham (CL-2) anchored off Norfolk Navy Base in Virginia. Two months later, on 18 January 1911, Ely landed his Curtiss Pusher airplane on a platform on the armored cruiser USS Pennsylvania (ACR-4) anchored in San Francisco Bay. On 9 May 1912, the first take off of an airplane from a ship while underway was made by Commander Charles Samson flying a Short Improved S.27 biplane "S.38" of the Royal Naval Air Service (RNAS) from the deck of the Royal Navy's pre-dreadnought battleship HMS Hibernia (1905), thus providing the first practical demonstration of the aircraft carrier for naval operations at sea. Seaplane tender support ships came next, with the French FRENCH SEAPLANE CARRIER Foudre of 1911.

Early in World War I, the Imperial Japanese Navy ship JAPANESE SEAPLANE CARRIER Wakamiya conducted the world's first successful ship-launched air raid: on 6 September 1914, a Farman aircraft launched by Wakamiya attacked the Austro-Hungarian cruiser SMS Kaiserin Elisabeth and the Imperial German gunboat Jaguar in Jiaozhou Bay off Qingdao; neither was hit. The first attack using an air-launched torpedo occurred on 2 August, when a torpedo was fired by Flight Commander Charles Edmonds from a Short Type 184 seaplane, launched from the seaplane carrier HMS Ben-my-Chree.

The first carrier-launched airstrike was the Tondern raid in July 1918. Seven Sopwith Camels were launched from the battlecruiser HMS Furious (47) which had been completed as a carrier by replacing her planned forward turret with a flight deck and hangar prior to commissioning. The Camels attacked and damaged the German airbase at Tondern, Germany (modern day Tønder, Denmark), and destroyed two zeppelin airships.

The first landing of an airplane on a moving ship was by Squadron Commander Edwin Harris Dunning, when he landed his Sopwith Pup on HMS Furious in Scapa Flow, Orkney on 2 August 1917. Landing on the forward flight deck required the pilot to approach round the ship's superstructure, a difficult and dangerous manoeuver and Dunning was later killed when his airplane was thrown overboard while attempting another landing on Furious. HMS Furious was modified again when her rear turret was removed and another flight deck added over a second hangar for landing aircraft over the stern. Her funnel and superstructure remained intact however and turbulence from the funnel and superstructure was severe enough that only three landing attempts were successful before further attempts were forbidden. This experience prompted the development of vessels with a flush deck and produced the first large fleet ships. In 1918, HMS Argus (I49) became the world's first carrier capable of launching and recovering naval aircraft.

As a result of the Washington Naval Treaty of 1922, which limited the construction of new heavy surface combat ships, most early aircraft carriers were conversions of ships that were laid down (or had served) as different ship types: cargo ships, cruisers, battlecruisers, or battleships. These conversions gave rise to the USS Langley (CV-1) in 1922, the US Lexington-class aircraft carriers (1927), Japanese JAPANESE AIRCRAFT CARRIER Akagi and JAPANESE AIRCRAFT CARRIER Kaga, and British Courageous-class aircraft carrier (of which Furious was one). Specialist carrier evolution was well underway, with several navies ordering and building warships that were purposefully designed to function as aircraft carriers by the mid-1920s. This resulted in the commissioning of ships such as the Japanese JAPANESE AIRCRAFT CARRIER Hōshō (1922), HMS Hermes (95) (1924, although laid down in 1918 before Hōshō), and FRENCH AIRCRAFT CARRIER Béarn (1927). During World War II, these ships would become known as fleet carriers.

World War II

The aircraft carrier dramatically changed naval warfare in World War II, because air power was becoming a significant factor in warfare. The advent of aircraft as focal weapons was driven by the superior range, flexibility, and effectiveness of carrier-launched aircraft. They had greater range and precision than naval guns, making them highly effective. The versatility of the carrier was demonstrated in November 1940, when HMS Illustrious (87) launched a long-range strike on the Italian fleet at their base in Taranto, signalling the beginning of the effective and highly mobile aircraft strikes. This operation in the shallow water harbor incapacitated three of the six anchored battleships at a cost of two torpedo bombers.

World War II in the Pacific Ocean involved clashes between aircraft carrier fleets. The Japanese surprise attack on the American Pacific fleet at Pearl Harbor naval and air bases on Sunday, 7 December 1941, was a clear illustration of the power projection capability afforded by a large force of modern carriers. Concentrating six carriers in a single unit turned naval history about, as no other nation had fielded anything comparable. In the "Doolittle Raid", on 18 April 1942, the US Navy carrier USS Hornet (CV-8) sailed to within 650 nmi of Japan and launched 16 B-25 Mitchell medium bombers from her deck in a demonstrative retaliatory strike on the mainland, including the capital, Tokyo. However, the vulnerability of carriers compared to traditional capital ships was illustrated by the sinking of HMS Glorious by German battleships during the Norwegian campaign in 1940.

This new-found importance of naval aviation forced nations to create a number of carriers, in efforts to provide air superiority cover for every major fleet in order to ward off enemy aircraft. This extensive usage led to the development and construction of 'light' carriers. Escort aircraft carriers, such as USS Bogue (CVE-9), were sometimes purpose-built but most were converted from merchant ships as a stop-gap measure to provide anti-submarine air support for convoys and amphibious invasions. Following this concept, light aircraft carriers built by the US, such as USS Independence (CVL-22) (commissioned in 1943), represented a larger, more "militarized" version of the escort carrier. Although with similar complement to escort carriers, they had the advantage of speed from their converted cruiser hulls. The UK 1942 Design Light Fleet Carrier was designed for building quickly by civilian shipyards and with an expected service life of about 3 years. They served the Royal Navy during the war, and the hull design was chosen for nearly all aircraft carrier equipped navies after the war, until the 1980s. Emergencies also spurred the creation or conversion of highly unconventional aircraft carriers. CAM ships were cargo-carrying merchant ships that could launch (but not retrieve) a single fighter aircraft from a catapult to defend the convoy from long range land-based German aircraft.

Postwar era

Before World War II, international naval treaties of 1922, 1930, and 1936 limited the size of capital ships including carriers. Since World War II, aircraft carrier designs have increased in size to accommodate a steady increase in aircraft size. The large, modern Nimitz-class aircraft carrier of US Navy carriers has a displacement nearly four times that of the World War II–era USS Enterprise (CV-6), yet its complement of aircraft is roughly the same—a consequence of the steadily increasing size and weight of individual military aircraft over the years. Today's aircraft carriers are so expensive that some nations which operate them risk significant economic and military impact if a carrier is lost.

Some changes were made after 1945 in carriers:
 * The angled flight deck was invented by Royal Navy Captain (later Rear Admiral) Dennis Cambell, as naval aviation jets higher speeds required carriers be modified to "fit" their needs.  Additionally, the angled flight deck allows for simultaneous launch and recovery.
 * Jet blast deflectors became necessary to protect aircraft and handlers from jet blast. The first US Navy carriers to be fitted with them were the wooden-decked Essex-class aircraft carriers which were adapted to operate jets in the late 1940s. Later versions had to be water-cooled because of increasing engine power.
 * Optical landing systems were developed to facilitate the very precise landing angles required by jet aircraft, which have a faster landing speed giving the pilot little time to correct misalignments, or mistakes. The first system was fitted to HMS Illustrious (87) in 1952.
 * Aircraft carrier designs have increased in size to accommodate continuous increase in aircraft size. The 1950s saw US Navy's commission of "supercarriers", designed to operate naval jets, which offered better performance at the expense of bigger size and demanded more ordnance to be carried on-board (fuel, spare parts, electronics, etc.).
 * The combination of increased carrier size, speed requirements above 30 kn, and a requirement to operate at sea for long periods mean that modern large aircraft carriers often use nuclear reactors to create power for propulsion, electricity, catapulting airplanes from aircraft carriers, and a few more minor uses.

