Cephalopod size

Cephalopods, which include squids and octopuses, vary enormously in size. The smallest are only about 1 cm long and weigh less than 1 g at maturity, while the giant squid can exceed 10 m in length and the colossal squid weighs close to half a tonne (0.5 tonne), making them the largest living invertebrates. Living species range in mass more than three-billion-fold, or across nine orders of magnitude, from the lightest hatchlings to the heaviest adults. Certain cephalopod species are also noted for having individual body parts of exceptional size.

Cephalopods were at one time the largest of all organisms on Earth, and numerous species of comparable size to the largest present day squids are known from the fossil record, including enormous examples of ammonoids, belemnoids, nautiloids, orthoceratoids, teuthids, and vampyromorphids. In terms of mass, the largest of all known cephalopods were likely the giant shelled ammonoids and endocerid nautiloids, though perhaps still second to the largest living cephalopods when considering tissue mass alone.

Cephalopods vastly larger than either giant or colossal squids have been postulated at various times. One of these was the St. Augustine Monster, a large carcass weighing several tonnes that washed ashore on the United States coast near St. Augustine, Florida, in 1896. Reanalyses in 1995 and 2004 of the original tissue samples—together with those of other similar carcasses—showed conclusively that they were all masses of the collagenous matrix of whale blubber.

Giant cephalopods have fascinated humankind for ages. The earliest surviving records are perhaps those of Aristotle and Pliny the Elder, both of whom described squids of very large size. Tales of giant squid have been common among mariners since ancient times, and may have inspired the monstrous kraken of Nordic legend, said to be as large as an island and capable of engulfing and sinking any ship. Similar tentacled sea monsters are known from other parts of the globe, including the Akkorokamui of Japan and Te Wheke-a-Muturangi of New Zealand. The Lusca of the Caribbean and Scylla in Greek mythology may also derive from giant squid sightings, as might eyewitness accounts of other sea monsters such as sea serpents.

Cephalopods of enormous size have featured prominently in fiction. Some of the best known examples include the giant squid from Jules Verne's 1870 novel Twenty Thousand Leagues Under the Seas and its various film adaptations; the giant octopus from the 1955 monster movie It Came from Beneath the Sea; and the giant squid from Peter Benchley's 1991 novel Beast and the TV film adaptation of the same name.

Due to its status as a charismatic megafaunal species, the giant squid has been proposed as an emblematic animal for marine invertebrate conservation. Life-sized models of the giant squid are a common sight in natural history museums around the world, and preserved specimens are much sought after for display.

Mantle length


Mantle length (ML) is the standard size measure for coleoid cephalopods (shell diameter being more common for nautiluses) and is almost universally reported in the scientific literature. The mantle is the cephalopod's "body", lying posterior to the head and enclosing the visceral mass and mantle cavity, the latter being used for locomotion by jet propulsion. Unless otherwise indicated, mantle length is measured dorsally over the midline of the mantle (sometimes specified as dorsal mantle length, DML). It is a straight-line measure, not measured over the curve of the body. In Decapodiformes (ten-limbed cephalopods), mantle length is measured from the anterior edge of the mantle (near the head), to the posterior end of the mantle or the apex of the united fins, whichever is longer. In Octopodiformes (eight-limbed cephalopods), the anterior edge of the mantle is not clearly delimited dorsally due to advanced head–mantle fusion, and mantle length is therefore taken from the midpoint between the eyes to the posterior end of the mantle. When ventral mantle length is meant instead of dorsal this is always specified as such and abbreviated VML.

As an indication of overall size, mantle length is generally considered more reliable than total length because cephalopod limbs may easily be stretched beyond their natural length and are often damaged or missing in preserved specimens (this is particularly true of the long tentacles of many squid species). Nevertheless, mantle length is not equally applicable to all species. Certain benthic octopuses such as Callistoctopus ornatus are able to elongate and retract their mantles and therefore mantle length measurements, even when taken from a live specimen, may vary considerably. Another problematic case is that of the gelatinous cirroteuthids, whose weakly muscled mantles are prone to substantial shrinkage during preservation. The interocular distance may be a more reliable standard for this group.

Total length
Total length (TL) is measured along the dorsal midline with the limbs outstretched and in line with the body axis. It is the greatest measurable extent of a specimen from the posterior end of the mantle or fins (or tail, if present ) to the apex of the longest limb. It is recommended that arms and tentacles be measured in a relaxed state so as not to exaggerate their length, but historically this practice was not always followed and some of the more extreme published giant squid measurements have been attributed to artificial lengthening of the tentacles. Although total length is often mentioned in relation to the largest cephalopod species, it is otherwise seldom used in teuthology. As with mantle length, it is a straight-line measure.

Total length is not to be confused with arm span—also known as arm spread, radial span, or radial spread—which may be much larger and is often reported for octopuses (for which the arms usually constitute the vast majority of the length). In squids, total length is inclusive of the feeding tentacles, which in some species may be longer than the mantle, head, and arms combined (chiroteuthids such as Asperoteuthis acanthoderma being a prime example).

A related measure is standard length (SL), which is the combined length of the mantle, head, and arms, excluding the often long feeding tentacles. This measure is particularly useful for species such as the giant squid, where almost the entire bulk of the animal takes up less than half of its total length.

Mass
Mass (often abbreviated WT for 'weight') is reported far less frequently than either mantle or total length, and accurate records do not exist for all of the large cephalopod species. It can also vary widely depending on the state of the specimen at the time of weighing (for example, whether it was measured live or dead, wet or dry, frozen or thawed, pre- or post-fixation, with or without egg mass, and so on).

<!--===Other size metrics=== Abbreviations used for measurements and counts are based on standardised acronyms in teuthology, primarily those defined by Roper & Voss (1983), with the exception of several found in older references (see Sweeney & Roper, 2001). https://www.tonmo.com/pages/measurements/

Arms and tentacles

 * arm circumference (AC): AC(I), AC(II), AC(III) and AC(IV) refer to measurements of specific arm pairs
 * arm diameter (AD): AD(I), AD(II), AD(III) and AD(IV) refer to measurements of specific arm pairs
 * arm length (AL): AL(I), AL(II), AL(III) and AL(IV) refer to measurements of specific arm pairs
 * arm sucker diameter (ASD)
 * carpus length (CaL)
 * club length (CL): usually refers to expanded portion at the apex of tentacle
 * club sucker diameter (CSD): usually largest; usually equivalent to LSD
 * club width (CW)
 * dactylus club length (DC)
 * hectocotylus length (HeL)
 * longest arm length (LAL)
 * largest sucker diameter (LSD): on tentacle club; usually equivalent to CSD
 * manus length (MaL)
 * tentacle circumference (TC): most often of tentacle stalk
 * tentacle club length (TCL)
 * tentacle diameter (TD): most often of tentacle stalk
 * tentacle length (TL)
 * tentacle sucker diameter (TSD): usually largest

