A standard squid has:
Two fins
A mantle
A head
8 arms and two tentacles, each endowed with hooks and/or suckers and sucker rings

A standard octopus has:
A mantle
A head
8 arms endowed with one or two rows of suckers (but never hooks or sucker rings); they have no tentacles

Some squid naturally lose the tentacles in post-larval stages, so that the adult possesses 8 arms only; some squid can have more than 2 fins.

Some octopuses (nine species in New Zealand waters) possess two well-developed fins. These nine species also have a well-developed pen (like a cuttlebone) in their mantle (primitive condition).

The major distinction between squid and octopus is that the suckers of squid are armed with hooks or sucker rings (or a combination of the two), while octopus have simple suckers without secondary armature

Size Popular press concentrates on the issue of size. There are several ways in which the size of a squid is reported (and misreported or exaggerated). Measured total length relaxed (post mortem), total length when the animal is alive (and outsretched), both including tentacles; estimated length (the one that got away); and weight.

Architeuthis dux
Architeuthis is frequently reported to attain a lotal length of 60 feet. The largest specimen known washed ashore on a New Zealand beach, Lyall Bay (Wellington) in the winter of 1887. It was a female and "in all ways smaller than any of the hitherto-described New Zealand species" (Kirk 1887); it measured 55 feet 2 inches in total length, exaggerated by great lengthening (stretching like rubber bands) of the very slight tentacular arms; its mantle length was 71 inches (1.8 m). A comparable-sized female (ML 1.8 m) measured post mortem and relaxed (by modern standards) would have a total length of ~ 32 feet.

Mantle length (as opposed to total length) is the standard measure in cephalopods. Architeuthis is not known to attain a mantle length in excess of 2.25 m. Standard Length (SL) is the length of a squid excluding the tentacles; in Architeuthis this measure very rarely exceeds 5 m. The rest of the animal's length, to a total length of 13 m, is made up of the two long tentacles. Of 105 specimens that we have examined, none has exceeded these figures (Fig. 7).

Architeuthis beaks recovered from the stomachs of sperm whales are smaller or equivalent to Architeuthis beaks recovered from specimens trawled in New Zealand waters. That is to say that no evidence exists for recognising larger specimens than those known from New Zealand. Moreover, it is most likely that a single species, Architeuthis dux, exists worldwide, so 'larger species' of Architeuthis do not occur. To perpetuate myths of more than one species of Architeuthis (up to 20 species have been reported), lengths of 60 feet and weights of up to a ton is a disservice to science.

Mesonychoteuthis hamiltoni
No mature Mesonychoteuthis is known. Based on the size of beaks recovered from sperm whale stomach contents it is estimated that it attains a mantle length of 2—4 m, which would render it considerably larger than Architeuthis (Fig. 8). The specimen that we will examine in Wellington will first be sexed (female squid often attain a larger size than male squid), then its state of reproductive maturity will be appraised. Both sex, an indication of whether a specimen has mated or not, and the stage of maturity provide insights into the realistic maximum size of a species.

The beaks will be extracted, measured and compared with the largest beaks of this species we have available in New Zealand, recovered from sperm whale stomach contents (Fig. 9). Should the beaks from sperm whale stomach contents be appreciably larger than those from the present carcass, then we can say that the animal does attain a considerably larger size. If not then it would appear that the reputed size that this animal attains has again been exaggerated, as it has for over a century with Architeuthis.

Unique characters
The tentacle club, the expanded distal portion of the tentacle, is endowed with two rows of swiveling hooks (Fig. 3)

The beaks are the largest known of any squid (Fig. 9), exceeding those of Architeuthis in size and robustness

The eyes are probably the largest in the animal kingdom (even larger than those of Architeuthis)

The relatively short arms are endowed with a combination of hooks and suckers.

Number of adult specimens reported: 6 (all but one recovered from sperm whale stomachs). Juveniles are not uncommon from surface waters to ~ 1000 m depth.

This species was first described on the basis of two arm (brachial) crowns recovered from sperm whale stomach contents (Robson 1925). Subsequently few specimens have been collected. A sub-mature female of mantle length (ML) 1.25 m is described by Voss (1980), and reference made to two other partial specimens (brachial crowns) in the collections of the United States National Museum, Smithsonian Institution. A further specimen, ML 1.05 m, was trawled in an opening-closing net (RMT8) at a depth of 2000—2200 m (Rodhouse & Clarke 1985). None, to our knowledge, have been reported subsequently.

Six specimens reported; no comprehensive description of the mature adult, male or female, is known.

Estimated mantle length: 2—4m; total length to 30 feet. Based on a combination of beak dimension, and both factory ship and photographic observations, Clarke (1986: 202) suggests Mesonychoteuthis attains a mantle length in excess of 2 m, possibly exceeding 4 m. Nesis (1987) reports mantle lengths of 2.0—2.25 m, but it is uncertain whether these measures are based on actual specimens, or estimates of size based on beak remains from stomach contents of sperm whales. This is a mantle length equivalent to that of the largest of 105 Architeuthis specimens examined by the authors.

The lower beak standard measurement of rostrum length (LRL) 22.0 mm reported for 1.05 m specimen (Rodhouse & Clarke 1985) is considerably shorter than the greatest LRL described for this species, 48.0 mm (a measure taken from the largest beak known from the stomach of sperm whales). Therefore Mesonychoteuthis obviously attains a size considerably exceeding 1.05 m ML.

Reference: http://www.tonmo.com/science/public/giantsquidfacts.php

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