The reproductive biology of the depressed river mussel, Pseudanodonta complanata (Bivalvia: Unionidae), with implications for its conservation

Pseudanodonta complanatais listed as 'Near Threatened' on the IUCN Red List (IUCN, 2006) and is a species of conservation priority on the UK Biodiversity Action Plan. The UK is host to some of the largest populations of this species, but little is known about their reproductive biology. Two populations were studied in the Great Ouse catchment and the Waveney and Yare catchment, East Anglia, UK. Both populations are reproductively active, producing viable glochidia. Small mussels (< 30 mm length) in both catchments indicate that recruitment is occurring. A short non-gravid period in May is followed by three months of glochidial formation in June, July and August; by September mussels contain mature glochidia ready for release the following April. Females brood between 5,000 and 50,000 glochidia, and this scales with mussel length L (mm) as: n(gloch)proportional to L-2.1. The sex ratio is skewed towards females (2.5 females:1 male), and males are larger than females. No hermaphrodites were found during the histological examination of the gonads of 24 mussels. The in vivo examination of demibranchs is shown to be an effective nonsacrificial means of determining sex and gravidity. Conservation recommendations include: minimizing management operations in river stretches containing large populations; avoiding forms of management which preferentially remove large mussels (i.e. males and the most fecund females); performing management during the non-gravid period to avoid causing the premature release of glochidia; leaving sufficient time between management operations for populations to recover; and temporarily translocating mussels to refugia during management operations.