Boring Molluscs

This is not a scientific grouping of bivalves, but that of species which share a basic life strategy of boring into wood or rock. They come from several genuses but share one thing in common their shell has evolved into a digging implement.

      Common Piddock Pholas dactylus

   Shipworm Teredo navalis

Piddocks

Piddocks are a relatively common bivalve in the UK, but are not so common in Cornwall due to the hardness of coastal rocks. They use the teeth like projections on the front edge of their shells, to grind the rock.

There are several piddock species which are not easy ask to differentiate. If they are found still in their burrows only the siphons may be seen, from which it is impossible to positively identify. The shells when they are washed ashore are so fragile that they rapidly are broken down.

Piddocks rely on their burrows for protection. Their shells are not only fragile and soon penetrated by predators, but they do not even completely enclose their soft body parts.

Common Piddock Pholas dactylus

The common piddock bores mud and even harder, shales. It carries this out by using its shell as a file to cut a burrow.

The shell has a distinctive shape but is fragile and does not last long after being stranded on the shore.

Shipworms

The name is a misnomer they are not worms but a bivalve. This is a understandable mistake as shipworms do not appear like other bivalves. The small shell at its head comprises only a small proportion of their body, the majority of the animal being soft and wormlike.

The shipworm attaches itself to wood including boats and piers at the larvae stage where it changes into a tiny adult. The shipworm burrows into the wood and from this point rapidly grows.

The shells’ surfaces are rough, and as the head turns side to side these surfaces abrade the wood. A tunnel is formed in the wood, its wall are then lined by a white chalky deposit.

As the tunnel is dug the abraded wood is digested. Shipworms possess the ability to break down cellulose, but they are primarily plankton feeders.

At their rear they have two siphons which protrude out of the tunnel opening. The siphons draw in and expel water. From this flow of water they obtain both their oxygen and food.

Shipworms are prone to drying out, to prevent this the siphons have pallets, which are small plates. These pallets can be pulled up tight by the siphons sealing off the tunnel, providing a water tight seal..

Shipworms are a pest with the extreme structural damage they can do, and was a very serious one before iron was used in shipping and pier construction.

Wood found on the strandline is worth investigating for the presence of shipworm. If found and broken open one will be surprised by the amount of infestation. There can be an intricate maze of tubes, as shipworms have the ability to sense the presence of other shipworms and will avoid breaking into their tunnels.

Shipworm Teredo navalis

The shipworm bores into timber digesting the wood as it goes. Timber washed ashore often contain this shellfish. The shell only comprises a small part (less than 1 cm) of the animal and is used as a rasp to cut the burrow. Running from the shell to the burrow’s opening is a syphon for feeding, waste and respiration. The syphon which can reach lengths over a foot, is enclose by a calcareous tube