Meeting Abstract

62.6  Wednesday, Jan. 6  Can Asymmetric Dispersal Explain the Maintenance of Larval Dimorphism in the Benthic Polychaete Streblospio benedicti ? ZAKAS, C*; HALL, D; Univ. of Georgia; Univ. of Georgia zakas1@uga.edu

Offspring dimorphism is an unusual life history occurrence yet it has evolved in a broad range of taxa. The rarity of this strategy suggests that the circumstances under which two offspring modes can be maintained within a species may be very narrow, or that such a strategy is unstable and indicates an evolutionary transition. Streblospio benedicti is one such species that exhibits a larval dimorphism where individuals in the same population produce either small (~70µm) or large (~150µm) planktonic offspring. Small offspring have an obligate planktonic phase that lasts 2-3 weeks before settlement. Large offspring are competent to settle in only 1-8 days. This difference in development time could potentially lead to large differences in larval dispersal ability, and suggests a trade-off between larval retention within a site and migration to a new site. Our objective is to determine whether the difference in migration and retention between the two morphs is sufficient to maintain the two modes within a population. We use an analytical population model to address this question. When dispersal between sites is asymmetric, implying that some suitable habitats within a population are upstream from others, the model predicts stable maintenance of both types for some parameter combinations. In order for an equilibrium to exist, small larvae migrating downstream must have higher survival than those migrating upstream, and the fecundity through retained larvae must be relative low for small larvae compared to large larvae. We conclude that the maintenance of larval dimorphism in S. benedicti could reasonably be explained by asymmetric migration among suitable habitats.