Advances in freshwater decapod systematics and biology

Freshwater habitats of the Caribbean island Jamaica are unique, in so far that they are not inhabited by freshwater Decapoda Reptantia with a long evolutionary history in fresh water, like crayfish or old lineages of freshwater crabs. Instead, a relatively young invasion and radiation of originally coastal crabs from the family Sesarmidae took place, resulting in currently ten endemic sesarmid species that are recognized from the island. Six of those have been described from Jamaican brooks and streams (river crabs), whereas the other four species thrive in caves and more terrestrial habitats. After establishing and describing the diversity of Jamaican river crabs at the species level, ongoing studies are designed to highlight presumed intraspecific differentiation within the recognized species, as a means of understanding the diversification and rapid speciation processes of this adaptive radiation. Here we use mitochondrial DNA sequences of the ND1 gene and morphometrics to document diversity within river crabs from western Jamaica so far considered to belong to Sesarma dolphinum, complementing a recent population genetics study with nuclear DNA. Distinct evolutionary lineages can be recognized, of which two are so clearly separated that they do not share mitochondrial haplotypes nor do they show any overlap in morphometry. Interestingly, these lineages co-occur within the same river system, allowing first insights into the mechanisms of differentiation of these crabs. Ecological restriction to upper reaches of rivers isolates the crabs in different tributaries and thus genetic connectivity is apparently more likely to be maintained in overland dispersal between headwaters than within the river system. The distinct evolutionary lineage from the southeastern range of the distribution area of S. dolphinum is here described as a new species in order to highlight its uniqueness and to make it a management unit. A nested clade analysis reveals that the genetic relationship of populations of S. dolphinum is the result of restricted gene flow with isolation by distance. A literature review compiles other published reports for freshwater Crustacea with different evolutionary lineages in the same water catchment.