Genetic population structure in two tropical sponge-dwelling shrimps that differ in dispersal potential

The spatial context in which host races of parasitic animals originate is a central issue in the controversial theory of sympatric speciation. Sponge-dwelling shrimps in the genus Synalpheus provide a good system for evaluating the possibility of resource-associated divergence in sympatry. I used allozyme electrophoresis to assess the genetic population structure of two Caribbean Synalpheus species sampled in 1988 to 1990 at a hierarchy of spatial scales. S. brooksi Coutière is a host-generalist, using several sponge species in an area, and develops directly, with no planktonic larval stage. G-tests and estimates of F _ST revealed highly structured populations in this species, with significant differentiation among samples from individual reefs within a region, and strong divergence among regions (Panama, Belize, Florida). Moreover, samples of S. brooksi taken from the two sponges Spheciospongia vesparium (Lamarck) and Agelas clathrodes (Schmidt) in Panama, and separated by 3 km, showed significant differentiation at both of the loci that were polymorphic in these populations. Genetic distances between these host-associated populations averaged >60% greater than distances between samples from the same host species and were comparable to, or greater tha, those for some inter-regional comparisons. These genetic data corroborate a previous finding of demographic differences between the same populations. The second species, S. pectiniger Coutière, occurs only in Spheciospongia vesparium, and produces swimming larvae. Although allele frequencies in this species differed significantly among the three regions, S. pectiniger showed no differentiation within regions, and significantly lower differentiation (F _ST) among regions than its direct-developing congener. These data suggest that genetic population structure in these two commensal crustaceans is related to dispersal potential, and that restricted dispersal may allow the divergence of host-associated populations on a local scale.