Slugs, Snails and Squids

Molluscs are Related to Annelids

© John Blatchford

The most familiar molluscs (snails, clams, octopus and the like) account for more than 90,000 species of the 93,00 or so living molluscs, and they make up almost a quarter of all the creatures that live in modern seas.

Mollusc Biology

The mollusc body is unsegmented, in other words it is not made of a series of repeated units. Instead it usually has a ‘head-foot’ region (often with eyes and muscular walking, swimming or burrowing bits), and another part that contains most of the internal organs and gills when they are present. Typical molluscs are protected by a tough shell – single in the snails, double in the bivalves, internal or reduced in most cephalopods, and made of several plates in the chitons.

Gastropods – Snails and Slugs

The majority of molluscs are snail-like. Most have a tough shell and creep around on their muscular foot scraping their food up using a special rasp-like structure called the ‘radula’. One group (possibly the most beautiful creatures in the sea – the nudibranchs) have lost their shell, and sometimes rely on ‘stinging cells’ for their protection. These cells are ‘captured’ when they eat stinging jellyfish and recycled, intact, to sit at the tips of the delicate frilly covering.

Bivalves – Clams and Oysters

While the gastropods creep over surfaces, the bivalves usually burrow into it. The two halves of their shell open up to allow a digging ‘foot’ to poke out. (See ‘Bivalve Digging’.) Their huge gills filter plankton and other nutrients from the water – they are ‘filter-feeders’.

Chitons

The chitons look rather like woodlice or sowbugs with armoured plates stuck over their backs. They live on the rocky seashore, and although there are over 1,000 living species, they are often overlooked. There is one notable exception, the ‘gumboot’ or ‘Giant Pacific’ chiton. This species is so large that it is difficult to miss! (See article image.) Like most gastropods – to which they are probably closely related – the chitons rasp food off the rocks.

Cephalopods – Octopus and Squid

There are more than 900 species of cephalopod (squid and octopus). They are active predators, usually near the top of the food chain. They grab their prey with suckered tentacles and dispatch it with a parrot-like beak. They are the most intelligent of the invertebrates (see ‘Octopus Biology’).

Mollusk Relations – Close to Worms

Both the molluscs (phylum Mollusca) and true worms (phylum Annelida) have an unusual larval form – the ‘trochophore’. For this reason they are thought to be closely related and are placed in a group known as the ‘Lophotrochozoa’. The fact that the annelids are segmented originally led zoologists to believe they might be closer to the arthropods, but molecular studies suggest that these two groups probably evolved their segmented nature independently.