Mantle

 * mantle length (ML): usually dorsal mantle length unless stated otherwise
 * dorsal mantle length (DML)
 * ventral mantle length (VML)
 * mantle width (MW): usually maximum
 * mantle thickness (MT)
 * fin length (FL)
 * fin width (FW)
 * tail length (TaL)

Head

 * buccal apparatus circumference (BAC)
 * buccal apparatus length (BAL)
 * eye diameter (EyD)
 * eye orbit diameter (EyOD)
 * head circumference (HC)
 * head length (HL): most often base of arms to edge of mantle
 * head width (HW)
 * lower rostral length of beak (LRL)
 * upper rostral length of beak (URL)

Funnel

 * funnel opening diameter (FuD)
 * funnel length (FuL)
 * funnel cartilage length (FuCL)
 * funnel cartilage width (FuCW)

Internal anatomy

 * egg diameter (ED)
 * NGL, nidamental gland length
 * PL, penis length
 * RaL, radula length
 * RaW, radula width
 * spermatophore length (SL)
 * spermatophore sac length (SSL)
 * gill length (GiL)
 * gladius (pen) length (GL)
 * gladius (pen) width (GW)
 * rachis length (RL)
 * rachis width (RW)

Historical and nonstandard

 * body circumference (BC): assumed to mean greatest circumference of mantle unless otherwise specified
 * body diameter (BD): assumed to mean greatest diameter of mantle
 * body length (BL): usually equivalent to mantle length, as head length is often given separately
 * "entire" length (EL): end of tentacle(s), often stretched, to posterior tip of tail; in contrast to WL, measured from end of arms to posterior tip of tail
 * "whole" length (WL): end of arms, often damaged, to posterior tip of tail; in contrast to EL, measured from end of tentacles to posterior tip of tail-->

Methods of size determination
In contrast to the vast majority of living cephalopods, which are wholly soft-bodied, size determination of the few surviving shelled species (in terms of shell diameter) is comparatively straightforward and can be accomplished with a high level of precision. Whatever the type of cephalopod, in the absence of whole specimens, size can often be estimated from only partial remains. For example, cephalopod beaks can be used for mantle length, total length and body mass estimation, and this method has notably been used to estimate the maximum size of the colossal squid. The lower rostral length (LRL) of the beak is often used for this purpose. The rostral length of the lower and upper beaks is the standard measure of beak size in Decapodiformes; hood length is preferred for Octopodiformes.

Mantle length has been estimated from video recordings of squid in the wild.

Hatchlings
Hatchlings of Idiosepius thailandicus, possibly the smallest extant cephalopod species at maturity, have a mantle length of around 1 mm. The closely related Idiosepius pygmaeus weighs only 0.00033 g upon hatching and increases in weight to 0.175 g as it reaches maturity in 50 days. Even smaller are the hatchlings of the commercially important Illex illecebrosus, with a mass of 0.00015 g. Hatchlings of the giant Pacific octopus (Enteroctopus dofleini)—one of the two largest octopus species—weigh 0.0253 g on average.

At the other extreme are nautiluses, which upon hatching typically have a shell diameter of 25 mm or more (depending on the species), the largest hatchling size among extant invertebrates. Hatchlings of Nautilus belauensis, one of the larger species, are estimated to weigh on the order of 5.9 g and mature at around 1.2 kg after almost 4000 days, or around 11 years.



Smallest adult size
The smallest adult size among living cephalopods is attained by the so-called pygmy squids, Idiosepius, and certain diminutive species of the genus Octopus, both of which weigh less than 1 g at maturity. Idiosepius thailandicus is perhaps the smallest of all, with females averaging 10.4 mm in mantle length and males 5.9 mm. Average wet weights are around 0.20 and 0.02 g, respectively.

Other tiny species include members of the bobtail squid family Sepiolidae; the myopsid squid genera Australiteuthis and Pickfordiateuthis; the oegopsid squid genera Abralia and Abraliopsis; the pygmy cuttlefish Sepia pulchra; and the ram's horn squid, Spirula spirula.

Male dwarfism
The octopod superfamily Argonautoidea is characterised by markedly dwarfed males. The four extant genera of the group are Argonauta, Haliphron, Ocythoe, and Tremoctopus, all of which are exclusively pelagic. The greatest disparity in the size of the sexes is seen in the blanket octopuses of the genus Tremoctopus. Norman et al. (2002) reported a fully mature male Tremoctopus violaceus measuring 2.4 cm in total length and weighing a mere 0.25 g. By comparison, the large females of this species reach total lengths of 2 m and probably some 10 kg in weight. This is the most extreme sexual size dimorphism known among non-microscopic animals, with mature females being at least 10,000 times heavier than males, and likely up to 40,000 times heavier. The related genera Argonauta and Ocythoe have similarly small males, but the females are not nearly as large as those of Tremoctopus, and the size dimorphism is therefore less pronounced. The females of the fourth argonautoid genus, Haliphron, are the largest of all up to length of 3.5 m and mass of 75 kg (and possibly the largest octopuses of any kind), but the males are also much larger, at up to 30 cm. So, the size dimorphism is least pronounced of all.

Extinct taxa
Numerous species of so-called micromorphic ammonites are known. Maximites from the Upper Carboniferous is the smallest known ammonoid. Adult specimens reached only 10 mm in shell diameter.

Scientifically validated records
Squids are the largest living cephalopods in terms of each of mantle length, total length, and mass, with the largest species by at least two of these measures being the colossal squid, Mesonychoteuthis hamiltoni. Reaching an estimated 3 m in mantle length and 10 m in total length, and weighing as much as 495 kg, this species is also the largest of all extant invertebrates. The only other squid that approaches these dimensions is the giant squid of the genus Architeuthis, with females up to 275 kg, 2.4 m in mantle length, and possibly as much as 15 m in total length, making it likely the longest of all cephalopods. The two largest octopus species—Enteroctopus dofleini and Haliphron atlanticus—can both exceed 70 kg, and the former has a maximum total length of more than 6 m. Cirrate (finned) octopods can also reach a large size, with the largest captured specimen likely being a Cirrothauma magna of 1.7 m total length and 33 cm mantle length, though observations from submersibles suggest that members of this group can exceed 4 m in total length.

Members of the other cephalopod groups are substantially smaller, although the largest cuttlefish can exceed 10 kg in weight and 50 cm in mantle length. Cephalopods of comparable size to the largest present day squid are known from fossil remains, including enormous examples of ammonoids, belemnoids, nautiloids, orthoceratoids, teuthids, and vampyromorphids.

