Do currents shape global patterns of hybrid richness in reef fishes?

Aim: Our main aim was to identify the distribution of, and potential mechanisms underpinning, hybrid-rich zones – regions with a disproportionate number of unique interspecific hybrids. We investigated whether coral reef fish hybrids coincided with factors such as phylogenetic relatedness, biogeographic barriers, species richness, geographic isolation, endemism, and oceanic currents. Location: Global. Time period: Contemporary. Major taxa studied: Coral reef fishes. Methods: We conducted a literature review and mapping to assess the taxonomic and global prevalence of hybridisation in coral reef fishes. We then fit Generalised additive models using a full-subsets and Bayesian framework to assess which variables are associated with hybrid-rich zones. Results: We found 143 unique interspecific coral reef fish hybrids involving 204 species – which accounts for approximately 7% of coral reef fish species, indicating that hybridisation is as common in the sea as it is on land. Characteristic coral reef fish families were not homogeneously represented in our dataset, with particularly colourful groups standing out. Mapping our dataset revealed that coral reef fish hybrids are found worldwide, though some ecoregions (e.g., the Christmas and Cocos (Keeling) Islands, South Kuroshio, Hawaii, and Eastern Philippines) are more hybrid-rich than others. Our analysis revealed that mean surface current velocity, phylogenetic relatedness, and geographic isolation were the best predictors of hybrid richness in a given location. Main conclusions: Phylogenetic distance between coral reef fish species may serve as a pre-condition for hybridisation to occur, lying between introgression and reproductive incompatibility. We also propose a novel mechanism, with oceanic currents driving long-distance larval dispersal events, transporting stray species to geographically remote sinks to maintain hybrid-rich zones.