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Life-History Variation in a Stream Population of Musculium partumeium (Bivalvia:Pisidiidae)

This study reports on the life-history traits of a stream population of the clam Musculium partumeium. The population displays two generations per year: a spring-born generation (SG) which reproduces twice (fall and following spring) and a fall-born generation (FG) which reproduces once in the spring. Traits for FG and SG generations are, respectively: maximum shell length, 5.1 and 6.1 mm; annual reproductive output (number of young born per average adult), 16 and 38; larval growth rate, 0.15 and 0.08 mm/wk; larval mortality, 67 and 48%; intrinsic rate of increase (r) 0.011 and 0.019/day. Compared with values for other populations of M. partumeium, our results were generally intermediate between those displayed by permanent pond populations (with 2 generations/year) and those of ephemeral pond populations (1 generation/year). This species appears to display a wide range of phenotypic plasticity in life-history traits.