LIFE-HISTORY PATTERNS IN THE BROODING FRESHWATER BIVALVE PISIDIUM (SPHAERIIDAE)

Abstract

Of all freshwater molluscs, the life history of sphaeriid bivalves has been one of the best studied, partly because they brood their young. In Pisidium, one of five genera in the Sphaeriidae, offspring develop and are released synchronously and therefore the reproductive state is easily determined from collections of adults. This study examines life-history trade-offs, morphological constraints on reproductive effort, intraclutch variation and brood production differences in different habitats in the pea clam, Pisidium. My results challenge previous ideas about life-history trade-offs in the pea clams and are as follows: (1) there is a trade-off between offspring number and size, larger embryos being found in smaller clutches when controlling for adult body size; (2) total embryo area is highly correlated with total gill area, but the measured maximum total area of embryos relative to gill area is rarely achieved; (3) intraclutch variance itself varies non-normally across Pisidium and does not appear to relate strongly to other measured variables; (4) creeks, ponds, springs and marshes appear to have higher relative per-clutch brood production than lakes in the best-sampled taxon, P. casertanum. Relative brood production is lower in high alpine lakes (above 3350 m), perhaps due to low dissolved organic material and ions.