Spatial and temporal variability in dispersal and population genetic structure of a rockpool alga

Abstract

We describe the population dynamics of a rockpool alga and identify a novel mechanism that allows dispersal where it would otherwise be limited. Fucus distichus L. is a monoecious seaweed restricted to high intertidal rockpools. It reproduces from December to mid-May in Maine, USA. Densities vary greatly among pools (0.05 to 80 individuals m 2 ) and extinctions or declines in abundances occur periodically. Gamete release occurs at low, neap tides when pools are isolated from bulk seawater. We proposed that genetic structure among rockpools would be high, and genetic diversity within pools would be low. We used 4 microsatellites to characterize genetic structure and diversity in pools at Schoodic (3 sites) and Chamberlain (2 sites) in Maine. Genetic differentiation existed between Schoodic and Chamberlain (F ST = 0.146), among sites (F ST = 0.095) and among pools within sites (F ST = 0.237). An excess of homozygotes characterized most pools. Many adults in each pool were classified as first generation migrants, indicating high levels of past gene flow. Current gene flow (recruitment) occurred among adjacent pools. Laboratory studies on zygote attachment, and field data on gamete release were used to model dispersal potential. Attachment of zygotes at cold seawater temperatures (5°C) was slow enough to permit dispersal for at least 5 high tides following gamete release. The unusual reproductive seasonality of F. distichus may be an adaptation that mitigates ecological and evolutionary bottlenecks associated with its patchy habitat.