Listed as Vulnerable on the IUCN Red List of Threatened Species, the Depressed River Mussel (Psuedanodonta complanate) has a serious case of the January blues.

Found throughout Europe, the UK appears to be a particular stronghold for remaining populations, including the Thames river catchment. However, pollution, dredging and competition from the invasive Zebra Mussel are taking their toll on this struggling mollusc.

Whilst the Depressed River Mussel doesn’t currently have much to be cheerful about, it is the shape of its greenish-brown double-hinged shell that actually gives this species its name. Like all Bivalves, the mussel is a filter feeder, drawing water through its gills to strain out tiny food particles like algae, bacteria and decaying organic matter. This makes it particularly sensitive to water quality - in fact, so sensitive that some European countries use freshwater mussels at water treatment works as a highly effective early-warning system.

This species has a fascinating lifecycle. Starting life as a tiny egg in its mother’s gills, once fertilised it develops into a tiny hooked larvae, which goes on to parasitize a fish by clinging to its gills and feeding on the filtered particles. The larva metamorphosizes into a juvenile mussel whilst still on its host fish, eventually detaching and settling on the riverbed to feed on passing detritus.

Often remaining in the same place on the riverbed for up to 100 years, the Depressed River Mussel is one of the longest-living species of mussel in the UK, and plays an important role in the river ecosystem by purifying the water through filter feeding. Key indicators of river health, severe population drops in rivers like the Thames signify degraded conditions and pollution incidents.

It is not all doom and gloom for the Depressed Mussel. An under-recorded species, scientists believe that it may be more numerous than once thought. The UK has also been designated as an important location for the survival of this species, so if we keep up the pressure on government and industry to clean up our rivers, the future could look brighter for this not-so-miserable mollusc.