The elongated bivalve shells and dark blue to purple colour of Common (or Blue) Mussels are a familiar sight on British shores. These molluscs live in the intertidal zone (this is the zone above water level at low tide, but underwater at high tide). They attach to rocks or other hard surfaces using byssus threads – strong filaments produced by glands in their feet that anchor them securely. Although they stay in one place for long periods of time, they are semi-sessile, meaning they can move to new locations if necessary. They are filter feeders, straining food particles from the water.

A group of Common Mussels might all look similar to each other, but the species does in fact have separate sexes. To reproduce, males and females release their sperm and eggs into the seawater. For each egg, there can be up to 10,000 sperm. Even so, many eggs do not become fertilized and, of those that do, only a few reach maturity.

It’s not uncommon to see Common Mussels in large groups, called aggregations, in which they use their byssus threads to attach to each other. The purpose of this may be to maximise reproductive success or improve protection from predators. Long-term persistent aggregations can form mussel beds of very dense populations. 

Common Mussels are, of course, eaten by humans, but are also enjoyed by seabirds, such as Oystercatchers, as well as crabs and starfish. Dog Whelks enjoy feasting on them, too, but the Mussels have a protection against them: they use the byssus threads, which are very sticky, to trap the Whelks and prevent them moving on, meaning they slowly starve.