INTRA VERSUS INTERSPECIFIC DIFFERENCES IN LARVAL SHELL SIZE AND THEIR MACROEVOLUTIONARY SIGNIFICANCE

Numerous phyla exhibit parallel divergent evolution in larval ecology across the Isthmus of Panama. Patterns within the bivalve family Pectinidae based on species occurrences generally agree with other groups: egg size is smaller and larval duration is longer in the eastern Pacific than in the Caribbean. However, regional variation is much greater in the Caribbean. This begs the question of whether these interspecific patterns are due to differential representation of species within regions as a result of differing environmental conditions and species tolerances, or reflect intraspecific patterns of variation.

We tested this by analyzing larval pII shell sizes within 8 species from the two oceans. The four Caribbean species exhibited higher variation in pII shell size than their congeners in the eastern Pacific. However, the strong interspecific environmental trend observed in the Caribbean was not observed in the intraspecific analyses. PII shell sizes of two of the four Caribbean species were significantly correlated with environmental conditions while the other two were not. Moreover, differences between oceans in larval shell sizes were much smaller relative to the interspecific pattern, though still significant between two of the four congeneric pairs. Our results support earlier studies that hypothesized within species variation should be relatively constant when compared to interspecific macroevolutionary patterns. Differences between regions and oceans are the result of differential representation in both abundance and occurrence of species, compounded by lower levels of within species variation.