Squid are a diverse group of invertebrates (animals without backbones). They come in a range of sizes. Some species mature at barely 1 cm, while the largest may grow to a total length approaching 27 m.
The largest invertebrate in the world is the giant squid (Architeuthis dux). The largest confirmed length of a giant squid is 18 m. However, found body parts indicate that they may grow up to 27 m long.
The giant squid has the largest eye in the animal kingdom. Its eye is about the size of a volleyball.
Colossal squids (Mesonychoteuthis hamiltoni) are slightly shorter than their giant cousins. But they are usually heavier, reaching almost 500 kg and approximately 6 m long.
Squid belong to a group of molluscs called cephalopods. This group also includes octopuses and cuttlefish.
There are approximately 70 species of cephalopods found in the Southern Ocean.
Most squid complete their life cycle — from tiny planktonic juveniles to mature adults — in approximately one or two years.
In warty squid reproduction, the female produces a huge ovary the size of a football. This process breaks down all muscle from her body wall. After spawning, they die. Their bodies float to the surface where they become important food for sea birds such as albatrosses.
Diet and feeding
Squid mainly eat fish and crustaceans. 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 in a day. They may increase in biomass by 10 to 15% per day in the first half of their life cycle. This figure drops to around 5% as they approach maturity.
Studying the diet of squid is difficult. Their oesophagus passes through their brain and is very narrow. This means that food particles must be chewed very finely, making them hard to identify. Both dietary analysis and squid fatty acid analysis are being used to determine squid prey species.
Many vertebrate predators depend on squid. Squid 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. They are also eaten by large fish such as toothfish.
Current research involves using statoliths (balance organs in the back of the squid head) to determine the age of individuals. Statoliths are composed of increments laid down daily (much like annual tree rings). This makes them powerful tools for assessing age, growth rates and hatch dates of key species.