Bigfin squid

Bigfin squids are a group of rarely seen cephalopods with a distinctive morphology. They are placed in the genus Magnapinna and family Magnapinnidae. Although the family was described only from larval, paralarval, and juvenile specimens, numerous video observations of much larger squid with similar morphology are assumed to be adult specimens of the same family.

The arms and tentacles of the squid are both extremely long, estimated at 4 to 8 m. These appendages are held perpendicular to the body, creating "elbows." How the squid feeds is yet to be discovered.

Magnapinna is thought to be the deepest-occurring squid genus, with sightings as deep as 6212 m below the surface, making it the only squid known to inhabit the hadal zone.

Taxonomy
Magnapinna is the sister group to Joubiniteuthis, another bizarre, little-known deep-sea squid with an unusual body plan and long arms. Both Magnapinna and Joubiniteuthis are monotypic genera within their own families, Magnapinnidae and Joubiniteuthidae respectively. They are also closely related to the "whip-lash squid" in the families Chiroteuthidae and Mastigoteuthidae. 

Physical specimens
The first record of this family comes from a specimen (Magnapinna talismani) caught off the Azores on 10 August 1883. Due to the damaged nature of the find, little information could be discerned, and it was classified as a mastigoteuthid, first as Chiroteuthopsis talismani and later as Mastigoteuthis talismani. In 1956, a similar squid (Magnapinna sp. C) was caught in the South Atlantic, but little was thought of it at the time. The specimen was illustrated in Alister Hardy's The Open Sea (1956), where it was identified as Octopodoteuthis sicula. During the 1980s, two additional immature specimens were found in the Atlantic (Magnapinna sp. A), and three more were found in the Pacific (Magnapinna pacifica). Researchers Michael Vecchione and Richard Young were the chief investigators of the finds, and eventually linked them to the two previous specimens, erecting the family Magnapinnidae in 1998, with Magnapinna pacifica as the type species. Of particular interest was the very large fin size, up to 90% of the mantle length, that was responsible for the animals' common name.

A single specimen of a fifth species, Magnapinna sp. B, was collected in 2006. Magnapinna sp. A was described as Magnapinna atlantica in 2006.

The genus was described from two juveniles and paralarva, none of which had developed the characteristic long arm tips. However, they did all have large fins, and were therefore named "magna pinna", meaning "big fin".

Sightings
The presumed adult stage of Magnapinna is known only from video observations from submersibles and ROVs; no physical specimens have yet been collected, leaving their exact identity unknown. These individuals and the collected juvenile specimens share the very large fins and the vermiform arm tips with no suckers, but the iconic elongated arm tips are known only from observed individuals. Although it has not been directly confirmed whether these squid are the same as the Magnapinna known from specimens, it is largely accepted that they are members of Magnapinnidae.

Although observations had been made over a decade earlier, adult bigfin squid only became known to science in 2001, when marine biology student Heather Holston sent footage of what she described as an "21-foot-long squid" to teuthologist Michael Vecchione. The footage had been recorded from an ROV in the Gulf of Mexico in January 2000 at the request of Holston's boyfriend Eric Leveton, who planned on showing it to her. Leveton was a structural engineer aboard the oil-drilling ship Millennium Explorer, who had happened to look into the ROV operation shack when the squid was observed by operators. Although Vecchione initially surmised from Holston's description that the footage might be the first video of a live giant squid (Architeuthis dux), he realized that the video itself portrayed a completely different squid that had no known identity.

Further discussions with other cephalopod researchers found no leads on the identity of the squid, and it was thus dubbed the "mystery squid" for a portion of time. Analysis by Vecchione et al of previous footage from submersibles found other video records of bigfin squid, the earliest from 1988. Around the same time, new high-quality footage of a bigfin squid was also recorded off Hawaii by the ROV Tiburon. In December 2001, Vecchione et al published a paper collating these observations; this was also the first paper to identify them as potential members of the Magnapinnidae, which had been named by Vecchione from the juvenile specimens a few years earlier. Independent of Vecchione's publication, Guerra et al published a paper the following year analyzing some of the early bigfin squid footage, and also identified them as potential adult magnapinnids.

