Cephalopods , which include squids and octopuses, vary enormously in size. The smallest are only about 1 centimetre 0. Living species range in mass more than three-billion-fold, [ nb 1 ] or across nine orders of magnitude , from the lightest hatchlings to the heaviest adults. Cephalopods were at one time the largest of all organisms on Earth, [ 6 ] 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, [ 7 ] 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 Reanalyses in and of the original tissue samplesβtogether with those of other similar carcasses βshowed 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. The Lusca of the Caribbean and Scylla in Greek mythology may also derive from giant squid sightings, [ 12 ] as might eyewitness accounts of other sea monsters such as sea serpents.
Cephalopods of enormous size have featured prominently in fiction. Due to its status as a charismatic megafaunal species, the giant squid has been proposed as an emblematic animal for marine invertebrate conservation. 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.
