Seasonal allocation of energy to respiration, growth and reproduction in the freshwater clams, Pisidium variabile and P. compressum (Bivalvia: Pisidiidae)

SUMMARY. 1. The ecological energetics of the pisidiid clams, Pisidium variabile and P. compressum , from permanent ponds in southwestern Ohio were studied from July 1978 to July 1980. Productivity (P; as g C m^?2 yr ^?1) was 1.82 (0.80 and 1.02 for the spring and autumn generations) and 4.22 (2.69 and 1.53 for the spring and autumn generations) for P. variabile and P. compressum , respectively.
2. The average biomass (B; as g C m^?2) of P. variabile was 0.54 (0.30 and 0.24 for the spring and autumn generations) and of P. compressum 1.09 (0.69 and 0.40 for the spring and autumn generations). P:B ratios for the spring and autumn generations of P. variabile were 2.7 and 4.2 and of P. compressum 3.9 and 3.8, which are below those reported for other iteroparous invertebrates.
3. Reproductive effort (Re:Px100) was 5.1 and 5.7 for the generations of P. variabile and 7.7 and 8.8 for the generations of P. compressum. These low values relative to other molluses can be attributed to a viviparous mode of reproduction and exposure to local environmental variability. In addition, the differences between the reproductive efforts of the spring and autumn generations of both species can be ascribed to season of birth and altometric constraints on reproductive capacity.
4. The data from this and related studies on pisidiid clams are used to generate several hypotheses concerning the evolution of life cycles in the family: (1) reproductive effort is limited by allometric constraints on clam size; (2) individuals of different ages can have different demographies and physiological rate functions; and (3) current one-dimensional life history theories are largely inappropriate when dealing with an organism that has a wide range of phenotypic responses to environmental variability as well as complex age structure.