Spatial aggregation , body size , and reproductive success in the freshwater mussel Elliptio complanata

The reproductive ecology of the freshwater, unionid mussel Elliptio complanata was studied by mapping a 6-m x 7-m segment of a population found in a uniform area of the sandy littoral zone of Lac de l'Achigan, Quebec. The contents of the marsupia were examined in mussels collected between spawning and larval release. Although unrelated to spatial aggregation, the number of ova carried by mussels varied with body size in a manner that suggests extremely late maturation followed by reproductive senescence in the largest mussels. Egg production was 1-2 orders of magnitude greater than that of other poikilotherms of equivalent mass. Fertilization success was strongly correlated with spatial aggregation, with complete fertilization failure found at local densities of < 10 mussels/m2, >50% successful when local densities were > 18 mussels/m2, and 100% successful only in patches where local densities exceeded 40 mussels/m2. Fertilization failure is probably frequent at mussel densities found in most lakes. Our data suggest that perturbations altering the density, aggregation, or size distribution of mussel populations may have serious consequences for the maintenance of viable populations.