Shell Mass, Biomass, Condition, and Reproductive State of the Invasive Bivalve Mytilopsis leucophaeata (Conrad, 1831) (Dreissenidae)

Basic information on shell dimensions, calcimass and periostracum mass, biomass, and the relation between the condition and reproduction of Mytilopsis leucophaeata was analyzed for a population in the North Sea Canal, the Netherlands. The mean dry flesh weight of individuals with a mean shell length of 12.5 mm showed a peak at the end of June. The mussel reached the highest mean dry flesh weight in the month before the spatfall started. The gonad index (GI) was high in June, followed by a period of decrease until September and a period of increase until November. The condition index (CI) peaked in June and decreased during the remainder part of the year. The range of CI was highest in May when the gonadal development progressed and was smallest in June when the gonadal development was complete. The CI decreased during the rest of the year. Quantile regressions showed a highly significant relation between CI and shell length. Based on the regression coefficients, the CI was maximal at a shell length of 11.4 mm and minimal at 12.5 mm. Condition index variation was highest within a shell length range of 10–15 mm.