Mussels

Common Mussel Mytilus edulis

The Common Mussel is the most common bivalve in Cornwall forming extensive beds on exposed rocky beaches. It is often the dominant species on beaches, as such it is used as a biological indicator for exposed rocky shores.

Often the beds are so overcrowded, that space for growth is restricted leading to stunted and distorted shells. Growth rates also vary depending on where the mussel is located. They can be in excess of 4 inches but are generally smaller. Higher up the shore the less the mussel is covered by the sea hence less time to feed leading to slower growth. Additionally in the beds, those on the outside grow faster however this benefit has to be weighed against heavier predation.

Mussels are adaptive and even on sandy beaches if there are rocks they will settle here, but run the risk of being smothered if the sand moves.

Their ability to form large beds on exposed shores may be due to the harsh conditions that they can survive while many of their potential predators cannot.

Mussels are filter feeders and require large quantities of water to pass through their system, just to get enough food to survive. This large flow means that any pollutant organic (sewage) or inorganic (e.g. heavy metals) pollutant tends to concentrate in their flesh, which if eaten by man will have dire results even death. The problem does not only have to be man made, there are seasonal algae blooms which on concentration can cause death if the shellfish are eaten.

Mussels have to be physically tough not only because they live on exposed beaches continually buffered by wave action, but also because they live in the intertidal zone. A priority is how to survive when exposed to the air by the tides. This they do by tightly closing their valves and trapping water within. Not only does this reduce desiccation but also aids protection from birds.

Mussels are sedentary fixing themselves to rock by byssal threads 2. These tough chitinous threads are produced as a liquid which then sets in sea water.

Their sedentary nature means that their only defence is the shell. One of the main predators are dog whelks which can be common on beaches with mussel beds. Dog whelks bore a hole through the shell and then suck out the soft parts. It is easy to find a mussel shell with a tiny round hole 1 drilled in it. Fortunately this process is not quick and sometimes the mussel will have enough time to attach a byssal thread onto the dog whelk’s shell. This may not save it, but the whelk is stuck and eventually it starves to death.

Horse Mussel Modiolus modiolus

This mussel grows considerably larger and is more rounded than the common mussel.

The horse mussel is mostly found outside the tidal zones but still can be found within them. They vary in colour from dark blue to purple.