Illustrated here are two deep sea squids, class Cephalopoda, phylum Mollusca.

Cephalopods are a group of exclusively marine molluscs that include squids, octopods, cuttlefish and the chambered Nautilus. These animals are important predators in the marine food chain and are generally adapted for a nectic lifestyle, with the exception of octopods which have taken up a benthic existance. Present day there are about 600 living species of cephalopods, but over ten times that many are known from the fossil record, where the group flourished in the Palaeozoic era.

Millions of years ago all cephalopods supposedly used the mantle to secrete an external shell for protection. Today this habit is continued only by two different 'living fossils' found in the deep sea realm, the several species of Nautilus in the south-west Pacific and the small squid Spirula sp. found at the upper edges of the abysses in the Atlantic and Indian Oceans. Spirula, the 'fag-end fish', rarely exceeds four cm in length and is the only extant cephalopod with a coiled internal shell (see above left).

The deep sea squids are a very important group ecologically, they are widespread and abundant and provide nutrition for many cetaceans, seals, fish and birds, aswell as being dominant predators themselves upon fish and crustacea. Squids and cuttlefish catch prey with their 8 arms and 2 prehensile tentacles, that are often covered with adhesive suckers, they then guide food into the beak-like jaws of the oral cavity.

Squids move rapidly through the water column by jet propulsion. They expel a jet of water from the mantle cavity through a funnel which can be directed to control the movement of the animal in the opposite direction. The streamlined squids can swim faster than any other invertebrate, reaching up to 25 miles per hour. Some squids have larger fins for swimming and several retain ammonium salts in their body which give them buoyancy. Shell bearing cephalopods, like Spirula and Nautilus, achieve a neutral buoyancy by sucking water out of the cavities within their chambered shells.

Light producing organs are found in many species of deep sea squid and may be spread over the ventral surface, on the under side of the eyes and liver, on the tips of tentacles to lure prey and in some cases a luminous shower of sparks may be produced to temporarily blind attackers. Lycoteuthis sp. (above right) is a small squid, 3.5cm in length, with light organs on its eyes and within its body, as seen through the transparent body wall.

Click here to view the 'Picture of the Week' archive.