Cephalopod Size

Cephalopods vary enormously in size. The smallest are only about long and weigh less than at maturity, while the largest—the giant and colossal squids—can exceed in length and weigh close to half a 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. The giant and colossal squids, for example, have the largest known eyes among living animals. Nilsson ''et al.'', 2012:683

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, Teichert & Kummel, 1960:6 though perhaps still second to the largest living cephalopods when considering tissue mass alone. Vermeij, 2016

Giant cephalopods have fascinated humankind since time immemorial. 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.

Size, and particularly maximum size, continues to be one of the most interesting aspects of cephalopod science to both the general public and researchers in the field. This is evidenced by the regular coverage given to the giant squid—and more recently, the colossal squid—in both the popular press and academic literature. 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.

Cephalopods vastly larger than either giant or colossal squids have been postulated at various times. Perhaps the most notable of these is the so-called St. Augustine Monster, a large carcass weighing several tonnes that washed ashore on the United States coast near St. Augustine, Florida, in 1896. Zoologist Addison Emery Verrill of Yale, at the time the country's foremost authority on cephalopods, was initially convinced that it represented a previously unknown species of gigantic octopus, and even proposed for it the scientific name ''Octopus giganteus''. However, having received tissue samples he quickly retracted his original opinion, identifying it instead as the remains of a whale. Nevertheless, the possible existence of such a gargantuan octopus was taken seriously well into the 20th century, until 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.

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 Sea'' 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.

Size in teuthology

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. Roper & Voss, 1983:58

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. O'Shea & Bolstad, 2008 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).

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.

Early life stages

Hatchlings

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

At the other extreme are nautiluses, which upon hatching typically have a shell diameter of 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 and mature at around 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 at maturity. '' Idiosepius thailandicus'' is perhaps the smallest of all, with females averaging in mantle length and males . Average wet weights are around , 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 in total length and weighing a mere . By comparison, the large females of this species reach total lengths of and probably some 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 (and possibly the largest octopuses of any kind), but the males are also much larger, at up to .

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 in shell diameter.

Maximum size

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 in mantle length and in total length, and weighing as much as , 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 , in mantle length, and possibly as much as in total length, making it likely the longest of all cephalopods. McClain ''et al.'', 2015 The two largest octopus species—'' Enteroctopus dofleini'' and '' Haliphron atlanticus''—can both exceed , and the former has a maximum total length of more than . Cirrate (finned) octopods can also reach a large size, with the largest captured specimen likely being a ''Cirrothauma magna'' of total length and mantle length, though observations from submersibles suggest that members of this group can exceed in total length.

Members of the other cephalopod groups are substantially smaller, although the largest cuttlefish can exceed in weight and 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 , its mantle length at , and its total length at . Once completely thawed the specimen was found to weigh , but to measure only in mantle length and in total length. nonymous 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 across and it had a mantle width of . The arms ranged in length from , while the two tentacles were around 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 and weighed , nonymous where as the submature female from 2003 had a LRL of . By comparison, the largest known colossal squid beak from a sperm whale stomach measured 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 . However, the scaling relationship for this species shows considerable latitude, as demonstrated by a beak of LRL extracted from an animal weighing only .

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. Paxton, 2016a This number has since increased substantially, with 57 specimens recorded from Japanese waters during an exceptional 15-month period between 2014 and 2015. Kubodera ''et al.'', 2016

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 and a "rarely encountered maximum length" of . Of the nearly 100 specimens examined by Clyde Roper, the largest was " long". O'Shea & Bolstad (2008) give a maximum total length of 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 . O'Shea, 2003a Older records of or more were likely exaggerated by stretching of the long feeding tentacles or resulted from inadequate measurement methods such as pacing. O'Shea & Bolstad, 2008; Roper & Shea, 2013:113 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 , 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". Bittel, 2016

Including the head and arms but excluding the tentacles (standard length, SL), the species very rarely exceeds according to O'Shea & Bolstad (2008). Paxton (2016a) considers to be the greatest reliably measured SL, based on a specimen reported by Verrill (1880a:192), and considers specimens of SL or more to be "very probable", but these conclusions have been criticised by giant squid experts. Greshko, 2016 O'Shea (2003a) put the maximum weight of female giant squid at , 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 , though heavier specimens have occasionally been reported (such as a specimen and a specimen). Roper & Jereb (2010a:121) give a maximum weight of up to , and "possibly greater". Discredited weights of as much as a 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 , but according to Roper & Jereb (2010h:266) this figure is wrong and stems from a typographical error, the correct mass being . Roper & Vecchione (1993) were however consistent in their use of the 61.4 kg figure. Another similarly large specimen—a female weighing —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 ; 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 . Some older records exceed this, such as the ML reported by Verrill (1876:237) from a specimen with a total length of (excluding the ends of the tentacles, which had been destroyed). Nesis (1987:192) likewise gave a maximum mantle length of , but Roper & Jereb (2010i:364) wrote that "this old record might be in error", with the species commonly growing to ML. Glaubrecht & Salcedo-Vargas (2004:66) provided a maximum total length of . Literature sources give a maximum weight of . 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 , if not . The largest animals are found off the western coast of South America; northern populations reach ML, and in general ML is more typical for the species. Roper ''et al.'', 2010b:301 Southern populations may have a total length approaching , and possibly up to . Again, specimens from the northern hemisphere are much smaller, with those off the Californian coast reaching total lengths of less than . The Humboldt squid commonly attains a weight of around and can reach a maximum of . 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). Cassell, 2005

''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

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