Fitness Consequences of Maternal Effects inStreblospio benedicti(Annelida: Polychaeta)

Abstract

SYNOPSIS. The degree to which a female partitions resources between fecundity and per offspring investment is a central question in life-history theory. Maternal effects may influence the nature of this tradeoff through their effect on per offspring investment and subsequent offspring fitness. The purpose of this study was to determine the effect of female age and size on brood size (number of offspring), per offspring investment, and fitness in the polychaete Streblospio benedicti . Early stage embryos were collected from brooding females of known age and size over a period of 100 days; these embryos were counted and analyzed for their C and N content. Female size had a positive effect on brood size; larger females produced larger broods. However, brood size decreased with female age (females did not increase in size after reaching sexual maturity). Brood size declined 20–46% between 60 and 160 days of age. During this same age period per offspring investment, measured in terms of C and N, increased by 25%. Offspring survivorship and size at two weeks post-release from the female were used as measures of offspring fitness. Offspring survivorship increased 28% between 60 and 160 days of age. Increased growth in offspring from older females resulted in a 23% increase in offspring size at two weeks. Including the maternal age effect in two population models for S. benedicti increased population growth rate (λ). Population growth was increased to a greater degree when the maternal effect was modeled by enhancing offspring survival compared to when fecundity was increased by the same proportional amount. This suggests that the maternal effect may be adaptive, particularly when conditions for offspring survival and growth are poor.