Life on Earth. David Attenborough
Brachiopods (Glottidia albida).
The lower part of the molluscan body is called the foot. Its owner moves itself about by protruding the foot from the shell and rippling its undersurface. Many species carry a small disc of shell on the side of it which, when the foot is retracted into the shell, forms a close-fitting lid to the entrance. The upper surface of the body is formed by a thin sheet that cloaks the internal organs and is appropriately called the mantle. In a cavity between the mantle and the central part of the body, most species have gills which are continually bathed by a current of oxygen-bearing water, sucked in at one end of the cavity and expelled at the other.
The shell is secreted by the upper surface of the mantle. One whole group of molluscs has single shells. The limpet, like Neopilina, produces shell at an equal rate right round the circumference of the mantle and so builds a simple pyramid. In other species, the front of the mantle secretes faster than the rear and creates a shell in a flat spiral, like a watch spring. In yet others, maximum production comes from one side so that the shell develops a twist and becomes a turret. The cowrie concentrates its secretion along the sides of the mantle, forming a shell like a loosely clenched fist. From the slit along the bottom, it protrudes not only its foot but two sections of its mantle which in life may extend over each flank of the shell and meet at the top. These lay down the marvellously patterned and polished surface characteristic of cowries.
Blue limpet (Patella coerulea), showing underside.
The single-shelled molluscs feed not with tentacles within the shell like the brachiopods but with a radula, a ribbon-shaped tongue, covered with rasping teeth. Some use it to scrape algae from the rocks. Whelks have developed a radula on a stalk which they can extend beyond the shell and use to bore into the shells of other molluscs. Through the holes they have drilled, they poke the tip of the radula and suck out the flesh of their victim. Cone shells also have a stalked radula but have modified it into a kind of gun. They slyly extend it towards their prey – a worm or even a fish – and then discharge a tiny glassy harpoon from the end. While the tethered victim struggles, they inject a venom so virulent that it kills a fish instantly and can even be lethal to human beings. They then haul the prey back to the shell and slowly engulf it.
A heavy shell must be something of a handicap when actively hunting, and some carnivorous molluscs have taken to a faster if riskier life by doing without it altogether and reverting to the lifestyle of their flatworm-like ancestors. These are the sea slugs (nudibranchs) and they are among the most beautiful and highly coloured of all invertebrates in the sea. Their long soft bodies are covered on the upper side with waving tentacles of the most delicate colours, banded, striped and patterned in many shades. Though they lack a shell, they are not entirely defenceless, for some have acquired secondhand weapons. These species float near the surface of the water on their feathery extended tentacles and hunt jellyfish. As the sea slug slowly eats its way into its drifting helpless prey, the stinging cells of the victim are taken into its gut, complete and unsprung. Eventually these migrate within the sea slug’s tissues and are concentrated in the tentacles on its back. There they give just the same protection to their new owners as they did to the jellyfish that developed them.
Other molluscs, such as mussels and clams, have shells divided into two valves lke those of a brachiopod and thus are known as bivalves. These creatures are much less mobile. The foot is reduced to a protrusion that they use to pull themselves down into the sand. For the most part, they are filter feeders, lying with valves agape, sucking water in through one end of the mantle cavity and squirting it out through a tubular siphon at the other. Since they do not need to move, great size is no disadvantage. Giant clams on the reef may grow to be a metre long. They lie embedded in the coral, their mantles fully exposed, a zigzag of brilliant green flesh spotted with black, which pulsates gently as water is pumped through it. They can certainly be quite big enough for a diver to put his foot into, but he would have to be very incautious indeed to get trapped. Powerful though the clam’s muscles are, it cannot slam its valves shut. It can only heave them slowly together, and that gives plenty of notice of its intentions. What is more, even when the valves of a really large specimen are fully closed, they only meet at the spikes on the edge. The gaps between them are so big that if you plunge your arm through into the mantle, the clam is quite unable to grip it – though the experiment is a little less unnerving if it is tried first with a thick stick.
Some filter feeders like the scallops do manage to travel – by convulsively clapping their valves together and so making curving leaps through the water. By and large, however, adult bivalves live rather static lives and the spreading of the species into distant parts of the seabed is carried out by the young. The molluscan egg develops into a larva, a minuscule animated globule striped with a band of cilia, which is swept far and wide by ocean currents. Then, after several weeks, it changes its shape, grows a shell and settles down. The drifting phase of its life puts it at the mercy of all kinds of hungry animals, from other stationary filter feeders to fish, so in order that its species can survive, a mollusc must produce great numbers of eggs. And indeed it does. One individual may discharge as many as 400 million.
One branch of the molluscs, very early in the group’s history, found a way of becoming highly mobile and yet retaining the protection of a large and heavy shell – they developed gas-filled flotation tanks. The first such creature appeared about 500 million years ago. Its flat-coiled shell was not completely filled with flesh as is that of a snail, but had its hind end walled off to form a gas chamber. As the animal grew, new chambers were added to provide sufficient buoyancy for the increasing weight. This creature was the nautilus, and we can get an accurate idea of how it and its family lived because a few nautilus species, just like Lingulella and Neopilina, have survived to the present day.
A nudibranch (Eubranchus tricolor) on the seabed of a Scottish loch.
One of these species, the pearly nautilus, grows to about 20 centimetres across. A tube runs from the back of the body chamber into the flotation tanks at the rear so that the animal can flood them and adjust its buoyancy to float at whatever level it wishes. The nautilus feeds not only on carrion but on living creatures such as crabs. It moves by jet propulsion, squirting water through a siphon in a variation of the current-creating technique developed by its filter-feeding relatives. It searches for its prey with the help of small stalked eyes and tentacles that are sensitive to taste. Its molluscan foot has become divided into some ninety long grasping tentacles which it uses to grapple with its prey. In the centre of them it has a horny beak, shaped like that of a parrot and capable of delivering a lethal, shell-cracking bite.
About 400 million years ago, after some 100 million years of evolution, the nautiluses gave rise to a variant group with many more flotation chambers to each shell, the ammonites. These became much more successful than their nautilus relatives, and today their fossilised shells can be found lying so thickly that they form solid bands in the rocks. Those of some species grew as big as lorry wheels. When you find one of these giants embedded in the honey-coloured limestones of central England or the hard blue rocks of Dorset, you might think that such immense creatures could do little but lumber massively across the seabed. But where erosion has removed the outer shell, the elegant curving walls of the flotation chambers that are revealed remind you that these creatures may well have been virtually weightless in water and able, like the nautilus, to jet-propel themselves at some speed through the water.
About 100 million years ago, the ammonite dynasty began to dwindle. Perhaps there were ecological changes that affected their egg-laying habits. Maybe new predators had appeared. At any rate, many species died out. Other lines gave rise to forms in which the shells were loosely coiled or almost straight. One group took the same path as the sea slugs did in more recent times and lost their shells altogether. Eventually all the shelled forms except the pearly nautilus disappeared. But some shell-less ones survived and became the most sophisticated and intelligent of all the molluscs, the squids and cuttlefish and the octopus. These are the cephalopods.
The relics of the cuttlefish’s ancestral shell can be found deep within