Colossal squid (Mesonychoteuthis hamiltoni)
Though a substantial number of colossal squid (Mesonychoteuthis hamiltoni) remains have been recorded (Xavier et al., 1999 collated 188 geographical positions for whole or partial specimens caught by commercial and scientific fisheries), very few adult or subadult animals have ever been documented, making it difficult to estimate the maximum size of the species. McClain et al. (2015) stated that only 12 "complete" specimens were known. The largest known complete specimen of the colossal squid was a mature female captured in the Ross Sea in February 2007. Its weight was initially estimated at 450 kg, its mantle length at 4 m, and its total length at 8 –. Once completely thawed the specimen was found to weigh 495 kg, but to measure only 2.5 m in mantle length and 4.2 m in total length. It is likely that the specimen, and particularly its tentacles, shrank considerably post mortem as a result of dehydration, having been kept in a freezer for 14 months. (As reported by the Museum of New Zealand Te Papa Tongarewa, specimens of Nototodarus sloanii, the New Zealand arrow squid, can shrink by as much as 22% when dehydrated with alcohol solutions. ) The colossal squid specimen contracted by a further 5% after several years in preservative fluid (first formalin and later propylene glycol). The fins of the 2007 Ross Sea specimen measured around 1.2 m across and it had a mantle width of 98.2 cm. The arms ranged in length from 0.85 to 1.15 m, while the two tentacles were around 2.1 m long. Beaks recovered from sperm whale stomachs indicate the existence of animals surpassing even the 2007 Ross Sea specimen. That specimen had a lower rostral length (LRL) of 42.5 mm and weighed 495 kg, where as the 300 kg submature female from 2003 had a LRL of 37 mm. By comparison, the largest known colossal squid beak from a sperm whale stomach measured 49 mm in LRL. Though the number of large colossal squid specimens known to science is too small to get a good idea of the relationship between beak size and overall body size, a beak of such enormity indicates a truly massive animal weighing perhaps as much as 600 –. However, the scaling relationship for this species shows considerable latitude, as demonstrated by a beak of 40 mm LRL extracted from an animal weighing only 160 kg.

Giant squid (Architeuthis dux)
The maximum size of the giant squid (Architeuthis dux) has long been a subject of both popular debate and academic inquiry. Unlike the colossal squid, the giant squid is known from a substantial number of mature specimens. The total number of recorded specimens (across all developmental stages) approaches a thousand, with approximately 700 documented, of which around 460 had been measured in some way. This number has since increased substantially, with 57 specimens recorded from Japanese waters during an exceptional 15-month period between 2014 and 2015. Based on a 40-year data set of more than 50 giant squid specimens, Roper & Shea (2013:114) suggest an average total length at maturity of 11 m and a "rarely encountered maximum length" of 14 –. Of the nearly 100 specimens examined by Clyde Roper, the largest was "46 ft long". O'Shea & Bolstad (2008) give a maximum total length of 13 m for females based on the examination of more than 130 specimens, measured post mortem and relaxed, as well as beaks recovered from sperm whales (which do not exceed the size of those found in the largest complete specimens). Steve O'Shea estimated the maximum total length for males at 10 m. Older records of 18 m or more were likely exaggerated by stretching of the long feeding tentacles or resulted from inadequate measurement methods such as pacing. O'Shea has stated that, given the available evidence, the highest upper bound he would consider plausible for the giant squid's total length would be 15 m, and that the likelihood that there exist 20-metre giant squid is "so exceedingly remote that you couldn't justify the effort in writing about it".

Including the head and arms but excluding the tentacles (standard length, SL), the species very rarely exceeds 5 m according to O'Shea & Bolstad (2008). Paxton (2016a) considers 9.45 m to be the greatest reliably measured SL, based on a specimen reported by Verrill (1880a:192), and considers specimens of 10 m SL or more to be "very probable", but these conclusions have been criticised by giant squid experts. O'Shea (2003a) put the maximum weight of female giant squid at 275 kg, based on the examination of some 105 specimens as well as beaks recovered from sperm whales (which do not exceed the size of those found in the largest complete specimens). Giant squid are sexually size dimorphic, with the maximum weight for males estimated at 150 kg, though heavier specimens have occasionally been reported (such as a 190 kg specimen and a 163 kg specimen ). Roper & Jereb (2010a:121) give a maximum weight of up to 500 kg, and "possibly greater". Discredited weights of as much as a tonne (1 tonne) or more are not uncommon in older literature (see below).

Other squid taxa
The third-heaviest extant squid species is Taningia danae, also known as the Dana octopus squid. The largest well documented specimen is a 160 cm ML mature female reported by Roper & Vecchione (1993) from the North Atlantic. The original paper gave the mass of this specimen as 61.4 kg, but according to Roper & Jereb (2010h:266) this figure is wrong and stems from a typographical error, the correct mass being 161.4 kg. Roper & Vecchione (1993) were however consistent in their use of the 61.4 kg figure. Another similarly large specimen—a female weighing 124 kg—was reported from northern Spanish waters by González et al. (2003:297) (see also initial reports ). In July 2010, a sperm whale was photographed off the Azorean island of Faial with a large squid—likely T. danae—in its mouth. The specimen's maximum width, from fin tip to fin tip, was estimated at 1.5 –; this would approximate its mantle length. Onykia robusta, previously known as Moroteuthis robusta and sometimes called the robust clubhook squid, has a mantle length of up to 200 cm. Some older records exceed this, such as the 91.5 in ML reported by Verrill (1876:237) from a specimen with a total length of 14 ft (excluding the ends of the tentacles, which had been destroyed). Nesis (1987:192) likewise gave a maximum mantle length of 230 cm, but Roper & Jereb (2010i:364) wrote that "this old record might be in error", with the species commonly growing to 160 cm ML. Glaubrecht & Salcedo-Vargas (2004:66) provided a maximum total length of 4 –. Literature sources give a maximum weight of 50 kg. There exist numerous published records of large individuals of this species. The Humboldt squid (Dosidicus gigas), also known as the 'jumbo squid', grows to a maximum mantle length of at least 120 cm, if not 150 cm. The largest animals are found off the western coast of South America; northern populations reach 100 cm ML, and in general 50 – ML is more typical for the species. Southern populations may have a total length approaching 2.5 m, and possibly up to 12 ft. Again, specimens from the northern hemisphere are much smaller, with those off the Californian coast reaching total lengths of less than 1.7 m. The Humboldt squid commonly attains a weight of around 20 – and can reach a maximum of 50 kg. There are anecdotal reports of much larger individual animals, including from diver Scott Cassell, who has dived with Humboldt squid over 300 times (reportedly more than any other person).