Modern navies that operate such aircraft carriers treat them as capital ships of fleets, a role previously held by the galleons, ships-of-the-line and battleships. This change took place during World War II in response to air power becoming a significant factor in warfare, driven by the superior range, flexibility and effectiveness of carrier-launched aircraft. Following the war, carrier operations continued to increase in size and importance, and along with, carrier designs also increased in size and ability. Some of these larger carriers, dubbed by the media as "supercarriers", displacing 75,000 tons or greater, have become the pinnacle of carrier development. Some are powered by nuclear reactors and form the core of a fleet designed to operate far from home. Amphibious assault ships, such as the Wasp-class amphibious assault ship and Mistral-class amphibious assault ship classes, serve the purpose of carrying and landing Marines, and operate a large contingent of helicopters for that purpose. Also known as "commando carriers" or "helicopter carriers", many have the capability to operate VSTOL aircraft.

The threatening role of aircraft carriers has a place in modern asymmetric warfare, like the gunboat diplomacy of the past. Carriers also facilitate quick and precise projections of overwhelming military power into such local and regional conflicts.

Lacking the firepower of other warships, carriers by themselves are considered vulnerable to attack by other ships, aircraft, submarines, or missiles. Therefore, an aircraft carrier is generally accompanied by a number of other ships to provide protection for the relatively unwieldy carrier, to carry supplies, re-supply (Many carriers are self-sufficient and will supply their escorts) and perform other support services, and to provide additional offensive capabilities. The resulting group of ships is often termed a carrier strike group, battle group, carrier group, or carrier battle group.

There is a view among some military pundits that modern anti-ship weapons systems, such as torpedoes and missiles, or even ballistic missiles with nuclear warheads have made aircraft carriers and carrier groups too vulnerable for modern combat.

like the German U24 of the conventional 206 class which in 2001 "fired" at the Enterprise during the exercise JTFEX 01-2 in the Caribbean Sea by firing flares and taking a photograph through its periscope or the Swedish Gotland which managed the same feat in 2006 during JTFEX 06-2 by penetrating the defensive measures of Carrier Strike Group 7 which was protecting USS Ronald Reagan.

Structure
Carriers are large and long ships, although there is a high degree of variation depending on their intended role and aircraft complement. The size of the carrier has varied over history and among navies, to cater to the various roles that global climates have demanded from naval aviation.

Regardless of size, the ship itself must house their complement of aircraft, with space for launching, storing, and maintaining them. Space is also required for the large crew, supplies (food, munitions, fuel, engineering parts), and propulsion. US aircraft carriers are notable for having nuclear reactors powering their systems and propulsion.

The top of the carrier is the flight deck, where aircraft are launched and recovered. On the starboard side of this is the island, where the funnel, air-traffic control and the bridge are located.

The constraints of constructing a flight deck affect the role of a given carrier strongly, as they influence the weight, type, and configuration of the aircraft that may be launched. For example, assisted launch mechanisms are used primarily for heavy aircraft, especially those loaded with air-to-ground weapons. CATOBAR is most commonly used on US Navy fleet carriers as it allows the deployment of heavy jets with full load-outs, especially on ground-attack missions. STOVL is used by other navies because it is cheaper to operate and still provides good deployment capability for fighter aircraft.

Due to the busy nature of the flight deck, only 20 or so aircraft may be on it at any one time. A hangar storage several decks below the flight deck is where most aircraft are kept, and aircraft are taken from the lower storage decks to the flight deck through the use of an elevator. The hangar is usually quite large and can take up several decks of vertical space.

Munitions are commonly stored on the lower decks because they are highly explosive. Usually this is below the waterline so that the area can be flooded in case of emergency.

Flight deck

As "runways at sea", aircraft carriers have a flat-top flight deck, which launches and recovers aircraft. Aircraft launch forward, into the wind, and are recovered from astern. The flight deck is where the most notable differences between a carrier and a land runway are found. Creating such a surface at sea poses constraints on the carrier. For example, the size of the vessel is a fundamental limitation on runway length. This affects take-off procedure, as a shorter runway length of the deck requires that aircraft accelerate more quickly to gain lift. This either requires a thrust boost, a vertical component to its velocity, or a reduced take-off load (to lower mass). The differing types of deck configuration, as above, influence the structure of the flight deck. The form of launch assistance a carrier provides is strongly related to the types of aircraft embarked and the design of the carrier itself.

There are two main philosophies in order to keep the deck short: add thrust to the aircraft, such as using a Catapult Assisted Take-Off (CATO-); and changing the direction of the airplanes' thrust, as in Vertical and/or Short Take-Off (V/STO-). Each method has advantages and disadvantages of its own:
 * Catapult Assisted Take-Off Barrier Arrested Recovery (CATOBAR): A steam- or electric-powered catapult is connected to the aircraft, and is used to accelerate conventional aircraft to a safe flying speed. By the end of the catapult stroke, the aircraft is airborne and further propulsion is provided by its own engines. This is the most expensive method as it requires complex machinery to be installed under the flight deck, but allows for even heavily loaded aircraft to take off.
 * Short Take-Off Barrier Arrested Recovery (STOBAR) depends on increasing the net lift on the aircraft. Aircraft do not require catapult assistance for take off; instead on nearly all ships of this type an upwards vector is provided by a ski-jump at the forward end of the flight deck, often combined with thrust vectoring by the aircraft. Alternatively, by reducing the fuel and weapon load, an aircraft is able to reach faster speeds and generate more upwards lift and launch without a ski-jump or catapult.
 * Short Take-Off Vertical-Landing (STOVL): On aircraft carriers, non-catapult-assisted, fixed-wing short takeoffs are accomplished with the use of thrust vectoring, which may also be used in conjunction with a runway "ski-jump". Use of STOVL tends to allow aircraft to carry a larger payload as compared to during VTOL use, while still only requiring a short runway. The most famous examples are the Hawker Siddeley Harrier and the BAe Sea Harrier. Although technically VTOL aircraft, they are operationally STOVL aircraft due to the extra weight carried at take-off for fuel and armaments. The same is true of the Lockheed F-35B Lightning II, which demonstrated VTOL capability in test flights but is operationally STOVL or in the case of UK uses "shipborne rolling vertical landing"
 * Vertical Take-Off and Landing (VTOL): Certain aircraft are specifically designed for the purpose of using very high degrees of thrust vectoring (e.g. if the thrust to weight-force ratio is greater than 1, it can take off vertically), but are usually slower than conventionally propelled aircraft due to the additional weight from associated systems.

On the recovery side of the flight deck, the adaptation to the aircraft load-out is mirrored. Non-VTOL or conventional aircraft cannot decelerate on their own, and almost all carriers using them must have arrested-recovery systems (-BAR, e.g. CATOBAR or STOBAR) to recover their aircraft. Aircraft that are landing extend a tailhook that catches on arrestor wires stretched across the deck to bring themselves to a stop in a short distance. Post-World War II Royal Navy research on safer CATOBAR recovery eventually led to universal adoption of a landing area angled off axis to allow aircraft who missed the arresting wires to "bolt" and safely return to flight for another landing attempt rather than crashing into aircraft on the forward deck.

