Cephalopod size

Cephalopods vary enormously in size. The smallest are only about 1 centimetre (0.39 in) long and weigh less than 1 gram (0.035 oz) at maturity, while the largest—the giant and colossal squids—can exceed 10 m (33 ft) in length and weigh close to half a tonne (1,100 lb), making them the largest living invertebrates. Similarly large cephalopods are known from the fossil record, including enormous examples of ammonoids, belemnoids, nautiloids, and vampyromorphids. In terms of mass, the largest of all known cephalopods were likely the giant shelled ammonoids and endocerids.

Size, and particularly maximum size, has been one of the most interesting aspects of cephalopod science to the general public. 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 (see Ellis, 1998; Roper & Shea, 2013; Paxton, 2016). On account of its status as a charismatic megafauna the giant squid has been proposed as an emblematic animal for marine invertebrate conservation (see Guerra et al., 2011).

Certain cephalopod species are noted for having individual body parts of exceptional size. The giant and colossal squids, for example, have the largest known eyes among living animals.

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).

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