Kondakovia longimana, sometimes known as the giant warty squid, is a little known species with a circum-Antarctic distribution in the Southern Ocean. The largest complete specimen, found floating at the surface off the South Orkney Islands, had a mantle length of 108 cm, but a damaged female specimen with an estimated mantle length of around 150 cm is known. The largest complete specimen had a wet weight of 29 kg. The species' maximum weight has been estimated at 50 kg.

Largest octopodes
The giant Pacific octopus (Enteroctopus dofleini) grows to more than 6.1 m in total length and at least 60 cm in mantle length. Cosgrove (1987) and Cosgrove & McDaniel (2009:69) gave a maximum confirmed weight of 71 kg for a live specimen collected in the mid-1960s. Norman et al. (2014:124) accept a maximum weight of at least 180 kg, which approximates the 182.3 kg reported for a specimen caught off Santa Barbara, California, in 1945, of which photographic evidence survives. No specimens approaching this size have been reported since the middle of the 20th century, with recent specimens very rarely exceeding 50 kg. It is possible that the maximum size of the species has decreased over this period, perhaps due to bioaccumulation of toxicants (see below).

In 2002, a giant specimen of Haliphron atlanticus, the seven-arm octopus, was caught by fishermen trawling at a depth of 920 m off the eastern Chatham Rise, New Zealand. This specimen, the largest of this species and possibly of all octopuses, was the first validated record of Haliphron from the South Pacific. It had a mantle length of 0.69 m, a total length of 2.90 m, and a weight of 61.0 kg, although it was incomplete. The total length of the specimen, when complete, has been estimated at 4 m, and its weight at 75 kg.

Extinct taxa
Certain extinct cephalopods rivalled or even exceeded the size of the largest living species. In particular, the subclass Ammonoidea is known to have included a considerable number of species that may be considered "giant" (defined by Stevens, 1988 as those exceeding 1 m in shell diameter). The largest confirmed ammonite, a specimen of Parapuzosia seppenradensis discovered in a German quarry in 1895, measures 1.742 m in diameter, though its living chamber is largely missing. The diameter of the complete shell has been estimated at 2.55 m, assuming the living chamber took up one-fourth of the outer whorl. Teichert & Kummel (1960:6) suggested an even larger original shell diameter of around 3.5 m for this specimen, assuming the body chamber extended for three-fourths to one full whorl. In 1971 a portion of an ammonite possibly surpassing this specimen was reportedly found in a brickyard in Bottrop, western Germany. A specimen found by Jim Rockwood, from the Late Triassic near Williston Lake, British Columbia, was said to measure more than 8 ft across, but was later determined to be a concretion. Heteromorph ammonites are known to have exceeded 1 m in length also, but since their shells were uncoiled to varying degrees, they were overall much smaller than the largest non-heteromorphs. The greatest lengths of all were reached by the orthocones of endocerid nautiloids such as Endoceras, which may have exceeded 8 m, although their maximum size is uncertain; while the largest well documented endocerid fossil is likely the 3-metre-long (3 m) shell fragment housed at the Museum of Comparative Zoology, Harvard University, there are published reports of even larger specimens. Teichert (1927) mentioned specimens up to 5 m long from the Middle Ordovician limestone of Estonia and Frey (1995:72) gave a maximum shell length of 6 m for the group. On the subject of endocerid size, nautiloid specialist Rousseau H. Flower wrote:

"They are not all large, by any means, but specimens twelve feet [12 ft] in length have been collected, and fragments of greater diameter indicate a much greater maximum length. I am not wholly inclined to discredit a report of an endoceroid found in a quarry near Watertown New York, which was measured before it was broken up and found to attain a length of 30 ft."

However, the uncoiled length of the largest ammonites far exceeds that of even these giant endocerids. Parapuzosia seppenradensis, the largest known ammonite species, had an estimated maximum unrolled shell length of around 60 ft. It was also possibly the heaviest of all known cephalopods, past or present, with an estimated live mass of 1456 kg, of which the shell would constitute 705 kg. By comparison, the largest endocerids may have weighed around 1000 kg. In terms of mass, these are the largest known invertebrates that have ever lived, though perhaps still second to the largest living cephalopods when considering tissue mass alone, since in shell-bearing species the vast majority of the living tissue is restricted to the body chamber, which occupies only a fraction of the internal shell volume. They might also be the largest—or at least longest—shell-bearing animals that have ever lived.

Cretaceous Yezo Group in Japan yields multiple taxa of large-sized cephalopod fossil remains. Oegopsid Yezoteuthis giganteus and Haboroteuthis poseidon are only known from jaw elements, which is close to size of that of giant squid. Species of Nanaimoteuthis, especially N. hikidai would be the largest known vampyromorph. Unnamed, incomplete upper jaw fossil that is approximately twice as large as that of the mature giant squid is also found there.

Misidentifications
The maximum sizes of certain cephalopod species, most notably the giant squid and giant Pacific octopus, have often been misreported and exaggerated. The literature on cephalopod size has been further muddied by the frequent misattribution of various squid specimens to the giant squid genus Architeuthis, often based solely on their large size. In the academic literature alone, such misidentifications encompass at least the oegopsid families Chiroteuthidae, Cranchiidae, Ommastrephidae, Onychoteuthidae, and Psychroteuthidae. This situation is further confused by the occasional usage of the common name 'giant squid' in reference to large squids of other genera.

Perhaps the most notable misidentification relates to a photograph taken some time before 1993 by diver H. Kubota off southern Japan. The image shows a large individual of Onykia robusta (previously known as Moroteuthis robusta), which appears to be sick or dying, alongside a diver in shallow water. A video of the same animal appeared in a Japanese made-for-television film. The image was published in the 1993 book European Seashells by Guido T. Poppe and Yoshihiro Goto, where it was identified as Architeuthis dux, the giant squid, and said to have been taken in the North Atlantic. If true, this image would represent the first known photograph of a live giant squid. In The Search for the Giant Squid (1998), Richard Ellis wrote:

"For a moment, I thought that some obscure photograph had captured the most elusive image in natural history. Fortunately for those who have devoted their lives to searching for Architeuthis, this was only an aberration, a case of mistaken identity."

It would be more than a decade before the true first photographs of a live giant squid in the wild were taken on 30 September 2004 by Tsunemi Kubodera and Kyoichi Mori. Kubodera and his team subsequently became the first to film a live adult giant squid on 4 December 2006, and the first to film a live giant squid in its natural habitat in July 2012. These milestones were preceded by the first footage of a live (paralarval) giant squid in 2001, and the first image of a live adult giant squid on 15 January 2002. Since then, live giant squid have been photographed and filmed on a number of occasions.