If the aircraft are VTOL-capable or helicopters, they do not need to decelerate and hence there is no such need. The arrested-recovery system has used an angled deck since the 1950s because, in case the aircraft does not catch the arresting wire, the short deck allows easier take off by reducing the number of objects between the aircraft and the end of the runway. It also has the advantage of separating the recovery operation area from the launch area. Helicopters and aircraft capable of vertical or short take-off and landing (V/STOL) usually recover by coming abreast of the carrier on the port side and then using their hover capability to move over the flight deck and land vertically without the need for arresting gear.

Staff and deck operations

Carriers steam at speed, up to 35 kn into the wind during flight deck operations to increase wind speed over the deck to a safe minimum. This increase in effective wind speed provides a higher launch airspeed for aircraft at the end of the catapult stroke or ski-jump, as well as making recovery safer by reducing the difference between the relative speeds of the aircraft and ship.

Since the early 1950s on conventional carriers it has been the practice to recover aircraft at an angle to port of the axial line of the ship. The primary function of this angled deck is to allow aircraft that miss the arresting wires, referred to as a bolter, to become airborne again without the risk of hitting aircraft parked forward. The angled deck allows the installation of one or two "waist" catapults in addition to the two bow cats. An angled deck also improves launch and recovery cycle flexibility with the option of simultaneous launching and recovery of aircraft.

Conventional ("tailhook") aircraft rely upon a landing signal officer (LSO, radio call sign 'paddles') to monitor the aircraft's approach, visually gauge glideslope, attitude, and airspeed, and transmit that data to the pilot. Before the angled deck emerged in the 1950s, LSOs used colored paddles to signal corrections to the pilot (hence the nickname). From the late 1950s onward, visual landing aids such as the optical landing system have provided information on proper glide slope, but LSOs still transmit voice calls to approaching pilots by radio.

Key personnel involved in the flight deck include the shooters, the handler, and the air boss. Shooters are naval aviators or naval flight officers and are responsible for launching aircraft. The handler works just inside the island from the flight deck and is responsible for the movement of aircraft before launching and after recovery. The "air boss" (usually a commander) occupies the top bridge (Primary Flight Control, also called primary or the tower) and has the overall responsibility for controlling launch, recovery and "those aircraft in the air near the ship, and the movement of planes on the flight deck, which itself resembles a well-choreographed ballet". The captain of the ship spends most of his time one level below primary on the Navigation Bridge. Below this is the Flag Bridge, designated for the embarked admiral and his staff.

To facilitate working on the flight deck of a US aircraft carrier, the sailors wear colored shirts that designate their responsibilities. There are at least seven different colors worn by flight deck personnel for modern United States Navy carrier air operations. Carrier operations of other nations use similar color schemes.

Deck structures

The superstructure of a carrier (such as the bridge, flight control tower) are concentrated in a relatively small area called an island, a feature pioneered on HMS Hermes (95) in 1923. While the island is usually built on the starboard side of the flight deck, the Japanese aircraft carriers JAPANESE AIRCRAFT CARRIER Akagi and JAPANESE AIRCRAFT CARRIER Hiryū had their islands built on the port side. Very few carriers have been designed or built without an island. The flush deck configuration proved to have significant drawbacks, primary of which was management of the exhaust from the power plant. Fumes coming across the deck were a major issue in USS Langley (CV-1). In addition, lack of an island meant difficulties managing the flight deck, performing air traffic control, a lack of radar housing placements and problems with navigating and controlling the ship itself.

Another deck structure that can be seen is a ski-jump ramp at the forward end of the flight deck. This was first developed to help launch short take off vertical landing (STOVL) aircraft take off at far higher weights than is possible with a vertical or rolling takeoff on flat decks. Originally developed by the Royal Navy, it since has been adopted by many navies for smaller carriers. A ski-jump ramp works by converting some of the forward rolling movement of the aircraft into vertical velocity and is sometimes combined with the aiming of jet thrust partly downwards. This allows heavily loaded and fueled aircraft a few more precious seconds to attain sufficient air velocity and lift to sustain normal flight. Without a ski-jump, launching fully-loaded and fueled aircraft such as the Harrier would not be possible on a smaller flat deck ship before either stalling out or crashing directly into the sea.

Although STOVL aircraft are capable of taking off vertically from a spot on the deck, using the ramp and a running start is far more fuel efficient and permits a heavier launch weight. As catapults are unnecessary, carriers with this arrangement reduce weight, complexity, and space needed for complex steam or electromagnetic launching equipment. Vertical landing aircraft also remove the need for arresting cables and related hardware. Russian, Chinese, and Indian carriers include a ski-jump ramp for launching lightly loaded conventional fighter aircraft but recover using traditional carrier arresting cables and a tailhook on their aircraft.

The disadvantage of the ski-jump is the penalty it exacts on aircraft size, payload, and fuel load (and thus range); heavily laden aircraft cannot launch using a ski-jump because their high loaded weight requires either a longer takeoff roll than is possible on a carrier deck, or assistance from a catapult or JATO rocket. For example, the Russian Sukhoi Su-33 is only able to launch from the carrier RUSSIAN AIRCRAFT CARRIER Admiral Kuznetsov with a minimal armament and fuel load. Another disadvantage is on mixed flight deck operations where helicopters are also present, such as on a US landing helicopter dock or landing helicopter assault amphibious assault ship. A ski jump is not included as this would eliminate one or more helicopter landing areas; this flat deck limits the loading of Harriers but is somewhat mitigated by the longer rolling start provided by a long flight deck compared to many STOVL carriers.

National fleets

The US Navy has the largest fleet of carriers in the world, with eleven supercarriers currently in service. China and India each have two STOBAR carriers in service. The UK has two STOVL carriers in service. The navies of France and Russia each operate a single medium-sized carrier. The US also has nine similarly sized Amphibious Warfare Ships. There are five small light carriers in use capable of operating both fixed-wing aircraft and helicopters; Japan and Italy each operate two, and Spain one.

Additionally there are eighteen small carriers which only operate helicopters serving the navies of Australia (2), Brazil (1), China (2), Egypt (2), France (3), Japan (4), South Korea (2), Thailand (1) and Turkey (1).

Algeria

 * Current

Kalaat Béni Abbès (L-474) is an amphibious transport dock of the Algerian National Navy with two deck-landing spots for helicopters.

Australia
The Royal Australian Navy operates two Canberra-class landing helicopter docks. The two-ship class, based on the Spanish vessel SPANISH SHIP Juan Carlos I and built by Navantia and BAE Systems Australia, represents the largest ships ever built for the Royal Australian Navy.
 * Current

HMAS Canberra (L02) underwent sea trials in late 2013 and was commissioned in 2014. Her sister ship, HMAS Adelaide (L01), was commissioned in December 2015. The Australian ships retain the ski-ramp from the Juan Carlos I design, although the RAN has not acquired carrier-based fixed-wing aircraft.

Brazil
In December 2017, the Brazilian Navy confirmed the purchase of HMS Ocean (L12) for (GBP) £84.6 million (equivalent to R$359.5M and US$113.2M) and renamed her BRAZILIAN AMPHIBIOUS ASSAULT SHIP Atlântico. The ship was decommissioned from Royal Navy service in March 2018. The Brazilian Navy commissioned the carrier on 29 June 2018 in the United Kingdom. After undertaking a period of maintenance in the UK, the ship travelled to its new home port, Arsenal de Marinha do Rio de Janeiro (AMRJ) in order to be fully operational by 2020. The ship displaces 21,578 tonnes, is 203.43 m long and has a range of 8000 nmi.
 * Current

Before leaving HMNB Devonport for her new homeport in Rio's AMRJ, Atlântico underwent operational sea training under the Royal Navy's Flag Officer Sea Training (FOST) program.

