Reproductive effort of winkles (Littorina spp.) with contrasted methods of reproduction

Abstract

Reproductive effort was compared in Littorina rudis (ovoviviparous), L. nigrolineata (benthic eggs with direct development), L. neritoides and L. littorea (planktonic eggs and larvae). Three indices of reproductive effort were used: the proportion of total production committed to reproduction per unit time, the cumulative proportion of total production committed to reproduction up to a given age, and the ratio of reproductive production per unit time to somatic biomass at the beginning of the unit time interval. The indices were plotted against age and the snails ranked in order of their reproductive efforts at equivalent ages. When plotted on axes of absolute time, all three indices ranked the snails in the order L. neritoides<L. rudis≈L. littorea<L. nigrolineata. The rank order of reproductive efforts on a time axis standardized for growth rate was L. littorea<L. rudis<L. nigrolineata≈L. neritoides, and on a time axis standardized for generation time was L. neritoides≈L. rudis<L. nigrolineata. The implications and relative merits of the different expressions of reproductive effort are discussed. No general relationship was found between the magnitude of reproductive effort and either reproductive type or population ecology among the Littorina spp., or among other intertidal prosobranchs reviewed from the literature. Semelparous species had among the highest reproductive efforts, but there was no clear separation in this respect from certain iteroparous species. Parental investment, measured as the energy content of an egg plus any accompanying jelly or capsular material, was one to two orders of magnitude greater in the species with direct development than in those with indirect development. There were significant differences between egg and hatchling sizes of L. rudis from three contrasted types of shore and these differences were thought to be correlated with the intensity of desiccation and predation on hatchlings rather than with demographic factors.