Specieswatch: depressed mussel disappears from stretch of Thames

The depressed mussel, Pseudanodonta complanata, has every reason to feel dejected. This endangered species has disappeared from the River Thames at Reading, once considered to be a stronghold of its kind. Poor river management and invasive species such as zebra mussel are pushing it to extinction, along with two of its other British cousins, the duck and painter’s mussels.

These two species have declined by 95% in this stretch of the Thames, according to a study by academics at the University of Cambridge. Only empty shells were found of the depressed mussel.

However, there is still hope for the species, so-called because of the shape of its shell rather than its mood. There are populations in the rivers Ouse, Waveney and Yare in East Anglia.

Its life cycle involves the female holding its eggs in its gills until they are fertilised from sperm released by males somewhere upstream. The eggs hatch into tiny mussels in their mother’s gills and grow hooks for the next stage. They float off downstream aiming to fasten themselves on to the gills of passing fish. Once lodged they filter food from the water the fish draws in until large enough to let go and live independently on the bottom of the river. Left alone, they can live 100 years.