On 12 November 2020, Atlântico was redesignated "NAM", for "multipurpose aircraft carrier" (Navio Aeródromo Multipropósito), from "PHM", for "multipurpose helicopter carrier" (Porta-Helicópteros Multipropósito), to reflect the ship's capability to operate with fixed-wing medium-altitude long-endurance unmanned aerial vehicles as well as crewed tiltrotor VTOL aircraft.

China
2 STOBAR carriers: 1 CATOBAR carrier: 3 Landing helicopter docks
 * Current
 * CHINESE AIRCRAFT CARRIER Liaoning (60,900 tons) was originally built as the Soviet Kuznetsov-class aircraft carrier carrier Varyag and was later purchased as a hulk in 1998 on the pretext of use as a floating casino, then towed to China for rebuild and completion. Liaoning was commissioned on 25 September 2012 and began service for testing and training. In November 2012, Liaoning launched and recovered Shenyang J-15 naval fighter aircraft for the first time. After a refit in January 2019, she was assigned to the North Sea Fleet, a change from her previous role as a training carrier.
 * CHINESE AIRCRAFT CARRIER Shandong (60,000–70,000 tons) was launched on 26 April 2017. She is the first to be built domestically, to an improved Kuznetsov-class design. Shandong started sea trials on 23 April 2018, and entered service in December 2019.
 * CHINESE AIRCRAFT CARRIER Fujian (80,000 tons) is a CATOBAR carrier which was under construction between 2015 and 2016 before being completed in June 2022. She is being fitted out as of 2022 and will commence service in 2023–2024.
 * A Type 075 LHD, CHINESE LANDING HELICOPTER DOCK Hainan was commissioned on 23 April 2021 at the naval base in Sanya. A second ship, Guangxi, was commissioned on 26 December 2021 and a third ship, Anhui, was commissioned in October 2022.

China has had a long-term plan to operate six large aircraft carriers with two carriers per fleet.
 * Future

China is planning a class of eight landing helicopter dock vessels, the Type 075 (NATO reporting name Yushen-class landing helicopter assault). This is a class of amphibious assault ship under construction by the Hudong–Zhonghua Shipbuilding company. The first ship was commissioned in April 2021. China is also planning a modified class of the same concept, the Type 076 landing helicopter dock, that will also be equipped with an electromagnetic catapult launch system.

Egypt
Egypt signed a contract with French shipbuilder DCNS to buy two Mistral-class amphibious assault ship helicopter carriers for approximately 950 million euros. The two ships were originally to be sold to Russia, but the deal was cancelled by France due to the Russian invasion of Ukraine in 2014.
 * Current

On 2 June 2016, Egypt received the first of two helicopter carriers acquired in October 2015, the landing helicopter dock EGYPTIAN SHIP Gamal Abdel Nasser. The flag transfer ceremony took place in the presence of Egyptian and French Navies' chiefs of staff, chairman and chief executive officers of both DCNS and STX France, and senior Egyptian and French officials. On 16 September 2016, DCNS delivered the second of two helicopter carriers, the landing helicopter dock EGYPTIAN SHIP Anwar El Sadat which also participated in a joint military exercise with the French Navy before arriving at her home port of Alexandria.

Egypt is so far the only country in Africa or the Middle East to possess a helicopter carrier.

France

The French Navy operates the 42,000-tonne nuclear-powered aircraft carrier, FRENCH AIRCRAFT CARRIER Charles de Gaulle. Commissioned in 2001, she is the flagship of the French Navy. The ship carries a complement of Dassault Rafale M and E-2C Hawkeye aircraft, EC725 Caracal and AS532 Cougar helicopters for combat search and rescue, as well as modern electronics and Aster missiles. She is a CATOBAR-type carrier that uses two 75 m C13-3 steam catapults of a shorter version of the catapult system installed on the US Nimitz-class aircraft carrier carriers, one catapult at the bow and one across the front of the landing area. In addition, the French Navy operates three Mistral-class amphibious assault ships.
 * Current

In October 2018, the French Ministry of Defence began an 18-month study for €40 million for the eventual future replacement of the FRENCH AIRCRAFT CARRIER Charles de Gaulle beyond 2030. In December 2020, President Macron announced that construction of the next generation carrier would begin in around 2025 with sea trials to start in about 2036. The carrier is planned to have a displacement of around 75,000 tons and to carry about 32 next-generation fighters, two to three E-2D Advanced Hawkeyes and a yet-to-be-determined number of unmanned carrier air vehicles.
 * Future

India

2 STOBAR carriers:
 * Current

INS Vikramaditya, 45,400 tonnes, modified Kiev class. The carrier was purchased by India on 20 January 2004 after years of negotiations at a final price of $2.35 billion. The ship successfully completed her sea trials in July 2013 and aviation trials in September 2013. She was formally commissioned on 16 November 2013 at a ceremony held at Severodvinsk, Russia.

INS Vikrant (2013), also known as Indigenous Aircraft Carrier 1 (IAC-1) a 45,000-tonne, 262 m aircraft carrier whose keel was laid in 2009. The new carrier will operate MiG-29K and naval HAL Tejas aircraft. The ship is powered by gas-turbines and has a range of 8000 nmi and deploys 10 helicopters and 30 aircraft. The ship was launched in 2013, sea-trials began in August 2021 and was commissioned on 02 September 2022.

India has plans for a third carrier, INS Vishal, also known as Indigenous Aircraft Carrier 2 (IAC-2) with a displacement of over 65,000 tonnes and is planned with a CATOBAR system to launch and recover heavier aircraft.
 * Future

India has also issued a request for information (RFI) to procure four Landing helicopter dock displacing 30,000-40,000 tons with a capacity to operate 12 medium lift special ops and two heavy lift helicopters and troops for amphibious operations.

Italy
2 STOVL carriers:
 * Current
 * ITALIAN AIRCRAFT CARRIER Giuseppe Garibaldi: 14,000-tonne Italian STOVL carrier, commissioned in 1985.
 * ITALIAN AIRCRAFT CARRIER Cavour: 30,000-tonne Italian STOVL carrier designed and built with secondary amphibious assault facilities, commissioned in 2008.

Italy plans to replace ageing aircraft carrier Giuseppe Garibaldi, as well as one of the San Giorgio-class amphibious transport dock landing helicopter docks, with a new amphibious assault ship, to be named ITALIAN LANDING HELICOPTER DOCK Trieste. The ship will be significantly larger than her predecessors with a displacement of 38,000 tonnes at full load. Trieste is to carry the F-35B Joint Strike Fighter. Meanwhile, Giuseppe Garibaldi will be transferred to Italian Space Operation Command for use as a satellite launch platform.
 * Future

Japan

In December 2018, the Japanese Cabinet gave approval to convert both Izumo-class destroyers into aircraft carriers for F-35B STOVL operations. The conversion of Izumo was underway as of mid-2020. The modification of maritime escort vessels is to "increase operational flexibility" and enhance Pacific air defense, the Japanese defense ministry's position is "We are not creating carrier air wings or carrier air squadrons" similar to the US Navy. The Japanese STOVL F-35s, when delivered, will be operated by the Japan Air Self Defense Force from land bases; according to the 2020 Japanese Defense Ministry white paper the STOVL model was chosen for the JASDF due the lack of appropriately long runways to support air superiority capability across all of Japanese airspace. Japan has requested that the USMC deploy STOVL F-35s and crews aboard the Izumo-class ships "for cooperation and advice on how to operate the fighter on the deck of the modified ships". On 3 October 2021, two USMC F-35Bs performed the first vertical landings and horizontal take-offs from JS Izumo, marking 75 years since fixed-wing aircraft operated from a Japanese carrier.
 * Current
 * 2 Izumo-class multi-purpose destroyers – 820 ft, 19,500-tonne (27,000 tonnes full load) STOVL carrier Izumo was launched August 2013 and commissioned March 2015. Izumo's sister ship, Kaga, was commissioned in 2017.

