If you have ever been down to a rocky beach or estuary in the UK you probably will have spotted a clump of common mussels (Mytilus edulis) on a rock or cliff face. If it has ever crossed your mind, as it used to for me, how this sessile (non-moving) animal survives and reproduces I hope this article will help you answer any questions you have.
The common mussel can be found from the mid-shore downwards on stones and rocks in estuaries and on the more exposed rocks on shorelines throughout the UK. They use byssal (protein) threads that are produced from the byssal gland, which is situated within the shell, to secure themselves to the rocks. If the conditions are right they often form into large beds with hundreds of individual mussels. These mussel beds can be found in places such as the Wash and Morecombe Bay.
Obviously, as adult mussels are anchored to their chosen site by their byssal threads, males and females cannot seek each other out to reproduce. Fertilisation occurs externally outside of the shell in the surrounding water column. This is one of the reasons why mussel form into beds as with so many individuals in close proximity, it increases the likelihood of successful fertilisation. Females release eggs into the water column during spring and early summer. Depending on the size of the individual female mussel they can release between 7 and 40 million eggs during the spawning season. The males simultaneously release sperm into the water column and fertilisation of the eggs occurs. Fertilised eggs then develop into small larvae. Mussel larvae go through two larval stages (trochophora and then veliger larval stage) that together last about 4 weeks. They then settle onto filamentous objects on the sea bed, such as seaweed, as juvenile mussels that are only 3mm long! During this juvenile stage although they are attached with only weak byssal threads and they can drift with the current on a long byssal thread, until they find the most suitable home. Any movement will finish by the time the mussel reaches 5cm long and from then on they will attach themselves securely with byssal threads and continue to grow and live for up to 24 years!
Mussels feed on phytoplankton and food particles found within the surrounding water. They do this by filtering up to 3 litres of sea water an hour through their siphons. The two siphons are lined with cilia (tiny hairs) that move and create a current of water moving in and out of the shell. Edible particles are directed to the digestive tract and inedible particles are pumped out with the waste water. This system of feeding is what makes mussels sensitive to water pollution, as toxins can be filitered out of the water column and can build up within their bodies. Therefore if they are heading towards our plates the toxin content within the water must be monitored.
Mussels have many predators that either target individuals during their larval stage in the water column or once they are settled on the sea bed. One of the mussels biggest predators is us! As a popular seafood they can be farmed, hand-picked or dredged. For those of us who are trying to eat environmentally friendly seafood, the farmed mussels currently have the best rating on the Marine Conservation Society sustainable fish list, which means we can continue to enjoy them guilt free. Hand-picked and dredged mussels are currently being assessed, so hopefully we should know shortly whether they are also sustainable fisheries.
Mussels have many other non-human predators to compete with. As larvae in the water column they are at risk of being eaten by the many plankton feeding species, such as juvenile fish. Once they are settled they are still at risk from both marine and terrestrial species. Sea birds such as oyster catchers and sea gulls pick off individual mussels during low tide. Marine predators include starfish, crabs and sea urchins and other mollusc species such as the common and dog whelk.
Blue mussel bed underwater- George Stoyle
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