Morphological divergence of Eastern Pacific and Caribbean isopods: effects of a land barrier and the Panama Canal

The Isthmus of Panama rose approximately 3 million years before the present (mybp) and isolated biotas in the tropical eastern Pacific from those in the Caribbean Sea. Populations that were split by the Isthmus and have evolved in allopatry since that time are known as geminates. The surf zone/beach isopod Excirolana braziliensis Richardson was examined between 1984 and 1989 to test the hypothesis that divergence in geminate isopod morphology has occurred, and that geminate divergence is greater than divergence between local populations from the same coastline. Three morphs of Excirolana braziliensis, one in the Caribbean and two in the eastern Pacific, were discovered using numerical taxonomic methods that adjust for body size. The two Pacific morphs have overlapping large-scale distributions, but those morphs are segregated on a smaller scale by beach. We inferred that one Pacific morph and one Caribbean morph were geminates, based on their relative similarity in shape, their geographical ranges, and natural history information about the organism's dispersal capabilities. The origin of the third morph probably predates the Isthmus of Panama, given its relative dissimilarity from the geminate morphs. The presumed geminates differ primarily with respect to the rostrum, antennae and one male reproductive structure. Divergence between geminates is greater than divergence between local populations of any morph along a coastline. Because only one morph occurs in the Caribbean, that region contains less morphological variation than the eastern Pacific, which contains two morphs. There was weak evidence that some introductions may have taken place in the last century from the Caribbean to the Pacific; however, introductions have not masked the pattern of divergence that has developed over millions of years.