 * 2 Hyūga-class helicopter destroyers – 19,000-tonne (full load) anti-submarine warfare carriers with enhanced command-and-control capabilities allowing them to serve as fleet flagships.

Russia

1 STOBAR carrier: Admiral Flota Sovetskogo Soyuza Kuznetsov: 55,000-tonne Kuznetsov-class aircraft carrier STOBAR aircraft carrier. Launched in 1985 as Tbilisi, renamed and operational from 1995. Without catapults she can launch and recover lightly fueled naval fighters for air defense or anti-ship missions but not heavy conventional bombing strikes. Officially designated an aircraft carrying cruiser, she is unique in carrying a heavy cruiser's complement of defensive weapons and large P-700 Granit offensive missiles. The P-700 systems will be removed in the coming refit to enlarge her below decks aviation facilities as well as upgrading her defensive systems.
 * Current

The ship has been out of service and in repairs since 2018. The current projection is that repairs will be completed and the ship will be transferred back to the Russian Navy sometime in 2024, however this may be pushed back to 2025 if issues arise during overhaul and testing.

The Russian Government has been considering the potential replacement of Admiral Kuznetsov for some time and has considered the Shtorm-class aircraft carrier as a possible option. This carrier will be a hybrid of CATOBAR and STOBAR, given the fact that she utilizes both systems of launching aircraft. The carrier is expected to cost As of 2020, the project had not yet been approved and, given the financial costs, it was unclear whether it would be made a priority over other elements of Russian naval modernization.
 * Future

A class of 2 LHD, Project 23900 is planned and an official keel laying ceremony for the project happened on 20 July 2020.

South Korea
Two Dokdo-class amphibious assault ship 18,860-tonne full deck amphibious assault ships with hospital and well deck and facilities to serve as fleet flagships.
 * Current

South Korea has set tentative plans for procuring two light aircraft carriers by 2033, which would help make the ROKN a blue water navy. In December 2020, details of South Korea's planned carrier program (CVX) were finalized. A vessel of about 40,000 tons is envisaged carrying about 20 F-35B fighters as well as future maritime attack helicopters. Service entry had been anticipated in the early 2030s. The program has encountered opposition in the National Assembly. In November 2021, the National Defense Committee of the National Assembly reduced the program's requested budget of 7.2 billion KRW and to just 500 million KRW (about $400K USD), effectively putting the project on hold, at least temporarily. However, on 3 December 2021 the full budget of 7.2 billion won was passed by the National Assembly. Basic design work is to begin in earnest starting 2022.
 * Future

Spain

SPANISH SHIP Juan Carlos I: 27,000-tonne, specially designed multipurpose strategic projection ship which can operate as an amphibious assault ship and aircraft carrier. Juan Carlos I has full facilities for both functions including a ski jump for STOVL operations, is equipped with the AV-8B Harrier II attack aircraft. Also, well deck, and vehicle storage area which can be used as additional hangar space, launched in 2008, commissioned 30 September 2010.
 * Current

Thailand

1 offshore helicopter support ship: HTMS Chakri Naruebet helicopter carrier: 11,400-tonne STOVL carrier based on Spanish SPANISH AIRCRAFT CARRIER Príncipe de Asturias design. Commissioned in 1997. The AV-8S Matador/Harrier STOVL fighter wing, mostly inoperable by 1999, was retired from service without replacement in 2006. As of 2010, the ship is used for helicopter operations and for disaster relief.
 * Current

Turkey
TCG Anadolu is a 27,079-tonne amphibious assault ship (LHD) of the Turkish Navy that can be configured as a 24,660-tonne V/STOL aircraft carrier. Construction began on 30 April 2016 by Sedef Shipbuilding Inc. at their Istanbul shipyard. TCG Anadolu was commissioned with a ceremony on 10 April 2023. The construction of a sister ship, to be named TCG Trakya, is currently being planned by the Turkish Navy.
 * Current

The Sikorsky S-70B Seahawk and the Bell AH-1 SuperCobra are the two main types of helicopters used on TCG Anadolu, with the occasional use of CH-47F Chinook helicopters of the Turkish Army during military exercises and operations. The AH-1W Super Cobras will eventually be complemented and replaced by the TAI T929 ATAK 2. The jet-powered, low-observable Bayraktar MIUS Kızılelma and the MALE Bayraktar TB3   are two UCAVs that are specifically designed and manufactured by Baykar Technologies to be used on TCG Anadolu. The maiden flight of TAI Anka-3 (also part of Project MIUS), a jet-powered, flying wing type UCAV with stealth technology, was successfully completed on 28 December 2023.

On 3 January 2024, the Turkish government approved the plan for the design and construction of a larger aircraft carrier.
 * Future

On 15 February 2024, the Design and Projects Office of the Turkish Navy announced that it will be a STOBAR aircraft carrier with an overall length of 285 m, beam of 68 m, draught of 10.1 m, and displacement of 60,000 tons. It will have a COGAG propulsion system and a maximum speed of 25 kn.

United Kingdom

Two 65,000-tonne Queen Elizabeth-class STOVL carriers which operate the F-35 Lightning II. HMS Queen Elizabeth (R08) was commissioned in December 2017 and HMS Prince of Wales (R09) in December 2019.
 * Current

Queen Elizabeth undertook her first operational deployment in 2021. Each Queen Elizabeth-class ship is able to operate around 40 aircraft during peacetime operations and is thought to be able to carry up to 72 at maximum capacity. As of the end of April 2020, 18 F-35B aircraft had been delivered to the Royal Navy and the Royal Air Force. "Full operating capability" for the UK's carrier strike capability had been planned for 2023 (two squadrons or 24 jets operating from one carrier). The longer-term aim remains for the ability to conduct a wide range of air operations and support amphibious operations worldwide from both carriers by 2026. They form the central part of the UK Carrier Strike Group.

The Queen Elizabeth-class ships are expected to have service lives of 50 years.
 * Future

United States
11 CATOBAR carriers, all nuclear-powered:
 * Current
 * Nimitz-class aircraft carrier: ten 101,000-tonne, 1092 ft fleet carriers, the first of which was commissioned in 1975. A Nimitz-class carrier is powered by two nuclear reactors providing steam to four steam turbines.
 * Gerald R. Ford-class aircraft carrier, one 100,000-tonne, 1106 ft fleet carrier. The lead of the class USS Gerald R. Ford (CVN-78) came into service in 2017, with another nine planned to replace the aging Nimitz-class ships.

