Unio pictorum | Painter’s Mussel

Introduction

The painter’s mussel (Unio pictorum) is a handsome freshwater bivalve of slow rivers and lakes, named centuries ago because its smooth, elongated shell was used by artists as a dish for holding paint. Smaller and more oval than the swan mussel, it lies in the riverbed siphoning food from the current. A characteristic animal of clean European waterways, it is a quiet engineer of water clarity and a link in the life of the stream.

Taxonomy

Unio pictorum is a mollusc in the class Bivalvia—the clams, oysters, and mussels—within the family Unionidae, the river mussels. It is a close relative of Mytilus edulis, the common mussel, sharing the bivalve plan of two hinged shells and filtering gills, but living in fresh water rather than the sea.

Physical Characteristics

The painter’s mussel has a thick, oval shell 6–12 cm long, greenish to brown, often with darker rays and a somewhat polished look. The two shells are hinged at one end and have visible hinge teeth. Inside, the body has a wedge-shaped foot used to burrow and a pair of siphons. The shell is heavier and more robust than the swan mussel’s, and the umbo—the oldest part near the hinge—is rounded and worn smooth.

Habitat

Painter’s mussels live in the beds of slow rivers, streams, lakes, and canals with clean, oxygenated water and a muddy or sandy bottom. They bury themselves in sediment with the rear end and siphons exposed to the current. They need good water quality and a steady flow of food, so they disappear from polluted or heavily silted waterways, marking them as a sign of a healthy river.

Diet

Like its relatives, the painter’s mussel is a filter feeder. It pumps water through its gills, trapping plankton, algae, and organic particles and expelling the cleared water. A bed of mussels can process a great volume of water, helping keep the river clear and tying up nutrients. It takes all its food from the passing current and asks nothing of other animals.

Reproduction

The female broods her tiny larvae, the glochidia, in a pouch in her gills and releases them into the flow. The glochidia must attach to the gills or skin of a host fish, where they briefly parasitise before dropping off to settle as young mussels. This fish-dependent stage scatters the next generation along the river, linking the mussel’s fortunes to those of the fish it rides.

Conservation

The painter’s mussel has declined across much of Europe with falling river quality, dams, and the loss of host fish. It is protected in several countries and is a valued indicator of unpolluted flowing water. Conserving clean, free-flowing rivers and their fish populations is the surest way to keep this gentle filter feeder in the streambed.

Fun Facts

• Its shell was once used by painters as a little dish for holding their pigments.
• Its larvae must hitch a ride on a fish’s gills to finish growing.
• A bed of mussels can filter huge amounts of river water as they feed.
• The shell is thicker and more polished than the swan mussel’s papery one.
• It is a living signal of clean, healthy flowing water.