Anatomy
The specimens in the videos looked very distinct from all previously known squids. Uniquely among cephalopods, the arms and tentacles were of the same length and looked identical (similar to extinct belemnites). The appendages were also held perpendicular to the body, creating the appearance of strange "elbows". Most remarkable was the length of the elastic tentacles, which has been estimated at up to 15–20 times the mantle length. This trait is caused by filament coiling of the tentacles, a trait that is rare among similar species. Estimates based on video evidence put the total length of the largest specimens at 8 m or more, with some estimates up to 12 m. Viewing close-ups of the body and head, it is apparent that the fins are extremely large, being proportionately nearly as big as those of bigfin squid larvae. While they do appear similar to the larvae, no specimens or samples of the adults have been taken. While their exact identity is unknown, all of the discovered specimens can be observed to have a brown-orange color body, translucent fins, near-white tentacles, and dark eyes. These species of squids are mainly identifiable by their long thin arms and specific colors. The squid also have a very unique brachial crown that sets them aside from the rest of other families that are known.

Feeding behaviour
Little is known about the feeding-behaviour of these squids. Scientists have speculated that the bigfin squid feeds by dragging their arms and tentacles along the seafloor and grabbing edible organisms from the floor. Alternatively, they may simply use a trapping technique, waiting passively for prey such as zooplankton to bump into their arms (see Cephalopod intelligence). The diet of the bigfin squid is unknown. However, cephalopods are known to feed on crustaceans, jellyfish, and even other cephalopods.

Observation timeline
The first visual record of an adult bigfin squid was in September 1988. The crew of the submersible Nautile encountered a bigfin squid off the coast of northern Brazil, 10.71517°N, -40.8905°W, at a depth of 4735 m. In July 1992, the Nautile again encountered these creatures, observing two individuals during a dive off the coast of Ghana at 3.66667°N, -2.5°W, first at 3010 m depth, and then again at 2950 m. Both were filmed and photographed. In November 1998, the Japanese crewed submersible Shinkai 6500 filmed another bigfin squid in the Indian Ocean south of Mauritius, at -32.75°N, 57.21667°W and 2340 m.

Eric Leveton's video, which was later shared with Vecchione, was taken from the remotely operated underwater vehicle (ROV) of the oil-drilling ship Millennium Explorer in January 2000, at Mississippi Canyon in the Gulf of Mexico (28.61667°N, -88°W) at 2195 m, and allowed for a size estimate. By comparison with the visible parts of the ROV, the squid was estimated to measure 7 m with arms fully extended. The Nautile filmed another Indian Ocean specimen at -19.53333°N, 65.86667°W and 2576 m, in the area of Rodrigues Island, in May 2000. In October 2000, the crewed submersible Alvin found another bigfin squid at 1940 m in Atwater Valley, Gulf of Mexico (27.57857°N, -88.50983°W).

These videos did not receive any media attention; most were brief and fairly blurry. In May 2001, approximately ten minutes of crisp footage of a bigfin squid were acquired by ROV Tiburon, causing a flurry of attention when released. These were taken in the Pacific Ocean north of Oʻahu, Hawaii (21.9°N, -158.2°W), at 3380 m. This video and the pre-2001 videos (which had not previously received much scientific attention) were documented by Vecchione et al in a paper that year, and some of the earlier footage was further analyzed by Guerra et al (2002).

On 11 November 2007, a bigfin squid was filmed off Perdido, a drilling-site owned by Shell Oil Company, located 200 smi off Houston, Texas in the Gulf of Mexico. The ROV that filmed the squid had originally been sent to retrieve drilling equipment from the seabed, and encountered the squid floating near a well. After being circulated within the oil industry, the footage was shared with National Geographic News to have its identity determined, and was released to the public in 2008. This video received significant online attention in the years since its filming. Observations of bigfin squid were made in the Great Australian Bight during towed camera and remote operated vehicle surveys in 2015 and 2017 respectively. In 2018, the first observations of a bigfin squid were made from the Southern Caribbean, off the coast of Colombia.

In March 2021, during the expedition to document the wreck of the USS Johnston, the submersible DSV Limiting Factor recorded footage of a juvenile bigfin squid from the Philippine Trench at a depth of 6212 m. This is the deepest observation of any squid, and rivalled only by some unidentified cirrate octopods from the same habitat as the deepest observation of any cephalopod. This makes Magnapinna the first squid known to inhabit the hadal zone.

On 9 November 2021, a video of a bigfin squid was captured at a ridge feature off the West Florida Escarpment by an ROV from the NOAAS Okeanos Explorer as part of the Windows to the Deep 2021 expedition. The squid was found at a depth of 2385 m, and its size is currently being measured using paired lasers.

ROV SuBastian of Schmidt Ocean Institute observed a bigfin squid in close proximity to a black smoker-type hydrothermal vent on 4 April 2023, during the "In Search of Hydrothermal Lost Cities" expedition. The squid was seen at a depth of 1931 m.