Nine amphibious assault ships carrying vehicles, Marine fighters, attack and transport helicopters, and landing craft with STOVL fighters for CAS and CAP: The current US fleet of Nimitz-class carriers will be followed into service (and in some cases replaced) by the Gerald R. Ford-class aircraft carrier. It is expected that the ships will be more automated in an effort to reduce the amount of funding required to maintain and operate the vessels. The main new features are implementation of Electromagnetic Aircraft Launch System (EMALS) (which replaces the old steam catapults) and unmanned aerial vehicles. In terms of future carrier developments, Congress has discussed the possibility of accelerating the phasing-out of one or more Nimitz-class carriers, postponing or canceling the procurement of CVN-81 and CVN-82, or modifying the purchase contract.
 * America-class amphibious assault ship: a class of 45,000-tonne amphibious assault ships, although the first two ships in this class, (Flight 0) do not have well decks, all subsequent ships (Flight I) will have well decks. Two ships are currently in service out of a planned 11 ships. Ships of this class can have a secondary mission as a light aircraft carrier with 20 AV-8B Harrier II, and in the future the F-35B Lightning II aircraft after unloading their Marine expeditionary unit.
 * Wasp-class amphibious assault ship: a class of 41,000-tonne amphibious assault ships, members of this class have been used in wartime in their secondary mission as light carriers with 20 to 25 AV-8Bs after unloading their Marine expeditionary unit. Seven ship currently in service of an original eight, with one lost to fire.
 * Future

Following the deactivation of USS Enterprise (CVN-65) in December 2012, the US fleet comprised 10 fleet carriers, but that number increased back to 11 with the commissioning of Gerald R. Ford in July 2017. The House Armed Services Seapower subcommittee on 24 July 2007, recommended seven or eight new carriers (one every four years). However, the debate has deepened over budgeting for the $12–14.5 billion (plus $12 billion for development and research) for the 100,000-tonne Gerald R. Ford-class carrier (estimated service 2017) compared to the smaller $2 billion 45,000-tonne America-class amphibious assault ships, which are able to deploy squadrons of F-35Bs. The first of this class, USS America (LHA-6), is now in active service with another, USS Tripoli (LHA-7), and 9 more are planned.

In a report to Congress in February 2018, the Navy stated it intends to maintain a "12 CVN force" as part of its 30-year acquisition plan.

Current museum carriers
A few aircraft carriers have been preserved as museum ships. They are:
 * USS Yorktown (CV-10) in Mount Pleasant, South Carolina
 * USS Intrepid (CV-11) in New York City
 * USS Hornet (CV-12) in Alameda, California
 * USS Lexington (CV-16) in Corpus Christi, Texas
 * USS Midway (CV-41) in San Diego, California
 * SOVIET AIRCRAFT CARRIER Kiev in Tianjin, China
 * SOVIET AIRCRAFT CARRIER Minsk in Nantong, China

Former museum carriers

 * INS Vikrant (1961) was moored as a museum in Mumbai from 2001 to 2012, but was never able to find an industrial partner and was closed that year. She was scrapped in 2014.
 * USS Cabot (CVL-28) was acquired for preservation and moored in New Orleans from 1990 to 2002, but due to an embezzlement scandal, funding for the museum never materialized and the ship was scrapped in 2002.

Future museum carriers

 * USS Tarawa (LHA-1) has a preservation campaign to bring her to the West Coast of the United States as the world's first amphibious assault ship museum.

Related lists

 * List of aircraft carriers
 * List of aircraft carriers in service
 * List of aircraft carriers by configuration
 * List of aircraft carriers of the Second World War
 * List of sunken aircraft carriers
 * List of amphibious warfare ships
 * List of carrier-based aircraft
 * List of Canadian Navy aircraft carriers
 * List of aircraft carriers of the People's Liberation Army Navy (China)
 * List of current French Navy aircraft carriers
 * List of German aircraft carriers
 * List of aircraft carriers of the Indian Navy
 * List of Italian Navy aircraft carriers
 * List of aircraft carriers of the Japanese Navy
 * List of aircraft carriers of Russia and the Soviet Union
 * List of active Spanish aircraft carriers
 * List of aircraft carriers of the Royal Navy
 * List of escort carriers of the Royal Navy
 * List of seaplane carriers of the Royal Navy
 * List of aircraft carriers of the United States Navy
 * List of aircraft carrier classes of the United States Navy
 * List of escort aircraft carriers of the United States Navy

title: Aircraft carrier


text:
Apicomplexa

The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an apicoplast—with an apical complex membrane. The organelle's apical shape (e.g., see Ceratium furca) is an adaptation that the apicomplexan applies in penetrating a host cell.

The Apicomplexa are unicellular and spore-forming. Most are obligate endoparasites of animals, except Nephromyces, a symbiont in marine animals, originally classified as a chytrid fungus, and the Chromerida, some of which are photosynthetic partners of corals. Motile structures such as flagella or pseudopods are present only in certain gamete stages.

The Apicomplexa are a diverse group that includes organisms such as the coccidia, gregarines, piroplasms, haemogregarines, and plasmodia. Diseases caused by Apicomplexa include:
 * Babesiosis (Babesia)
 * Malaria (Plasmodium)
 * Cryptosporidiosis (Cryptosporidium parvum)
 * Cyclosporiasis (Cyclospora cayetanensis)
 * Cystoisosporiasis (Cystoisospora belli)
 * Toxoplasmosis (Toxoplasma gondii)

The name Apicomplexa derives from two Latin words—apex (top) and complexus (infolds)—for the set of organelles in the sporozoite. The Apicomplexa comprise the bulk of what used to be called the Sporozoa, a group of parasitic protozoans, in general without flagella, cilia, or pseudopods. Most of the Apicomplexa are motile, however, with a gliding mechanism that uses adhesions and small static myosin motors. The other main lines of this obsolete grouping were the Ascetosporea (a group of Rhizaria), the Myxozoa (highly derived cnidarian animals), and the Microsporidia (derived from fungi). Sometimes, the name Sporozoa is taken as a synonym for the Apicomplexa, or occasionally as a subset.

Description

The phylum Apicomplexa contains all eukaryotes with a group of structures and organelles collectively termed the apical complex. This complex consists of structural components and secretory organelles required for invasion of host cells during the parasitic stages of the Apicomplexan life cycle. Apicomplexa have complex life cycles, involving several stages and typically undergoing both asexual and sexual replication. All Apicomplexa are obligate parasites for some portion of their life cycle, with some parasitizing two separate hosts for their asexual and sexual stages.

Besides the conserved apical complex, Apicomplexa are morphologically diverse. Different organisms within Apicomplexa, as well as different life stages for a given apicomplexan, can vary substantially in size, shape, and subcellular structure. Like other eukaryotes, Apicomplexa have a nucleus, endoplasmic reticulum and Golgi complex. Apicomplexa generally have a single mitochondrion, as well as another endosymbiont-derived organelle called the apicoplast which maintains a separate 35 kilobase circular genome (with the exception of Cryptosporidium species and Gregarina niphandrodes which lack an apicoplast).

All members of this phylum have an infectious stage—the sporozoite—which possesses three distinct structures in an apical complex. The apical complex consists of a set of spirally arranged microtubules (the conoid), a secretory body (the rhoptry) and one or more polar rings. Additional slender electron-dense secretory bodies (micronemes) surrounded by one or two polar rings may also be present. This structure gives the phylum its name. A further group of spherical organelles is distributed throughout the cell rather than being localized at the apical complex and are known as the dense granules. These typically have a mean diameter around 0.7 μm. Secretion of the dense-granule content takes place after parasite invasion and localization within the parasitophorous vacuole and persists for several minutes.
 * Flagella are found only in the motile gamete. These are posteriorly directed and vary in number (usually one to three).
 * Basal bodies are present. Although hemosporidians and piroplasmids have normal triplets of microtubules in their basal bodies, coccidians and gregarines have nine singlets.
 * The mitochondria have tubular cristae.
 * Centrioles, chloroplasts, ejectile organelles, and inclusions are absent.
 * The cell is surrounded by a pellicle of three membrane layers (the alveolar structure) penetrated by micropores.

Replication:
 * Mitosis is usually closed, with an intranuclear spindle; in some species, it is open at the poles.
 * Cell division is usually by schizogony.
 * Meiosis occurs in the zygote.

Mobility:

Apicomplexans have a unique gliding capability which enables them to cross through tissues and enter and leave their host cells. This gliding ability is made possible by the use of adhesions and small static myosin motors.

Other features common to this phylum are a lack of cilia, sexual reproduction, use of micropores for feeding, and the production of oocysts containing sporozoites as the infective form.