Giant squid
Reports of giant squid (Architeuthis dux) specimens reaching or even exceeding 18 m in total length are widespread, but no animals approaching this size have been scientifically documented in recent times. This is despite there being hundreds of specimens available for study (c. 700 documented as of 2015, of which c.  460 measured in some way ), including numerous recent examples, such as the 57 specimens recorded from Japanese waters over a 15-month period in 2014–2015. It is now thought likely that such lengths were achieved by great lengthening of the two long feeding tentacles, analogous to stretching elastic bands, or resulted from inadequate measurement methods such as pacing.

On the subject of the oft-cited maximum size of 18 metres—or 60 feet—Dery (2013) quoted giant squid experts Steve O'Shea and Clyde Roper:

If this figure [45 ft or 45 ft] seems a little short of the Brobdingnagian claims made for Architeuthis in most pop-science stories about the animal, that's probably because virtually every general-interest article dutifully repeats the magic number of 60 feet.

Steve O'Shea deplores the media's perpetuation of what he believes to be a credulity-straining exaggeration, based on the 19th-century biologist Thomas Kirk's eyeball estimate of a specimen's length. In a comment on the final draft of this article, O'Shea wrote, "Kirk paced it, in his own words, for he had no ruler/measure handy, and I believe this misrepresentation has been perpetuated enough; if they were foot-on-foot, as in heel directly to toe, I would accept 57 (or 58, whatever the precise figure was), but I think perpetuating this as fact any longer is doing a disservice to science."

Roper, in his comments on the final draft of this article, was even more conservative, writing, "there are no confirmed records of giant squid longer than about 45 feet [45 ft] total length. Most are in the 25–35-foot [25 –] range. I have examined specimens in museums and laboratories around the world—perhaps a 100 or so—and I believe the 60-foot number comes from fear, fantasy, and pulling the highly elastic tentacles out to the near breaking point when they are measured on the shore or on deck."

Largest reported animals
Paxton (2016a) investigated the maximum size of Architeuthis by performing a statistical analysis using data from literature records of giant squid specimens. He selected what he regarded as the largest size records for each of mantle length (ML), standard length (SL), and total length (TL). Paxton's study has been criticised by giant squid experts, who have called into question the reliability of some of the selected literature records.

For mantle length, Paxton (2016a:83) considered the 11 ft reported by Dell (1952:98) as the "longest measured", though "more reliably" the 9 ft ML specimen from Lyall Bay, New Zealand, documented by Kirk (1880:312). Paxton added: "A 4.5 m specimen from Mauritius is often mistakenly cited but consultation of the primary paper (Staub, 1993) reveals an ill-defined length which is clearly not ML." The greatest measured ML of a giant squid recovered from a sperm whale is either the 2.4 m reported by Keil (1963:320) (though Paxton writes: "the account is confused and the 2.4 m figure probably refers to the head and ML combined") or the 6 ft of a specimen that had been swallowed whole off the Azores, detailed by Clarke (1955:589) and Clarke (1956:257). The "longest visually estimated" ML, according to Paxton, is the c. 100 ft of a specimen apparently observed in the North Atlantic off Portugal, attributed to a personal communication with T. Lipington. A more modest 4 m ML is also given, based on a sighting in the Indian Ocean sourced to the TV documentary of Lynch (2013).

For standard length (excluding the tentacles), Paxton (2016a:83) cited the 31 ft of the "Three Arms specimen" documented by Verrill (1880a:192) as the "longest measured". Among specimens recovered from sperm whales, the longest "definitely measured" SL is the 16 ft reported by Clarke (1956:257) and the longest "visually estimated" SL is the c. 9 m attributed to a photograph of a sperm whale with giant squid remains in its jaws, though Paxton conceded that it is "[n]ot clear how much/what portion of body was eaten". For the "longest visually estimated", more extreme supposed SLs of c. 175 ft and c.  100 ft are cited to Starkey (1963) and Ellis (1998a:246), respectively (the latter an eyewitness account by Dennis Braun). Paxton treated these last two size estimates as SLs as opposed to TLs because "squid do not generally leave their tentacles exposed except when grabbing prey and this appears to be the case for Architeuthis".

For total length, Paxton (2016a:83) considered three records as candidates for the "longest measured": the 19 m specimen of Berzin (1972:199), the 55 ft specimen described by Kirk (1888) as Architeuthis longimanus—a strangely proportioned animal that has been much commented on—and the 55 ft "Thimble Tickle specimen" reported by Verrill (1880a:191), which is often cited as the largest giant squid ever recorded. Of the last one, Paxton wrote: "Sometimes mistakenly cited as 17.37 m but the source is clear that it is 55 ft long." The first two records, particularly that of Berzin, are more questionable, as Paxton explained:

"The accuracy of the two longest measured TLs of 19 and 16.81 m from a specimen found in the gut of a sperm whale from the Indian Ocean and from the specimen from New Zealand in 1887, respectively, should also be questioned but again are certainly not impossible. The New Zealand specimen (named Architeuthis longimanus Kirk, 1888) clearly has the largest ratio of TL to ML ever known in Architeuthis [...] which led [ O'Shea & Bolstad, 2008 ] to suggest that the length was paced out and/or there was extensive post-mortem stretching. However, a re-reading of the original paper suggests that the specimen, although initially paced out, was actually measured, nevertheless the TL is at the edge of the 99.9% prediction interval range [...] and so it was certainly an unusual specimen. Berzin's (1972) Indian Ocean claim is suspect because of the roundness of the figure, the lack of detailed measurements and because in an associated photo, the mantle (whose length was not given) does not look very large compared to the men in the image. Consequently the measurement, if accurate, would represent another animal with very long tentacles."



However, as Paxton (2016a:86) pointed out, the genetic analysis of Winkelmann et al. (2013)—which concluded that there is likely a single, globally-distributed species of Architeuthis—did not encompass these two specimens, and it is therefore possible that there exists a second, as yet unsampled, giant squid species with proportionately longer tentacles.

The 19 m total length of the Berzin specimen was later confirmed to be erroneous; according to Valentin Yukhov, who was involved in the specimen's discovery, it should have read 9 m. The misprint was reproduced in the English translation published the following year and was later propagated in a number of papers on giant squid. With the Berzin specimen not being as large as originally reported, the longest giant squid recovered from a sperm whale is the 34 ft TL individual recorded by Clarke (1956:257) (this specimen also has the longest confirmed ML and SL of any giant squid from a sperm whale). Paxton considered the "longest visually estimated" TL to be the 60 ft published by Murray (1874:121), from an eyewitness account by fisherman Theophilus Picot, who claimed to have struck the floating animal from his boat, causing it to attack. Picot managed to hack off one of its tentacles, which was subsequently examined by a number of authors.

