Come on a dive with Paul Naylor as he takes a closer look at the vibrant and fascinating range of molluscs lurking on a typical British seabed
Think of molluscs and chances are it is shellfish such as limpets, whelks, scallops and mussels that spring to mind. While some of them taste rather nice smothered with cheese sauce or garlic butter, these animals do not at first sight seem to be of particular interest to us divers.
Such an assumption, however, not only overlooks the fascinating habits of these animals but ignores the fact that some of their relatives in the mollusc group, such as the dazzling sea slugs and the remarkable cuttlefish, are among the most interesting of marine animals.
The range of molluscs is huge top of the range is the cuttlefish, with its close relatives the octopus and squid, representing the peak of achievement as far as invertebrates (animals without backbones) are concerned.
Many other molluscs, while they may be colourful or intriguing, are rather slow and fairly simple animals with just a basic nervous system zoologists sometimes classify them as more primitive than crustaceans and worms.
To get an idea of the range of molluscs that can be found within a small area, consider a single dive in our home waters, out of a rocky cove in the West Country or Wales, for example. Entering the water from the shore, the first animal we spot may well be a mollusc such as a limpet or dog-whelk.
Both creatures share the basic mollusc features of a soft body, comprising a muscular foot and a mass of various digestive and reproductive tissues on top of that, all covered by a hard shell.
The shell of a mollusc isn't a suit of armour like that worn by crust-aceans such as the crab. Molluscs, unlike crustaceans, never have to cast off their shell or re-form it to provide growing room. It acts more like a hat (albeit one so large that the whole animal can hide inside it!) which grows gradually with the rest of the body.
The first sightings on our dive show obvious variation in shell shape (the dog-whelk has a twisted shell, the limpet a plain one), but there are also considerable differences in lifestyle. Both animals possess a feeding mechanism unique to molluscs, a "radula". This is a little like a rasping tongue that runs over a set of pulleys, but they use it for different purposes. The dog-whelk's radula drills into other animals such as barnacles, while the limpet's is used to rasp algae off rock surfaces.
Limpets seen under water while diving at high tide are, in fact, usually grazing. As the tide recedes, they return to their "home base", a patch of rock that they have ground their shell to fit, to avoid drying out when exposed to air. Look out for these home bases, and the owners grazing nearby, next time you dive from a limpet-strewn shore.
As we venture into slightly deeper water on our mollusc-spotting dive, we might well encounter a colourful nudibranch sea slug hungrily creeping over a rock covered with hydroids. Nudibranchs are closely related to limpet-like creatures, but seem different because they have no shell.
Their gills are unprotected and appear as frilly rosettes or long finger-like projections on their backs. Nudibranch means ''naked gill''.
Without the protection of a hard shell, these sea slugs have to resort to other methods of protecting their slow-moving and soft bodies. This often involves exuding unpleasant substances, and the brilliant colours of some nudibranchs are thought to signal their toxicity, warning potential predators to stay away.
Many nudibranchs eat creatures such as sea anemones and hydroids, which are armed with stinging cells for catching prey and seeing off predators. Far from being deterred, the slugs ingest the stinging cells without triggering them and may even then use the cells for their own defence, by positioning them near the surface of their bodies.
Nudibranchs also have interesting sex lives. Each individual can act as a male and female at the same time, so that when a pair mates an exchange of sperm takes place.
The sex organs are located on the right-hand side of the body, so when you see two slugs with their right-hand sides next to each other, stay away if you're easily shocked! The distinctive egg ribbons of nudi-branchs can be found entwined around hydroids, rocks and weed.
On a muddy patch in deeper water we may come across scallops. If we approach carefully, the beautiful patterning on the soft tissues, the slender tentacles and iridescent eye spots can all be observed.
Scallops are bivalve molluscs, so called because they have two halves or "valves" to their shell. They are unusual bivalves in that they can swim, admittedly in a fairly rudimentary fashion but adequate to evade predatory starfish.
Most bivalves lead a fairly stationary life, either anchored to rocks, like mussels, or buried in sediment, like razor-shells, cockles and clams. The only signs of the burrowing molluscs are their water intake and outlet openings, just visible at the surface of a muddy seabed.
Bivalves have little need to move, because they are highly efficient filter-feeders, pumping large volumes of water through their bodies and extracting planktonic food. It's this life-style that can make those collected from polluted waters a serious food-poisoning risk.
Swimming back to shore across sandy patches interspersed with seaweed-covered rocks, especially if our dive is in the early summer, we might be fortunate enough to come across a cuttlefish. This beautiful creature can jet away rapidly, leaving a cloud of ink to confuse any pursuer but, more often, it will hover just above the seabed and eye us suspiciously for a few moments before eventually retreating.
As it swims over different areas of seabed, a cuttlefish can change colour almost instantly to match its surroundings, taking on a dark brown shade over kelp and turning almost white over sand. If it comes to rest on gravel, it can match the pattern perfectly.
This is a more skillful colour-change artist than a chameleon. Even when the overall colour appears steady, the whole of a cuttlefish's skin will shimmer or flicker, and complex display patterns can be generated during courting.
Cuttlefish engaged in courtship are a particularly wonderful sight, because their preoccupation leads them to disregard a watching diver.
The rapid changes in shade and colour are made possible by cells called chromatophores, which are full of pigment and can expand or contract under nervous control.
The nervous systems of cuttlefish are complex and include a proper brain not for them the scattered nerve centres of simpler molluscs. They can even learn to solve puzzles and, though such comparisons tend to be misleading, are often reckoned to be as intelligent as rats or cats.
As we reach the shore at the end of our mollusc-spotting dive, we contemplate the difficulty of acknowledging that the cuttlefish is a relative of the humble limpet.
The cone shape of the limpet can be visualised lying on its side to form the outline of the cuttlefish body, but it takes a lot of imagination. The limpet's simple foot has become the tentacles of the cuttlefish and a proper head has developed (cuttlefish and their relatives, the octopus and squid, are classed as cephalopods, which means "head-foot").
Part of the body cavity has become an organ for jet propulsion. There is no hard outer shell, but an internal "cuttlebone" that functions as a buoyancy compensator.
It is quite a transformation but, from the humblest to the most sophisticated, molluscs are fascinating creatures that deserve special attention. How many will you see on your next dive?
Appeared in DIVER - December 1998