Coevolutionary Implications of Obligate Commensalism in Sea Turtles: the Case of the Genus Hyachelia Barnard, 1967 (Crustacea, Amphipoda).

Obligate commensalism in the marine environment and its evolutionary role are still poorly understood. Although sea turtles may serve as ideal substrates for epibionts, within amphipods, only the genus Hyachelia evolved in obligate commensalism with turtles. Here, we report a new host record for Hyachelia lowryi on the hawksbill turtle and describe a larger distribution of the genus in the Atlantic Ocean on green and loggerhead turtles. Hyachelia spp. were sampled from nesting sites of Caretta caretta and feeding grounds of Eretmochelys imbricata and Chelonia mydas along the Brazilian coast. Insights regarding the coevolution of this remarkable genus with its hosts based on molecular analyses are inferred based on mitochondrial (COI) and nuclear (18SrRNA) genes using new and previously available sequences from the infraorder Talitrida. Divergence times for Hyachelia are around the Cretaceous (~127.66 Mya), corresponding to an ancient origin and in agreement with modern green turtle (Chelonioidea) radiation. Later, diversification of Hyachelia species is dated at about 26 Mya, suggesting a coevolutionary association between amphipods and Carettini/Chelonini sea turtles.