Hybridization among Caribbean damselfish species correlates with habitat degradation

Caribbean damselfish species (genus Stegastes) are permanently territorial, display obvious aggressive behavior related to territory defense, and have divergent habitat preferences for primary territory substrates. Territory quality is directly tied to variation in reproductive success among individuals, which suggests that ecological isolation plays a key role in the maintenance of species boundaries in this genus. If so, hybridization is expected to occur among closely-related damselfish species when there is a limitation in the availability of preferred primary habitat. To test this prediction, we collected damselfish from both high disturbance (Jamaica) and low disturbance (Barbados) coral reef environments, and looked for evidence of hybridization among species based on both morphological variation and discordance between mitochondrial and nuclear genetic markers. Consistent with the hypothesis that species boundaries in this system are maintained by strong substrate preferences and aggressive territoriality, we found evidence for extensive hybridization in Jamaica, where intense hurricanes have led to considerable habitat degradation over the last 30 years. In light of ongoing climate change, this result suggests that hybridization leading to a blurring of ecological species boundaries may represent an increasingly common yet underappreciated threat to many coral reef fish species.

► mtDNA and nuclear genealogies reveal hybridization among Caribbean damselfishes.
► Hybridization is correlated with habitat degradation caused by hurricane damage.
► Loss of habitat isolation leads to a blurring of ecological species boundaries.