Perhaps the largest of all recorded giant squid specimens was the one found floating at the surface off Saint-Gilles, Réunion, on 4 March 2016. Although due to its great size the specimen could not be retrieved in its entirety, the head and arm crown were saved. Crucially, this meant the beak could be measured to estimate the mantle length and total length of the specimen. Using different allometric scaling equations, the lower rostral length of the beak, at 19.74 mm, gave an estimated dorsal mantle length of 215.3 – and this, in turn, was used to estimate the total length at 11.025 –.

Supposed sucker scars
More extreme and outlandish giant squid size claims—belonging firmly in the realm of cryptozoology—have appeared in the works of authors such as Bernard Heuvelmans, Willy Ley, and Ivan T. Sanderson. The existence of these gargantuan squids is often supported by reference to the giant circular scars sometimes found on sperm whales, which are assumed to have been inflicted by the suckers of struggling giant squid. Sometimes these claims are accompanied by extrapolations of body size based on the isometric upscaling of a "typical" giant squid. However, such scars are not necessarily of squid origin and may instead represent fungal growths or bite marks, with sea lampreys (Petromyzon marinus) being one possible source. Even in the case of genuine giant squid sucker marks it is possible that subsequent skin growth has enlarged them well beyond their original dimensions. Nevertheless, claims of enormous sucker scars are widespread in the literature. Richard Ellis collected some of "the more egregious examples" in his book, The Search for the Giant Squid. These include the claim of Dozier (1976) that "an ordinary giant squid of 50 feet [50 ft] leaves teeth-ringed sucker marks measuring between three and four inches [3 - 4 in] across on a whale, but sperm whales have been captured with tentacle marks 18 inches [18 in] across." L. Harrison Matthews's monographic treatment of the sperm whale, published in 1938, includes the following: "Nearly all male Sperm whales carry scars caused by the suckers and claws of large squids, scars caused by suckers up to 10 cm. [10 cm] in diameter being common. The claw marks take the form of scratches 2–3 m. [2 –] in length, and appear to be of more frequent occurrence than sucker marks". Ellis (1998a:142) wrote that this 10 cm figure is "so much larger than any other recorded sucker dimensions that one suspects some sort of error, either in measuring or in transcription."

The subject was covered in some detail by Wood (1982:192):

Measurements of 90 ft, 130 ft and even 200 ft have been conjectured for giant squids from the size of sucker marks found on the skins of captured sperm whales, but it is dangerous to place too much reliance on this evidence. Verrill says the largest suckers on the tentacles of a 32 ft long specimen measured 1$1/4$ in [1.25 in] in diameter, and those on a 52-footer [52 ft] about 2 in. Daniel (1925), however, examined sucker marks on the head of one cachalot which measured 3$1/2$ in [3.5 in] across, and others measuring up to 5 in in diameter have been found on the skins of sperm whales captured in the North Atlantic. Ivan Sanderson (1956) goes even further and claims that sucker marks over 18 in have been found on the heads of cachalots, but he does not explain how the poor whales managed to escape from the clutches of such colossi!

The general consensus of opinion is that exceptionally large sucker marks, i.e. over 2 in in diameter, are old scars that have increased in size as the sperm whale grew. Perhaps the most extreme published claim, ridiculed by Ellis (1998a:142), appeared in Willy Ley's 1959 book, Exotic Zoology: "Toothed whales, vomiting in death struggle, have shown evidence of still larger kraken; in one case a 6-foot [6 ft] piece of tentacle, with a diameter of 2 feet [2 ft; emphasis in original], has been claimed. Another claim goes for marks on the skin of such a whale, looking like the mark of a sucking disk over 2 feet [2 ft] in diameter".

Marine biologist Frederick Aldrich, who personally examined more than a dozen giant squid specimens, wrote that his largest specimen from Newfoundland bore tentacular suckers "approximately two inches [2 in] in diameter" but that "[s]uckers and their toothed armament of over twelve inches [12 in] in diameter have been found in the stomachs of sperm whale as indigestible wastes". This led him to entertain the idea of giant squid over 150 ft long, and even to suggest a binomial name for this super-sized species, were it ever to be discovered: Architeuthis halpertius.

By comparison, giant squid suckers normally reach a maximum diameter of only a few centimetres. Based on a detailed examination of a number of large specimens from New Zealand waters, Förch (1998:55) wrote that "[t]he largest suckers [...] on the sessile arms are a very constant 21 – in external diameter". In giant squid the largest suckers of all are found on the central portion of the tentacular club, called the manus, and among the specimens examined by Förch (1998:53) these reached a maximum diameter of 28 –. Clarke (1980) wrote: "I have not yet seen conclusive evidence to suggest that sucker scars are larger than 3.7 cm across". According to Roper & Boss (1982:97), the largest suckers of the tentacular clubs reach 5.2 cm in diameter.

Mass estimates
It is now accepted that the giant squid has a maximum mass of several hundred kilograms, but the literature is rife with claims of much greater weights. Clarke (1966), for example, put the mass of the largest giant squid specimens at around 1 tonne. Similarly, Richard Ellis wrote: "Where [giant] squid carcasses have actually been weighed, it appears that the longest ones—in the 50-foot [50 ft] range, for example—weigh about a ton [1 short ton]." Much greater estimates of giant squid mass can be found in, for example, Natural History of Marine Animals by MacGinitie & MacGinitie (1949): "two arms of Architeuthis that were 42 ft long were found, and if one reconstructed a body [...] the squid to which these arms belonged was 4.6 ft in diameter and 24 ft long, with an overall measurement of 66 ft. It would have weighed about 42$1/2$ tons [42.5 short ton]." They added that a 55 ft specimen, such as the one reported from Thimble Tickle, "would have weighed 29$1/4$ or 30 tons [29.25 or] including the tentacles—a truly noble animal, being a little more than one-fifth the weight of the largest whale and larger than the whale sharks and basking sharks, the largest of all fishes". Ellis characterised these estimates as "unfounded exaggerations". In the revised edition of Natural History of Marine Animals, published in 1968, the authors reduced their estimate to less than 8 tonnes.

Bernard Heuvelmans believed that "there must be Architeuthis weighing more than 5 tons, and some even larger ones which must weigh between 2 and 27 tons, the normal weight being around 8 tons. There are good reasons to believe that there may even exist specimens twice as long as that of Thimble Tickle, which, depending upon their girth, might have weighed between 16 and 216 tons, but more likely around 64 tons." Ellis, who considered these estimates "utterly ridiculous", wrote:

"Heuvelmans commits a fundamental error in calculating the weight of some of these monsters when he writes that 'the density of living creatures is only slightly higher than that of water ... a decimetre of living flesh weighs about as much as a litre of water.' That may be true for some other living creatures, but the flesh of Architeuthis, saturated with ammonium chloride, is lighter than water, and the giant squid is neutrally buoyant. (This is believed to be the reason that dead or dying squid are found floating at the surface or washed up on the beach.) His assumption, therefore, that the 55-foot-long [55 ft] Thimble Tickle squid would have 'probably weighed near 24 tons' is patently erroneous."

