Anodonta cygnea | Swan Mussel

Introduction

The swan mussel (Anodonta cygnea) is the giant of European fresh water—a long, brown, paper-thin bivalve that can reach over 20 cm, named for its size rather than any real connection to swans. It lies half-buried in the mud of lakes, slow rivers, and canals, filtering the water and forming a quiet but important part of the wetland floor. Long-lived and sluggish, it is one of the most impressive of all freshwater molluscs.

Taxonomy

Anodonta cygnea is a mollusc in the class Bivalvia—the clams, oysters, and mussels—within the family Unionidae, the river mussels. It is a relative of Mytilus edulis, the common mussel, and shares the bivalve body plan of two hinged shells and feathery, filtering gills, though it lives in fresh water rather than the sea.

Physical Characteristics

The swan mussel has a thin, elongated, triangular shell 15–25 cm long, sometimes more, coloured olive to brown and often encrusted with silt. The two shells are hinged at one end and meet at a blunt posterior point; inside, the soft body has a large pale foot and a pair of siphons. Unlike many mussels it has no teeth on the hinge, and the shell is remarkably light for its size because it is so thin.

Habitat

Swan mussels live in the soft bottoms of lakes, ponds, slow rivers, and canals with clean, well-oxygenated water and a muddy or sandy substrate. They bury the rear of the shell in sediment and extend their siphons into the water above. They need stable, unpolluted water and are absent from fast, silty, or eutrophic streams, so their presence signals a healthy freshwater body.

Diet

Like other mussels, the swan mussel is a filter feeder. It draws water in through one siphon, traps plankton, algae, and organic particles on its gills, and expels the cleaned water through the other. A large mussel can filter many litres a day, helping keep the water clear. It asks only for a current of food-laden water passing over its buried body.

Reproduction

Swan mussels have a remarkable reproduction. The female broods her larvae—tiny “glochidia”—in a pouch in her gills and releases them into the water. The glochidia must latch onto the gills or fins of a passing fish, where they parasitise briefly before dropping off to settle as small mussels. This odd dependence on fish lets the young travel to new stretches of water.

Conservation

The swan mussel has declined in parts of its range because of pollution, siltation, and the loss of the host fish its larvae need. It is sensitive to falling water quality and is protected in some regions. As a long-lived filter feeder it is a good indicator of healthy lakes and rivers, and conserving clean, slow freshwater helps it persist.

Fun Facts

• It can grow longer than a hand and live for decades, slowly filtering the water.
• Its tiny larvae must grip onto a fish’s fin to complete their development.
• The shell is so thin and light it feels almost like paper for its size.
• It spends life half-buried in mud with only its siphons showing.
• A single mussel can clean litres of water every day as it feeds.