Cephalopod size

Hatchlings

Hatchlings of Idiosepius thailandicus, possibly the smallest extant cephalopod species at maturity, have a mantle length of around 1 mm (0.039 in) (Nabhitabhata, 1998:32). The closely related Idiosepius pygmaeus weighs only 0.00033 g (1.2×10⁻⁵ oz) upon hatching and increases in weight to 0.175 g (0.0062 oz) as it reaches maturity in 50 days (Wood & O'Dor, 2000:93). Even smaller are the hatchlings of the commercially important Illex illecebrosus, with a mass of 0.00015 g (5.3×10⁻⁶ oz) (O'Dor et al., 1986:59; Wood & O'Dor, 2000:93). Hatchlings of the giant Pacific octopus (Enteroctopus dofleini)—one of the two largest octopus species—weigh 0.0253 g (0.00089 oz) on average (Cosgrove & McDaniel, 2009:88).

At the other extreme are nautiluses, which upon hatching typically have a shell diameter of 25 mm (0.98 in) or more (depending on the species), the largest hatchling size among extant invertebrates (Grulke, 2014:105). Hatchlings of Nautilus belauensis, one of the larger species, are estimated to weigh on the order of 5.9 g (0.21 oz) and mature at around 1.2 kg (2.6 lb) after almost 4000 days, or 11 years (Wood & O'Dor, 2000:93).

Smallest adults

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 gram (0.035 oz) at maturity (Boletzky, 2003:19). Idiosepius thailandicus is perhaps the smallest of all, with females averaging 10.4 mm (0.41 in) in mantle length and males 5.9 mm (0.23 in) (Nabhitabhata, 1998:28). Average wet weights are around 0.20 g (0.0071 oz) and 0.02 g (0.00071 oz), respectively (Nabhitabhata, 1998:28).

Other tiny species include members of the various Sepiolidae genera; 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 Argonautoida is characterised by markedly dwarfed males (Boletzky, 1999:24; Boletzky, 2003:20; Norman et al., 2002:733). 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 (0.94 in) in total length and weighing a mere 0.25 g (0.0088 oz). By comparison, the large females of this species reach total lengths of 2 m (6.6 ft) and probably some 10 kg (22 lb) 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 (Norman et al., 2002:733; Fairbairn, 2007:3). 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 30 cm (12 in) (Norman et al., 2002:733).

Extinct taxa

Numerous species of so-called micromorphic ammonites are known (see Kennedy & Cobban, 1990).

Maximum size

The largest living cephalopods in terms of mantle length, total length and mass are all squid, of which the largest species by at least two of these measures is the colossal squid, Mesonychoteuthis hamiltoni. Reaching an estimated 3 m (9.8 ft) in mantle length and 10 m (33 ft) in total length, and weighing as much as 495 kg (1,091 lb), this species is also the largest of all extant invertebrates (Rosa & Seibel, 2010; McClain et al., 2015). The only other squid that approaches these dimensions is the giant squid of the genus Architeuthis, with females up to 275 kg (606 lb), 2.4 m (7.9 ft) in mantle length, and possibly as much as 15 m (49 ft) 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 70 kg (150 lb), and the former has a maximum total length of more than 6 m (20 ft). Members of the other cephalopod groups are rather small by comparison, although the largest cuttlefish can exceed 10 kg (22 lb) in weight and 50 cm (1.6 ft) 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, and vampyromorphids.

The maximum sizes of certain cephalopod species, most notably the giant squid, have often been misreported and exaggerated. Reports of giant squid specimens reaching or even exceeding 18 m (59 ft) in 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; Paxton, 2016), including numerous recent examples, such as the 57 specimens recorded from Japanese waters over a 15-month period in 2014–2015 (Kubodera et al., 2016). 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 (O'Shea & Bolstad, 2008; Roper & Shea, 2013:113).

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 (see Sanderson, 1956; Heuvelmans, 1958; Ley, 1959). 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 scaling 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 (Wood, 1982:193). 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 (Wood, 1982:192).

