Freshwater Mussels

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The above is a Swan Mussel (Anodonta cygnea) found in lakes whilst below is the Pearl Mussel (Margaritifer margaritifer) . The latter is associated with fast flowing water with low calcium content. Also rivers that are rocky with some gravel or sand present so that it can burrow. The photo below shows it within the natural habitat with just the siphons showing.

IDENTIFICATION: There are wide range of freshwater bivalves but the mussels are especially large and impressive. They can be quite inconspicuous as the merge in with the background environment. The two species illustrated here can grow up to 15 cm long. The shells show growth rings and the hinge has a strong ligament to hold the two symmetrical shells together. The lining of the inside is mother-of-pearl and the Pearl Mussel may actually contain a freshwater pearl. They are small, 1 - 2 mm. Opening up a mussel reveals two thickened muscles which clamp the shells together. Another thick muscle is the muscular foot while the mantle which secretes the shell covers the inside of both shells. There are a pair of short siphons that are just visible when the mussel opens.
Swan Mussel is the largest species.

The Swan Mussel is found in large ponds, lakes and slow moving water such as canals. Here they burrow into the mud with just the siphon tips exposed. Widespread across Central Europe and the UK. Pearl Mussels by contrast live in fast flowing water, amongst rocks where they attach themselves inside cracks, crevices and gravels with the siphons exposed. An upland species with a more restricted distribution around Europe.

ECOLOGY: All the mussels rest on the bottom drawing water into the body through one of the siphons. Using the gills, oxygen is removed and they also filter finely suspended particles out of the water for food. Waste is expelled through a narrower siphon (to eject it more forcefully away). The food is mixed with mucus and moved to the mouth by beating, fine hairs called cilia. The thick muscular foot is active and can be used for pulling the animal along or burrowing.

The Swan and Pearl Mussel have a fascinating life cycle. After the fertilisation of the eggs they hatch inside the mussel to form a larva called a glochidium (glochidia plural). This happens during the winter and the larvae are released in spring. The glochidia are unique because as they are released sticky strings trail behind and are an adaptation to attach to a fish. Most will die but some will get caught on to a fish. The larva then becomes an external parasite on the fish, clamping on to it to feed on blood. Over several weeks it will be dispersed away from the parent and eventually it drops off the fish to develop into an adult mussel.

Classification:
Kingdom	Animal
Phylum	Mollusca
Class	Bivalva
Order