On the subject of the Thimble Tickle specimen's mass, Wood (1982:190) referred to the work of Soviet zoologist and writer Igor Akimushkin:

"According to Dr Igor Akimushkin (1965), the Russian teuthologist, a 12 m long giant squid will weigh 1 tonne [1 tonne] if the head, mantle and arms combined make up half the total length. Since there is a cubic relationship between the linear dimensions of Architeuthis and its volume or weight, this means the Thimble Tickle monster must have scaled about 2.8 tonnes [2.8 tonne] (i.e. the weight of a large bull hippopotamus), although 2 tonnes [2 tonne] is probably a more realistic figure."

Giant Pacific octopus
The maximum size of the giant Pacific octopus (Enteroctopus dofleini) has long been a source of debate in the scientific community, with dubious reports of specimens weighing hundreds of kilograms.

Largest reported animals
In 1885, reporting on the longest octopus specimen reliably recorded up to that point, renowned malacologist William Healey Dall wrote:

"In 1874 I speared an octopus in the harbor of Iliuliuk, Unalashka, which was afterward hung, by a cord tied around the body immediately behind the arms, to one of the stern davits of the coast survey vessel under my command. As soon as the animal died and the muscles relaxed, I noticed that the tips of the longer tentacles just touched the water. On measuring the distance with a cord, I found it to be sixteen feet [16 ft], giving the creature a spread from tip to tip of the longest pair of arms, of not less than thirty-two feet [32 ft]. The arms toward the tips were all exceedingly slender, but rather stout toward the body, which was somewhat over a foot [1 ft] long. The largest suckers were two and a half inches [2.5 in] in diameter; the whole creature nearly filled a large washtub. Parts of this specimen are now in the U. S. national museum."

In an article for the National Marine Fisheries Service summarising knowledge on the giant Pacific octopus, High (1976:17–18) wrote:

Several octopuses in excess of 100 pounds [100 lb] have been encountered and captured. Much larger ones have been reported but, like the Loch Ness Monster, these usually elude the careful photographer or scientist. Most octopuses weigh less than 70 pounds [70 lb] with a stretched length of 15 feet [15 ft] or less. Overall length between arms is not a suitable measure because of the animal's unusual elasticity.

In the late 1950s, I interviewed a Canadian commercial diver, Jock MacLean of Prince Rupert, B.C. He reported capturing an immense creature weighing 600 pounds [600 lb] and measuring 32 feet [32 ft] from arm tip to top. MacLean's photographs, unfortunately, were of poor quality. Smaller animals, to 400 pounds [400 lb], were occasionally taken in his commercial octopus fishing endeavor.

Hochberg & Fields (1980:436) referenced the same specimen, writing: "the largest specimen on record with a total arm spread of 9.6 m [9.6 m] and a weight of 272 kg [272 kg]". These figures are only estimates, however, as—contrary to the above quotation from High (1976:17–18)—it appears that this specimen was never collected and measured. Murray Newman, director of the Vancouver Aquarium for 37 years, quoted Jock MacLean in his 1994 memoir, Life in a Fishbowl: "Next year [1957] in the same place, I saw one, maybe thirty-two feet [32 ft] across and six hundred pounds [600 lb]. Didn't go for her, though, no place to keep her!" Nevertheless, the misleadingly precise metric conversion of 272 kg (for 600 lb) and the imprecise conversion of 9.6 m (for 32 ft; naively employing a conversion factor of 0.3 instead of 0.3048) gained wide acceptance as the maximum recorded dimensions of the giant Pacific octopus, and have been much repeated.

Jock MacLean is also reported to have captured a 198 kg animal with an arm span of 8.5 m near Port Hardy, British Columbia, in March 1956. Another giant specimen was caught off Santa Barbara, California, in 1945. Its weight was recorded as 182.3 kg and the surviving photograph makes it possible to estimate its total length at more than 3 m and arm span at 6 –. In a book dedicated to the giant Pacific octopus, Cosgrove & McDaniel (2009:72) summarised knowledge on the species's maximum size as follows:

"The specimen William Dall speared in 1885 [sic] at Iliuliuk had the largest radial span of any giant Pacific octopus ever measured. Jock MacLean's 1956 Port Hardy behemoth was the biggest ever weighed. The Santa Barbara specimen photographed in 1945 was the second heaviest. It would appear that octopuses weighing as much as 272 kg and with radial spans of over 9 m are within the realm of possibility, but have never actually been documented by both measuring and weighing."

Possible diminution in size
No specimens approaching these extreme sizes have been reported since the middle of the 20th century. This lack of giant individuals is corroborated by commercial octopus fishers; none of those interviewed by Cosgrove & McDaniel (2009) had caught a single animal weighing more than 57 kg in the previous 20 years, among many thousands harvested over that period. Octopus specialist Roland Anderson, a biologist with the Seattle Aquarium for more than 30 years, had long sought, unsuccessfully, to find a giant Pacific octopus weighing more than 100 lb. In an attempt to raise a truly enormous specimen, Anderson fed a number of captive males ad libitum. The heaviest animal (nicknamed 'Big') attained a peak weight of 43 kg and its largest suckers measured 7.9 cm in diameter. Anderson suggested the species might now be maturing at a smaller size as a result of toxicant bioaccumulation, which could explain the lack of truly gigantic specimens in recent times. In particular, high concentrations of heavy metals and PCBs have been identified in the digestive glands of wild giant Pacific octopuses, likely originating from their preferred prey, the red rock crab (Cancer productus). A preliminary study found that aquarium animals fed equal quantities of raw sea food and live C. productus (caught locally in Elliott Bay) matured at a smaller size, reached a lower maximum weight (27 kg mean), and had higher concentrations of most heavy metals, than those fed solely on raw sea food (36 kg mean, including the aforementioned 43 kg specimen).

Largest species by measure
Cephalopod size can be quantified in various ways. Some of the most common size measures are covered below. The following four tables list only extant species; extinct taxa are treated separately at the end.

Mantle length


The list of largest cephalopods by mantle length is dominated by squids, with more than twenty species exceeding the largest-bodied octopuses and cuttlefish. The largest of all is the colossal squid (Mesonychoteuthis hamiltoni) with an estimated maximum mantle length of 3 m (Roper & Jereb, 2010c:173). Even greater mantle lengths have historically been reported for the giant squid (Architeuthis dux), but these have been discredited (see O'Shea & Bolstad, 2008).

