Physical description and related species
Squid are a diverse group of invertebrates (animals without backbones), and range in size from barely two and a half centimetres (Idiosepius sp.) to a total length approaching 18 metres (giant squid Architeuthis sp.).
The largest invertebrate in the world is the giant squid, which can grow to 15 m.
The giant squid has the largest eye in the animal kingdom. It is much bigger than the eye of a blue whale! That is about the size of a volleyball.
Squid belong to a group of molluscs called cephalopods, which also includes octopuses, cuttlefish and their allies.
There are approximately 70 species of cephalopods found in the Southern Ocean.
The warty squid reproduces by the female producing a huge ovary the size of a football while breaking down all muscle from her body wall. After spawning they die, with spent individuals floating to the surface where they become important dietary items for sea birds such as albatrosses.
Most squid complete their life cycle – from tiny planktonic juveniles to mature adults – in approximately one year.
Diet and feeding
Squid eat mainly fish and crustaceans, although they are also known to be cannibalistic and may feed on each other, especially when caught in nets. It is thought that squids can routinely eat 30% or more of their body weight a day and actually increase in biomass by 10–15% per day in the first half of their life cycle. This figure drops to 5% or so as they approach maturity.
Dietary studies of squid are difficult because their oesophagus is very narrow as it passes through their brain, therefore food particles must be chewed up very finely and are hard to identify. Both dietary analysis and squid fatty acid analysis are being used to determine squid prey species.
Many vertebrate predators depend heavily on squid, which is second only to krill as a food source in the Southern Ocean. Animals such as the grey-headed albatross and the sperm whale (the largest of the toothed whales) feed almost entirely on squid.
The warty squid Moroteuthis ingens is one of the species most preyed-upon by vertebrate predators in the subantarctic.
Current research involves using statoliths (balance organs in the back of the squid head) for determining the age of individuals. Statoliths are composed of increments laid down daily (much like annual tree rings). They thus serve as powerful tools for assessing age, growth rates and hatch dates of key species.