Mussels are a wide group of bivalve organisms with several found on the shore. The two shells are held by a ligament that forces the shell open when the muscle relaxes. Bivalves are filter feeders, they use a siphon to draw in water, it is then filtered and expelled through a second siphon. The gills are used both to take in diffused oxygen from this water and perform the filtering action for food. Mussels attach themselves to rocks by tough threads these are secreted from a gland in the foot known as the byssus. The outer layer of the shell (periostracum) can develop spines in some species.
The usual mussel at Bude is the edible (Mytilus edulis), this lacks spines and is frequently seen in large groups, it is tolerant of freshwater and so is often is found on the beach where a stream crosses suitable rocks where other species struggle to live. Dog whelks, oystercatchers and large crabs munch on mussels, the whelks drill into the shell and use chemicals to dissolve the internal organs before sucking them out. The mussel is not defenceless against whelk attacks, it is thought an individual being attacked can release chemicals that cause others to release threads that attach the whelk to the rock so immobilising it. This doesn't work on oystercatchers though and the birds happily settle on mussel beds at low tide. Research has found that mussels that are not yet attached will move away from areas of high whelk density and clump together, it is thought they can detect the pheromones of whelks. The whelks stay at the edge of large concentrations of mussels trying to avoid this trap, this is very much an arms race in action.
Mussels release larvae into the water, these are planktonic and move freely for up to four weeks before settling on hydroids or seaweeds. Here they grow before releasing themselves once again while still maintaining an attachment, similar to a spider on a thread. Once they reach 1-2mm (another four weeks) they then settle onto rocks and attach. Strangely some do settle straight onto existing mussel beds at earlier stages of life.
Mussels reach maturity in the first year and live for several years, those that live higher up the shore grow more slowly and have been recorded living over 15 years. Mussel shells provide homes to other organisms, the pea crab and polychaete worms are known to be lodgers.
Mussels look fairly uninteresting but hopefully, after reading this, you will see them in a new light.
References -Fish & Fish a student’s guide to the seashore.