Life history of the freshwater mussel Unio tumidiformis (Bivalvia: Unionidae) in a temporary Mediterranean‐type stream

Mediterranean-type streams are characterized by great seasonal and annual variation in flow. We studied the biology of the freshwater mussel Unio tumidiformis in such a stream, the Vascao River in southern Portugal, during a period of great interannual variation in hydrology. We studied growth patterns of the mussels between 2002–2008, and the reproductive cycle between 2005–2006. Life-history parameters were calculated and related to environmental variables and fish community patterns. Mark–recapture growth data confirmed that observed annuli were formed annually and are therefore reliable for use in growth studies of U. tumidiformis. The von Bertalanffy growth constant K and the asymptotic length L∞ were negatively correlated. K varied between 0.20 and 0.58 and was positively correlated with factors related to eutrophication. The maximum observed age was 7 years, and the maximum observed length was 52 mm. There was no apparent sexual dimorphism, and sexual maturity was reached at the age of 2 years. Fecundity was low, between 1500–15000 glochidia per female, and positively correlated with body length. Only one annual gametogenic cycle was identified, leading to a short-term reproductive period in spring (tachyticity). The reproductive cycle was similar in the two years sampled, between which hydrology and water quality differed greatly. The presence of glochidia was synchronized with the period when the proportion of fish hosts (genus Squalius) was higher in the fish community. Unio tumidiformis seems to be well adapted to the natural variability in flow in temporary Mediterranean-type streams. Modifications of the natural flow regime caused by climate change or increased water exploitation may lead to rapid declines in mussel populations.