Crustaceans and molluscs (or mollusks) are both considered invertebrates yet are no more closely related to each other than they are to us. What they have in common, though, are hard outer coverings. You will have seen the shells of crabs and snails if you have ever visited a beach.
With molluscs especially their shells can be extraordinarily ornate, vividly coloured and sometimes very large. Looking very pretty is, however, not the main purpose. Molluscs are a highly diverse animal phylum, mostly marine but with a good number on land and in freshwater too. They are currently divided into 4 (sometimes 5) classes: Gastropoda, Polyplacophora, Bivalvia and Cephalopoda.
Gastropods are what people usually think of as molluscs. These include snails, slugs, whelks and limpets. Apart from slugs, which have either completely lost the shell or have only a tiny vestigial remnant, all gastropods have a single coiled shell that grows with the animal.
The shell's main purpose is usually protection; from predators in completely aquatic species and also to prevent land based ones from drying out. Gastropods do not have a completely waterproof skin and dehydration is a constant danger. Slugs manage to get round this on land by favouring damp habitats.
These are a smaller group commonly referred to as chitons. They are all marine and their shells are a series of articulated plates covering their bodies, The purpose of these is also protection.
Bivalves are more familiar than chitons to most of us, especially as many end up on our plates. They include all the two shelled molluscs such as clams, mussels, cockles, scallops, shipworms and razor shells. All are aquatic with most living in the oceans. As with the previous two classes their shells protect them from predators and also help them dig and provide support.
Cephalopods are arguably the most interesting of the mollusc classes. Octopi, cuttlefish, pearly nautilus and squid are the most intelligent invertebrates there are, some being roughly equivalent to a cat. Octopi in fact have been found to use tools and squid have a complex system of communcation. Most have shells, but these are usually internal and their function is like ours; to provide support for the soft tissues in larger animals.
Because they are not restricted by an external shell, and sometimes because they have internal support, certain cephalopods can become very large indeed. The giant squid is an example, reaching as it does a phenomenal 14 metres.
Crustaceans refers to a more specific group. They are a subphylum in the phylum Arthropoda, which includes insects, spiders and scorpions. Familiar crustaceans are crabs, shrimps, lobsters, barnacles and the land based woodlice.
These too are mostly marine, with a few freshwater and land based species. All classes have a hard outer covering usually referred to as an exoskeleton (external skeleton) rather than a shell. Its purpose underwater is to protect the animal from predators and to provide support. On land as with molluscs the additional purpose is to protect the animal from drying out.
There are a few species, known as hermit crabs, that take on the shells of molluscs. This is for much the same purposes as the exoskeletons of other crustaceans but mollusc shells are tougher than exoskeletons so hermit crabs have an advantage in the protection department. They are however very vulnerable when they have to switch to a new shell.
The multitude of shells that mollucs and crustaceans grow is mesmerising. Any walk along the coast reveals thousands upon thousands of empty gastropod, bivalve and crustacean shells, perhaps with a few from cuttlefish too. These sometimes extraordinary structures were essential to the survival of the animals they once belonged to.