Meeting Abstract
Broadcast-spawning coral species have wide geographic ranges, spanning strong environmental gradients, but it is unclear how much spatially varying selection these gradients actually impose. Strong divergent selection might present considerable barrier for demographic exchange between disparate reef habitats. Here, we investigated whether the cross-shelf gradient (nearshore – offshore – deep) is associated with spatially varying selection in two common coral species, Montastrea cavernosa and Siderastrea siderea, in the Florida Keys. We used 2bRAD to genotype 20 juveniles and 20 adults from each of the three reef zones, planning to look for signatures of selection occurring within a single generation. What we found instead was completely unexpected. Each species turned out to be composed of four (M. cavernosa) or even five (S. siderea) genetically distinct races, with gene flow between them highly reduced in 30-50% of the genome while being essentially unrestricted in the rest of the genome. Each species includes two sympatric races that are only found in the deep (20 m) habitat, while other races are found only in the shallower reefs (3-7 m). The two “shallow” races of M. cavernosa are also specialized for either nearshore or offshore: comparison between adult and juvenile cohorts indicates that cross-shelf migrants are 3 times less likely to reach adulthood than local recruits. In conclusion, cryptic genetic subdivision, often associated with environmental specialization, appears to be very common in these two coral species, which might in part explain why they are still ecologically successful in the Florida Keys. In addition, our results show that deep reefs cannot serve as refugia from climate change since corals there tend to be highly environmentally specialized.