Transposons appear to be rare in this phylum, but have been identified in the genera Ascogregarina and Eimeria.

Life cycle
Most members have a complex lifecycle, involving both asexual and sexual reproduction. Typically, a host is infected via an active invasion by the parasites (similar to entosis), which divide to produce sporozoites that enter its cells. Eventually, the cells burst, releasing merozoites, which infect new cells. This may occur several times, until gamonts are produced, forming gametes that fuse to create new cysts. Many variations occur on this basic pattern, however, and many Apicomplexa have more than one host.

The apical complex includes vesicles called rhoptries and micronemes, which open at the anterior of the cell. These secrete enzymes that allow the parasite to enter other cells. The tip is surrounded by a band of microtubules, called the polar ring, and among the Conoidasida is also a funnel of tubulin proteins called the conoid. Over the rest of the cell, except for a diminished mouth called the micropore, the membrane is supported by vesicles called alveoli, forming a semirigid pellicle.

The presence of alveoli and other traits place the Apicomplexa among a group called the alveolates. Several related flagellates, such as Perkinsus and Colpodella, have structures similar to the polar ring and were formerly included here, but most appear to be closer relatives of the dinoflagellates. They are probably similar to the common ancestor of the two groups.

Another similarity is that many apicomplexan cells contain a single plastid, called the apicoplast, surrounded by either three or four membranes. Its functions are thought to include tasks such as lipid and heme biosynthesis, and it appears to be necessary for survival. In general, plastids are considered to have a common origin with the chloroplasts of dinoflagellates, and evidence points to an origin from red algae rather than green.

Subgroups
Within this phylum are four groups — coccidians, gregarines, haemosporidians (or haematozoans, including in addition piroplasms), and marosporidians. The coccidians and haematozoans appear to be relatively closely related.

Perkinsus , while once considered a member of the Apicomplexa, has been moved to a new phylum — Perkinsozoa.

Gregarines

The gregarines are generally parasites of annelids, arthropods, and molluscs. They are often found in the guts of their hosts, but may invade the other tissues. In the typical gregarine lifecycle, a trophozoite develops within a host cell into a schizont. This then divides into a number of merozoites by schizogony. The merozoites are released by lysing the host cell, which in turn invade other cells. At some point in the apicomplexan lifecycle, gametocytes are formed. These are released by lysis of the host cells, which group together. Each gametocyte forms multiple gametes. The gametes fuse with another to form oocysts. The oocysts leave the host to be taken up by a new host.

Coccidians

In general, coccidians are parasites of vertebrates. Like gregarines, they are commonly parasites of the epithelial cells of the gut, but may infect other tissues.

The coccidian lifecycle involves merogony, gametogony, and sporogony. While similar to that of the gregarines it differs in zygote formation. Some trophozoites enlarge and become macrogamete, whereas others divide repeatedly to form microgametes (anisogamy). The microgametes are motile and must reach the macrogamete to fertilize it. The fertilized macrogamete forms a zygote that in its turn forms an oocyst that is normally released from the body. Syzygy, when it occurs, involves markedly anisogamous gametes. The lifecycle is typically haploid, with the only diploid stage occurring in the zygote, which is normally short-lived.

The main difference between the coccidians and the gregarines is in the gamonts. In the coccidia, these are small, intracellular, and without epimerites or mucrons. In the gregarines, these are large, extracellular, and possess epimerites or mucrons. A second difference between the coccidia and the gregarines also lies in the gamonts. In the coccidians, a single gamont becomes a macrogametocyte, whereas in the gregarines, the gamonts give rise to multiple gametocytes.

Haemosporidia

The Haemosporidia have more complex lifecycles that alternate between an arthropod and a vertebrate host. The trophozoite parasitises erythrocytes or other tissues in the vertebrate host. Microgametes and macrogametes are always found in the blood. The gametes are taken up by the insect vector during a blood meal. The microgametes migrate within the gut of the insect vector and fuse with the macrogametes. The fertilized macrogamete now becomes an ookinete, which penetrates the body of the vector. The ookinete then transforms into an oocyst and divides initially by meiosis and then by mitosis (haplontic lifecycle) to give rise to the sporozoites. The sporozoites escape from the oocyst and migrate within the body of the vector to the salivary glands where they are injected into the new vertebrate host when the insect vector feeds again.

Marosporida
The class Marosporida Mathur, Kristmundsson, Gestal, Freeman, and Keeling 2020 is a newly recognized lineage of apicomplexans that is sister to the Coccidia and Hematozoa. It is defined as a phylogenetic clade containing Aggregata octopiana Frenzel 1885, Merocystis kathae Dakin, 1911 (both Aggregatidae, originally coccidians), Rhytidocystis sp. 1 and Rhytidocystis sp. 2 Janouškovec et al. 2019 (Rhytidocystidae Levine, 1979, originally coccidians, Agamococcidiorida), and Margolisiella islandica Kristmundsson et al. 2011 (closely related to Rhytidocystidae). Marosporida infect marine invertebrates. Members of this clade retain plastid genomes and the canonical apicomplexan plastid metabolism. However, marosporidians have the most reduced apicoplast genomes sequenced to date, lack canonical plastidial RNA polymerase and so provide new insights into reductive organelle evolution.

Ecology and distribution
Many of the apicomplexan parasites are important pathogens of humans and domestic animals. In contrast to bacterial pathogens, these apicomplexan parasites are eukaryotic and share many metabolic pathways with their animal hosts. This makes therapeutic target development extremely difficult – a drug that harms an apicomplexan parasite is also likely to harm its human host. At present, no effective vaccines are available for most diseases caused by these parasites. Biomedical research on these parasites is challenging because it is often difficult, if not impossible, to maintain live parasite cultures in the laboratory and to genetically manipulate these organisms. In recent years, several of the apicomplexan species have been selected for genome sequencing. The availability of genome sequences provides a new opportunity for scientists to learn more about the evolution and biochemical capacity of these parasites. The predominant source of this genomic information is the EuPathDB family of websites, which currently provides specialised services for Plasmodium species (PlasmoDB), coccidians (ToxoDB),  piroplasms (PiroplasmaDB), and Cryptosporidium species (CryptoDB). One possible target for drugs is the plastid, and in fact existing drugs such as tetracyclines, which are effective against apicomplexans, seem to operate against the plastid.

Many Coccidiomorpha have an intermediate host, as well as a primary host, and the evolution of hosts proceeded in different ways and at different times in these groups. For some coccidiomorphs, the original host has become the intermediate host, whereas in others it has become the definitive host. In the genera Aggregata, Atoxoplasma, Cystoisospora, Schellackia, and Toxoplasma, the original is now definitive, whereas in Akiba, Babesiosoma, Babesia, Haemogregarina, Haemoproteus, Hepatozoon, Karyolysus, Leucocytozoon, Plasmodium, Sarcocystis, and Theileria, the original hosts are now intermediate.

Similar strategies to increase the likelihood of transmission have evolved in multiple genera. Polyenergid oocysts and tissue cysts are found in representatives of the orders Protococcidiorida and Eimeriida. Hypnozoites are found in Karyolysus lacerate and most species of Plasmodium; transovarial transmission of parasites occurs in lifecycles of Karyolysus and Babesia.

Horizontal gene transfer appears to have occurred early on in this phylum's evolution with the transfer of a histone H4 lysine 20 (H4K20) modifier, KMT5A (Set8), from an animal host to the ancestor of apicomplexans. A second gene—H3K36 methyltransferase (Ashr3 in plants)—may have also been horizontally transferred.

