Growth dynamics in natural populations of Sphaeriidae clams (Sphaerium, Musculium, Pisidium)

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Abstract
Analyses of life history samples of 5 spp. of sphaeriid clams collected at least monthly for 1 yr from 10 different habitats show that growth of all larval stages stops when parents cease to grow. During the log phase of growth, larvae grow slower than parents in temporary and permanent pond populations of M. lacustre, P. casertanum and P. variabile, as fast as parents in a river population of S. fabale and as fast or faster than parents in 5 populations of M. securis. Species with slow larval growth rates are usually semelparous and univoltine but can be iteroparous by precocious birth of larvae and multivoltine by accelerated growth of semelparous individuals. Species with rapid larval growth rates are usually iteroparous because larvae grow faster than parents and (or) there is precocious birth of larvae. There is greater mortality of larvae during early stages than during later stages of larval development. The greatest numbers of larvae per parent usually occur in early winter and (or) late spring: winter and summer kills often result in small litter sizes.