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 (see Ellis, 1998; Salcedo-Vargas, 1999; Glaubrecht & Salcedo-Vargas, 2004). This situation is further confused by the occasional usage of the common name 'giant squid' in reference to large squid of other genera (see Mitsukuri & Ikeda, 1895; Meek & Goddard, 1926; Clarke & Robson, 1929; Rees, 1950; Nesis, 1970).

Debate has also surrounded the maximum reported dimensions of some other species, including the giant Pacific octopus (Enteroctopus dofleini), with dubious reports of specimens weighing hundreds of kilograms. The large size of this species made it the focus of octopus wrestling championships, which reached the height of their popularity on the West Coast of the United States in the 1960s (see High, 1976:17; Norman, 2000:217). In contrast to these wholly soft-bodied cephalopods, 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 and body weight estimation (see Clarke, 1962; Wolff, 1981; Wolff, 1984; Gröger et al., 2000), 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.

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

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"; it lies posterior to the head and encloses 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. 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; Ellis, 1998:106; Glaubrecht & Salcedo-Vargas, 2004:62). 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 (Roper & Voss, 1983:55).

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 (9.8 ft) (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

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; see Arkhipkin et al., 2015) to the apex of the longest limb (Roper & Voss, 1983:58). 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 (Roper & Young, 1972:205).

Total length is not to be confused with arm span, also known as radial span, 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).

The longest scientifically documented specimens belong to the giant squid, with a maximum total length of 14–15 m (46–49 ft) (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

Cephalopod mass 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).

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 (1,091 lb). 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 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 (0.0039 in), 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.

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 (3.3 ft) in shell diameter). 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 Cameroceras and Endoceras, which may have exceeded 8 m (26 ft), although their maximum size is uncertain. 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 18 m (60 ft). It was also possibly the heaviest of all known cephalopods, past or present, with an estimated live mass of 1,456 kg (3,210 lb) (Landois, 1898:27). By comparison, the largest endocerids may have weighed around 1,000 kg (2,200 lb) (Teichert & Kummel, 1960:6). In terms of mass, these are the largest known invertebrates that have ever lived (Grulke, 2014:124), though perhaps still second to the largest living cephalopods when considering tissue mass alone (Vermeij, 2016). They might also be the largest—or at least longest—shell-bearing animals that have ever lived (Vermeij, 2016).

Eyes

The giant and colossal squids have the largest recorded eyes of any living animal, with a maximum diameter of at least 27 cm (11 in) and a 9 cm (3.5 in) pupil (Nilsson et al., 2012:683). This is three times the size of the largest fish eyes—up to 90 mm (3.5 in) in swordfish—and more than twice the diameter of the largest whale eyes—up to 109 mm (4.3 in), 61 mm (2.4 in), and 55 mm (2.2 in) in blue, humpback, and sperm whales, respectively—which are the largest among vertebrates (Nilsson et al., 2012:683). A large colossal squid caught in 2014 and dissected at the Museum of New Zealand Te Papa Tongarewa reportedly had eyes 35 cm (14 in) across (Farquhar, 2014).

Only the extinct ichthyosaurs are known to have approached these dimensions, with some species having eyes up to 35 cm (14 in) in diameter (Nilsson et al., 2012:687). 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 (Schmitz et al., 2013:45). Many sources state that the vampire squid (Vampyroteuthis infernalis) has the largest eyes of any animal relative to its size, with a 15 cm (5.9 in) specimen having eyes around 2.5 cm (0.98 in) in diameter (Ellis, 1996:177; though see Young et al., 2015).

Neurons

Squid giant axons can exceed 1 mm (0.039 in) 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 (0.059 in), and those of Loligo forbesii can also exceed 1 mm (Adelman & Gilbert, 1990:102). 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 (Scully, 2008). Squid giant axon diameters do not necessarily correlate with overall body size; those of the giant squid (Architeuthis dux) are only 0.137–0.21 mm (0.0054–0.0083 in) thick (Adelman & Gilbert, 1990:102).

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 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 (see Llinás, 1999).

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 (Ellis, 1998:149; Barrat, 2015). 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 (see Kubodera et al., 2006). 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 (see Barrat, 2015). 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 (Kubodera et al., 2006:1033).

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 (Arkhipkin & Laptikhovsky, 2010:299; Walker, 2010). 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 (Arkhipkin & Laptikhovsky, 2010:300).