MORAN, A.L.; Univ. of North Carolina, Chapel Hill: Egg Size Evolution in the Tropical American Oceans

The rise of the Central American Isthmus had strong effects on the marine biota of Neogene tropical America, including widespread extinctions of marine taxa in the Western Atlantic (WA) and life history shifts among marine species with free-living larvae. In both echinoderms and bivalves, egg sizes are larger in the WA than in the eastern Pacific (EP) when comparisons are made between species pairs in which one inhabits the WA and its closest relative is found in the Eastern Pacific (EP) (= �geminate� species). Because the WA is much less productive than the EP, large eggs in the WA may represent an adaptation to offset the poorer larval feeding environment in that ocean. Here I use both phylogenetic data and the fossil record to test the hypothesis that present-day egg size patterns in bivalve geminates evolved in response to the well-documented drop in productivity that occurred in the WA around the time of Isthmus closure. Phylogenetic ancestral character state reconstruction using 17 arcid species indicated that for all geminate pairs, each living Pacific species has smaller eggs than the pair�s common ancestor. Data from the fossil record also support the EP as the site of the most egg size evolution; in one geminate lineage, egg size has been large in the WA from the Miocene to the present, indicating that evolutionary change likely occurred as egg size reductions in the EP. These data suggest that if egg size of planktotrophic species does indeed evolve in response to changing oceanic productivity, downward shifts in egg size in response to increased productivity are as or more likely to occur than increases in egg size in response to lowered productivity.