Total length
The longest scientifically documented specimens belong to the giant squid, with a maximum total length of 14 – (Roper & Shea, 2013:114). Despite its proportionally shorter tentacles, the colossal squid may rival the giant squid in total length, but the species's size limits are uncertain because only a handful of mature specimens have been recorded.

Mass
The heaviest known cephalopod, and the largest living invertebrate, is the colossal squid. The largest recorded specimen of this species, caught in the Ross Sea in 2007, weighed 495 kg. However, its beak is not the largest known from this species; even bigger colossal squid beaks have been recovered from the stomachs of sperm whales, indicating that this species can grow larger still.

Shell diameter
Nautiluses are the only extant cephalopods with a true external shell; in other groups the shell has been internalised or lost completely. Internal shells include the cuttlebones of cuttlefish, the gladii of squids and the vampire squid, the winged shells of cirrate octopods, and the spiral shells of Spirula. Additionally, females of the octopus genus Argonauta secrete a specialised paper-thin eggcase in which they reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal (see Finn, 2013).

Cephalopod shell diameter is of interest to teuthologists and conchologists alike. The Registry of World Record Size Shells, the most comprehensive publication on maximum shell size in marine molluscs, specifies that specimens "should be measured with vernier type calipers and should reflect the greatest measurable dimension of the shell in any direction including any processes of hard shell material produced by the animal (i.e. spines, wings, keels, siphonal canals, etc.) and not including attachments, barnacles, coralline algae, or any other encrusting organisms" (Pisor, 2008:14). Unlike most other measures of cephalopod size, shell diameter can be determined with a high degree of precision and usually leaves little room for ambiguity. For this reason it is usually recorded to the nearest one-tenth of a millimetre (0.1 mm), as is standard in conchology.

When the Registry of World Record Size Shells changed ownership in 2008 it was launched as an online database in addition to its print publication. Subsequent rule changes meant that all records required photographic verification. Over time, older records for which photographic evidence could not be obtained were removed from the database. As a result, some records from older editions of the registry actually exceed the size of the current official record holders, sometimes by considerable margins. Where this has occurred, the largest recorded size across all editions is shown first and any discrepancies or competing records are noted thereafter. Where a reliable literature record surpasses all specimens ever included in the registry, this is given instead and the registry record(s) noted thereafter. Pisor (2008) was the fifth and final print edition of the registry published prior to the rule change, and Barbier et al. (N.d.) is the current, continuously updated online database. The registry only covers the shells of nautiluses and Spirula and the eggcases of Argonauta.

Extinct taxa




Eyes
The giant and colossal squids have the largest recorded eyes of any living animal, with a maximum diameter of at least 27 cm and a 9 cm pupil. This is three times the size of the largest fish eyes—up to 90 mm in swordfish—and more than twice the diameter of the largest whale eyes—up to 109 mm, 61 mm, and 55 mm in blue, humpback, and sperm whales, respectively—which are the largest among vertebrates. A large colossal squid caught in 2014 and dissected at the Museum of New Zealand Te Papa Tongarewa had eyes some 35 – across. There are unconfirmed reports from the 19th century of giant squid eyes up to 40 cm across. Only the extinct ichthyosaurs are known to have approached these dimensions, with some species having eyes up to 35 cm in diameter.

Despite their size, the eyes of giant and colossal squids do not appear to be disproportionately large; they do not deviate significantly from the allometric relationship seen across other squid species. Sepiolids are noted for having exceptionally large eyes, which are much bigger relative to their mantle length than those of the giant squid; the same is true of Histioteuthis species. Gonatids and the loliginids Loligo and Lolliguncula also have proportionately somewhat larger eyes than Architeuthis. Some sources state that the vampire squid (Vampyroteuthis infernalis) has the largest eyes of any animal relative to its size, with a 15 cm specimen having eyes around 2.5 cm in diameter.



There is some debate in the scientific community as to the evolutionary reason behind the extremely large eyes of giant and colossal squids. Nilsson et al. (2012) and Nilsson et al. (2013) argue that it is an anti-predator adaptation for enhanced detection of sperm whales, with the squids picking up plankton bioluminescence triggered by moving whales, perhaps from distances exceeding 120 m. Schmitz et al. (2013a) and Schmitz et al. (2013b) contend that their eyes are so large due to a phylogenetically conserved developmental pattern that governs the relative dimensions of squids and their eyes, and that any fitness benefits their size may confer in terms of predator avoidance are the result of exaptation ("pre-adaptation").

Neurons
Squid giant axons can exceed 1 mm in diameter: 100 to 1000 times the thickness of mammalian axons. The axons of the Humboldt squid (Dosidicus gigas) are exceptional in that they can reach a diameter of as much as 1.5 mm, and those of Loligo forbesii can also exceed 1 mm. Such was the importance of Humboldt squid to electrophysiology research that when the animals migrated out of reach of Chilean fishermen in the 1970s "it led to the demise of a world-class electrophysiology laboratory" based there. Squid giant axon diameters do not necessarily correlate with overall body size; those of the giant squid (Architeuthis dux) are only 0.137 – thick.



The squid giant synapse is the largest chemical junction in nature. It lies in the stellate ganglion on each side of the midline, at the posterior wall of the squid's muscular mantle. Activation of this synapse triggers a synchronous contraction of the mantle musculature, causing the forceful ejection of a jet of water from the mantle. This water propulsion allows the squid to move rapidly through the water and, in the case of the so-called 'flying squids', even to jump through the surface of the water (breaking the air–water barrier) to escape predators. Many essential elements of how all chemical synapses function were first discovered by studying the squid giant synapse.

Photophores
Taningia danae, a very large octopoteuthid squid, possesses "lemon-sized" yellow photophores at the tips of two of its arms, which are the largest known light-emitting organs in the animal kingdom. Video footage shot in 2005 in deep water off Japan shows T. danae emitting blinding flashes of light from these photophores as it attacks its prey. A pair of muscular lids surrounds each photophore and it is the withdrawal of these lids that produces the flashes. A large individual filmed from a remote submersible off Hawaii in 2015 can clearly be seen opening the lids to reveal its photophores. It is believed that this highly manoeuvrable squid uses bright flashes to disorientate potential prey. The flashes may also serve to illuminate prey for easier capture or play a role in courtship and/or territorial displays.

Reproductive organs
Extreme penis elongation has been observed in the deep water squid Onykia ingens. When erect, the penis may be as long as the mantle, head, and arms combined. As such, deep water squids have the greatest known penis length relative to body size of all mobile animals, second in the entire animal kingdom only to certain sessile barnacles.