Meeting Abstract

S3-1.4  Wednesday, Jan. 4  Ecological triggers and evolutionary consequences of alternative larval types in sea slugs KRUG, P.J.*; GORDON, D.; California State University, Los Angeles

Sea slugs in clade Sacoglossa evolve lecithotrophy at a high rate, and five species express larval dimorphisms. Poecilogony may facilitate shifts in larval type and could promote speciation, but we have little understanding of how variable development affects marine macroevolution. I present data for two sea slugs with contrasting forms of poecilogony: Alderia willowi, in which larval type changes seasonally, and Costasiella ocellifera, with populations fixed for planktonic or non-planktonic larvae. Native to Californian mudflats, A. willowi is unique because larval type can vary among clutches laid by the same mother. Field populations shift from lecithotrophy in summer and fall to planktotrophy in winter. In lab experiments, juvenile rearing environment strongly influenced the type of offspring produced by adults. Slugs raised at high temperature and full salinity (dry, summer conditions) produced lecithotrophic larvae, whereas slugs reared under cool temperatures and low salinity (wet, winter conditions) laid planktotrophic eggs. This is the first demonstration of seasonal polyphenism in larval type, and may be an adaptation to annual closures of Californian estuaries during the dry season. In contrast, the Caribbean C. ocellifera is planktotrophic in all but two populations that produced entirely non-planktonic larvae over a six-year study period. Lecithotrophic populations are self-recruiting, and show strong pre- and post-mating reproductive isolation in crosses with slugs from nearby planktotrophic demes. These results indicate that mating barriers quickly form in populations that lack a dispersive stage and receive immigrants from planktotrophic demes, likely to prevent outbreeding depression. This is the first experimental evidence that poecilogony may drive speciation in the sea, and could explain why sister species in diverse taxa often differ in larval type.