Blood-borne genera
Within the Apicomplexa are three suborders of parasites:
 * suborder Adeleorina—eight genera
 * suborder Laveraniina (formerly Haemosporina)—all genera in this suborder
 * suborder Eimeriorina—two genera (Lankesterella and Schellackia)

Within the Adelorina are species that infect invertebrates and others that infect vertebrates. The Eimeriorina—the largest suborder in this phylum—the lifecycle involves both sexual and asexual stages. The asexual stages reproduce by schizogony. The male gametocyte produces a large number of gametes and the zygote gives rise to an oocyst, which is the infective stage. The majority are monoxenous (infect one host only), but a few are heteroxenous (lifecycle involves two or more hosts).

The number of families in this later suborder is debated, with the number of families being between one and 20 depending on the authority and the number of genera being between 19 and 25.

History
The first Apicomplexa protozoan was seen by Antonie van Leeuwenhoek, who in 1674 saw probably oocysts of Eimeria stiedae in the gall bladder of a rabbit. The first species of the phylum to be described, Gregarina ovata in earwigs intestines, was named by Dufour in 1828. He thought that they were a peculiar group related to the trematodes, at that time included in Vermes. Since then, many more have been identified and named. During 1826–1850, 41 species and six genera of Apicomplexa were named. In 1951–1975, 1873 new species and 83 new genera were added.

The older taxon Sporozoa, included in Protozoa, was created by Leuckart in 1879 and adopted by Bütschli in 1880. Through history, it grouped with the current Apicomplexa many unrelated groups. For example, Kudo (1954) included in the Sporozoa species of the Ascetosporea (Rhizaria), Microsporidia (Fungi), Myxozoa (Animalia), and Helicosporidium (Chlorophyta), while Zierdt (1978) included the genus Blastocystis (Stramenopiles). Dermocystidium was also thought to be sporozoan. Not all of these groups had spores, but all were parasitic. However, other parasitic or symbiotic unicellular organisms were included too in protozoan groups outside Sporozoa (Flagellata, Ciliophora and Sarcodina), if they had flagella (e.g., many Kinetoplastida, Retortamonadida, Diplomonadida, Trichomonadida, Hypermastigida), cilia (e.g., Balantidium) or pseudopods (e.g., Entamoeba, Acanthamoeba, Naegleria). If they had cell walls, they also could be included in plant kingdom between bacteria or yeasts.

Sporozoa is no longer regarded as biologically valid and its use is discouraged, although some authors still use it as a synonym for the Apicomplexa. More recently, other groups were excluded from Apicomplexa, e.g., Perkinsus and Colpodella (now in Protalveolata).

The field of classifying Apicomplexa is in flux and classification has changed throughout the years since it was formally named in 1970.

By 1987, a comprehensive survey of the phylum was completed: in all, 4516 species and 339 genera had been named. They consisted of:
 * Class Conoidasida
 * Subclass Gregarinasina p.p.
 * Order Eugregarinorida, with 1624 named species and 231 named genera
 * Subclass Coccidiasina p.p
 * Order Eucoccidiorida p.p
 * Suborder Adeleorina p.p
 * Group Hemogregarines, with 399 species and four genera
 * Suborder Eimeriorina, with 1771 species and 43 genera
 * Class Aconoidasida
 * Order Haemospororida, with 444 species and nine genera
 * Order Piroplasmorida, with 173 species and 20 genera
 * Other minor groups omitted above, with 105 species and 32 genera

Although considerable revision of this phylum has been done (the order Haemosporidia now has 17 genera rather than 9), these numbers are probably still approximately correct.

Jacques Euzéby (1988)
Jacques Euzéby in 1988 created a new class Haemosporidiasina by merging subclass Piroplasmasina and suborder Haemospororina.
 * Subclass Gregarinasina (the gregarines)
 * Subclass Coccidiasina
 * Suborder Adeleorina (the adeleorins)
 * Suborder Eimeriorina (the eimeriorins)
 * Subclass Haemosporidiasina
 * Order Achromatorida
 * Order Chromatorida

The division into Achromatorida and Chromatorida, although proposed on morphological grounds, may have a biological basis, as the ability to store haemozoin appears to have evolved only once.

Roberts and Janovy (1996)
Roberts and Janovy in 1996 divided the phylum into the following subclasses and suborders (omitting classes and orders): These form the following five taxonomic groups:
 * Subclass Gregarinasina (the gregarines)
 * Subclass Coccidiasina
 * Suborder Adeleorina (the adeleorins)
 * Suborder Eimeriorina (the eimeriorins)
 * Suborder Haemospororina (the haemospororins)
 * Subclass Piroplasmasina (the piroplasms)
 * 1) The gregarines are, in general, one-host parasites of invertebrates.
 * 2) The adeleorins are one-host parasites of invertebrates or vertebrates, or two-host parasites that alternately infect haematophagous (blood-feeding) invertebrates and the blood of vertebrates.
 * 3) The eimeriorins are a diverse group that includes one host species of invertebrates, two-host species of invertebrates, one-host species of vertebrates and two-host species of vertebrates. The eimeriorins are frequently called the coccidia. This term is often used to include the adeleorins.
 * 4) Haemospororins, often known as the malaria parasites, are two-host Apicomplexa that parasitize blood-feeding dipteran flies and the blood of various tetrapod vertebrates.
 * 5) Piroplasms where all the species included are two-host parasites infecting ticks and vertebrates.

Perkins (2000)

Perkins et al. proposed the following scheme. It is outdated as the Perkinsidae have since been recognised as a sister group to the dinoflagellates rather that the Apicomplexia:
 * Class Aconoidasida
 * Conoid present only in the ookinete of some species
 * Order Haemospororida
 * Macrogamete and microgamete develop separately. Syzygy does not occur. Ookinete has a conoid. Sporozoites have three walls. Heteroxenous: alternates between vertebrate host (in which merogony occurs) and invertebrate host (in which sporogony occurs). Usually blood parasites, transmitted by blood-sucking insects.
 * Order Piroplasmorida

 * Class Conoidasida
 * Subclass Gregarinasina
 * Order Archigregarinorida
 * Order Eugregarinorida
 * Suborder Adeleorina
 * Suborder Eimeriorina
 * Order Neogregarinorida
 * Subclass Coccidiasina
 * Order Agamococcidiorida
 * Order Eucoccidiorida
 * Order Ixorheorida
 * Order Protococcidiorida
 * Class Perkinsasida
 * Order Perkinsorida
 * Family Perkinsidae

The name Protospiromonadida has been proposed for the common ancestor of the Gregarinomorpha and Coccidiomorpha.

Another group of organisms that belong in this taxon are the corallicolids. These are found in coral reef gastric cavities. Their relationship to the others in this phylum has yet to be established.

Another genus has been identified - Nephromyces - which appears to be a sister taxon to the Hematozoa. This genus is found in the renal sac of molgulid ascidian tunicates.

Evolution
Members of this phylum, except for the photosynthetic chromerids, are parasitic and evolved from a free-living ancestor. This lifestyle is presumed to have evolved at the time of the divergence of dinoflagellates and apicomplexans. Further evolution of this phylum has been estimated to have occurred about. The oldest extant clade is thought to be the archigregarines.

These phylogenetic relations have rarely been studied at the subclass level. The Haemosporidia are related to the gregarines, and the piroplasms and coccidians are sister groups. The Haemosporidia and the Piroplasma appear to be sister clades, and are more closely related to the coccidians than to the gregarines. Marosporida is a sister group to Coccidiomorphea.

Janouškovec et al 2015 presents a somewhat different phylogeny, supporting the work of others showing multiple events of plastids losing photosynthesis. More importantly this work provides the first phylogenetic evidence that there have also been multiple events of plastids becoming genome-free.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Apicomplexa