Blocks of Identity by Descent Reveal the Dual Role Played by Reef Fish Larvae for Retention and Dispersal.

Isolation by distance, whereby genetic distance between individuals or populations increases with geographic distance, is a fundamental population genetic pattern that is observed across a broad range of taxa and spatial scales. This pattern can be driven by dispersal limitation and be thereby informative about dispersal rate ( σ ), a parameter of both basic and applied interest. Yet isolation by distance is typically directly estimated from microsatellite or single nucleotide polymorphism data, which does not permit the direct estimation of σ or allow to disentangle dispersal limitation from other processes. Blocks of identity by descent (IBD) provide another lens through which isolation by distance may be viewed, and the opportunity to overcome these limitations. Here, we apply this framework to the barred hamlet (Hypoplectrus puella), a reef fish with a 3-week planktonic larval phase. We consider two spatial scales, a continuous stretch of 200 km of reef along the Mesoamerican Barrier Reef in Belize and the Greater Caribbean across > 3000 km. The isolation by distance patterns show a good fit with theoretical expectations in Belize, and provide a recent σ estimate of 31 km / gen (95% confidence interval 23-40). The data also show a good fit with theoretical expectations at the Greater Caribbean scale, which is remarkable given the patchy nature of coral reef habitat at this scale, and provide a recent σ estimate of 248 km / gen (95% confidence interval 231-265). These results highlight the dual role played by coral reef fish larvae, which mediate both local retention and long-distance dispersal.