Maintenance of species boundaries within social aggregations of ecologically similar goby sister species

The maintenance of species boundaries when opportunities for admixture are abundant, is a poorly understood phenomenon for many taxa. While many mechanisms for maintaining species boundaries have been described their relative importance depends largely on the particulars of the system in question. Aggregating social behavior can be a means to keep sympatric sister species distinct if it leads to segregation during reproduction. The widespread Caribbean reef gobies Coryphopterus personatus and C. hyalinus are sympatric sister species with nearly identical morphology that spend their entire adult lives in shoals in which reproduction occurs. To date no studies have investigated whether shoals are species-specific, which would be expected if aggregating behavior helps to maintain species boundaries. To address this, the species of individual fishes collected from 16 shoals were identified using morphology, mitochondrial sequence data, and microsatellite allele frequencies. Levels of admixture between the species were also assessed. Shoals were generally composed of both species in similar proportions to their relative abundances on the reef, where the shoals were found, indicating that the species are not behaviorally segregating. For most specimens, morphological, mitochondrial, and nuclear data were congruent with a single species, but 18 individuals showed disagreements with microsatellite genotypes of 16 suggesting some level of historic/contemporary admixture. Of these, two were identified as likely first- or second-generation hybrids or backcrosses. Despite co-occurrence and evidence of some gene flow, the two species show little admixture overall suggesting that microscale differences in breeding site selection, allochrony, and/or cryptic mate choice may play an important role in the maintenance of species boundaries despite cooccurrence well within the range typically thought of as sympatry.