Shipworms, mussels and piddocks

Teredo norvegica, the common shipworm

Shipworm and piddocks are the boring bivalves

This group have evolved to use the edge of their valves to bore into wood and even into rock. The anterior end of the shell is usually serrated to aid the boring action.

Teredo norvegica, the common shipworm (left)

Teredo norvegica became known as the shipworm because its shell is greatly reduced, (it lies towards the bottom of the image), and does not contain all of its body. The rest of the body lies in a chalky lining secreted as it burrows, so it does look worm-like.

At one the time it was not understood how shipworms could have evolved before man started building boats and structures in the sea. It takes millions of years for animals to evolve and man has been building boats for only a few thousand years. However now it is known that timber is quite plentiful in the oceans especially near great river estuaries and after storms, so the evolution of the shipworm did not need the presence of boats.

The larva settles on a piece of timber and starts to burrow. As it burrows the foot grips the side of the tunnel and the siphons are protected by two calcareous flaps. Food is obtained through the siphons, but the shipworm has the ability to digest the cellulose of the wood it drills into. It can cause considerable damage to wooden ships.

The oval piddock, Zirphaea crispata

The oval piddock, Zirphaea crispata, right, bores into rock. As with the shipworm, the foot holds the animal in place while the rocking backwards and forwards of the valves rubs away the rock. The valves do get worn down by this action, but every now and then the mantle edge creeps over the edge of the valve and deposits another layer of chalk.

Zirphaea crispata, the rock boring oval piddock
Pholas, a boring bivavle drawing showing siphon and cutting teeth

Pholas dactylus, the European piddock.

The European piddock is found in the N. E. Atlantic, North Sea and English Channel. It bores into soft rocks, e.g. chalk, sandstone and clay. It can be up to 12 cm long, excluding the siphon which can add another 12 - 24 cm to its length. In the dark it phosphoresces a blue/green light.

On the left is a drawing of Pholas sp. showing the rock-cutting teeth and the siphon.

Pholas dactylus, European piddock embedded in rock

Mytilus edulis, the common edible mussel

Mussels

Mussels attach themselves to surfaces by a thin guy rope called a byssus (see right). These are placed on the anchoring surface by the foot (see left), and enable the mussel to withstand battering from the sea.

The scar inside an empty mussel shell shows where the adductor muscle pulling the two shells together anchors.

Mussels are both freshwater (see below) and marine, and are filter feeders. Water enters through the syphon. Compare the very muscular foot of the common cockle (top) with that of the mussel. The mussel uses its foot much less. Mussels often live in large groups and can cause problems for coastal power stations by blocking the pipes of their cooling systems.

Mytilus edulis, the common edible mussel, diagram showing body parts

Margaritifera margaritifera, freshwater mussel

Above is Margaritifera margaritifera, the pearl mussel, a freshwater mussel. Fully grown specimens can reach 150 mm long, and can live fore 190 years. Large pearls are rare; most pearls are around 2 mm in diameter. It is found in large, fast rivers with sand or gravel beds in which it buries itself. Recently it has declined through pearl collecting.

On the right is Anodonta cygnea, the swan mussel. The shell when fully grown can be 15 cm long and olive brown. It is found in firm mud in canal bottoms, slow river and lakes.

The eggs are produced from June to August, as many as 500 000 per individual. They are kept in the outer gills which now become brood pouches, and are fertilised by sperm carried on a water current from a nearby male.

After 9 months they leave the brood pouch and become parasites of fish for 3 months. When a miniature mussel drops off the fish and, hopefully, lands in mud.

Anodonta cygnea, swan mussel, diagram showing body parts

Pisidium sp., pea-shell cockle

On the left is Pisidium sp., the pea-shell cockle or Pea mussel. It has just one siphon. It is common in sand and mud of all types of fresh water including marshes, ditches and mountain tarns.

It is hermaphrodite and viviparous producing fully developed young. It is very small, just 3 - 5 mm, and buff coloured.