Duck mussel Anodonta anatina

Several species of mussel live in the Baltic Sea. They all have a flat body covered by two calcium carbonate shells, which are attached together by a hinge. Mussel’s gills have evolved into ctenidia, specialized organs which they use for feeding and breathing. They also have a leg, which they use for moving.

Duck mussel half burrowed into sand in sea bottom.
Duck mussel have a breathing and a discard holes for air and food. Photo by Pauliina Ahti, Metsähallitus.

Duck mussel is yellowish-brown, and the largest of the bivalve mollusks found in the Bothnian Bay. It can grow up to 10 cm wide. It is a freshwater species, and the most common freshwater mussel in the sea. It tolerates some salinity, and it can be found in the river estuaries along the coast. It is also found in the archipelago in the northern Bothnian Bay. Duck mussel occur widely in northern and central Europe and its IUCN status is least concerned.

Duck mussel live on soft-bottomed sea shores where they get their leg pushed inside the bottom substrate. They move up to some meters per day with their leg and form grooves as they travel across the bottom.

Sandy bottom with a circular trail done by a mussel.
Duck mussel leaves makings on the bottom when it moves. Photo by Essi Keskinen, Metsähallitus.

When young, the duck mussel is hermaphrodite, meaning it has both sexes. In small water bodies, duck mussel remains hermaphrodite, which ensures reproduction as they can fertilize themselves if necessary. In larger water bodies, duck mussels evolve into either males or females as they grow older. At the time of reproduction, the male pours his milt into water, where the female sucks it through her ctenidia.

During the winter, fertilized egg cells develop into larvae. The mussel releases the larvae from its shell cavity when a fish swim nearby. The larvae have barbs at the edges of their shells, which they use to attach themselves to the skin and fins of the fish. The larvae feed on the fish for a few months and then detach and dig into the bottom where they grow up.

Duck mussel's ctenidia showing, otherwise covered up with sand.
Duck mussel burrowed into the bottom sand. Photo by Suvi Saarnio, Metsähallitus.

Duck mussel’s mouth is in the inside cavity and it eats plankton and organic matter that floats inside with the water. The food particles attach to the surface of the gills and small lashes, that are covering the gills, move them towards the mouth. By eating the floating matter, the mussels are cleaning the water. Mussels’ work is really important, and it is calculated that blue mussels (Mytilus trossulus) can go through entire mass of water in Baltic Sea once a year. However, in the Bothnian Bay, there aren’t any blue mussels, because the salinity of the water is too low. Duck mussels are doing the same job here, their efforts just haven’t been calculated.

Blue mussels.
Blue mussels attach themselves to rocks and form dense blue mussel beds. There are no blue mussels in the Bothnian Bay. Photo by Essi Keskinen, Metsähallitus.

Name “duck mussel” comes from a habit of people to feed this mussel to ducks. There used to be such large and widespread populations of this mussel that it was commonly used. Finnish name “pikkujärvisimpukka” = “small lake mussel” indicates that the mussel likes fresh waters and that there are also bigger mussels in lakes. Swedish name “vanlig dammussla” = “common dame mussel” tells that it’s a common mussel in Sweden.

Duck mussel on the bottom, picture from the side.
Even though duck mussels do move, they do it so slowly that they can be a growing platform for algae and water mosses. Photo by Noora Kantola, Metsähallitus.

Written by Anna Antinoja

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