RAD-sequencing highlights polygenic discrimination of habitat ecotypes in the panmictic American eel (Anguilla rostrata)

The two primary ways that species respond to heterogeneous environments is through local adaptation and phenotypic plasticity. The American eel (Anguilla rostrata) presents a paradox; despite inhabiting drastically different environments, the species is panmictic. Spawning takes place only in the southern Sargasso Sea in the Atlantic Ocean. Then, the planktonic larvae (leptocephali) disperse to rearing locations from Cuba to Greenland, and juveniles colonize either freshwater or brackish/saltwater habitats, where they spend 3–25 years before returning to the Sargasso Sea to spawn as a panmictic species. Depending on rearing habitat, individuals exhibit drastically different ecotypes. In particular, individuals rearing in freshwater tend to grow slowly and mature older and are more likely to be female in comparison to individuals that rear in brackish/saltwater. The hypothesis that phenotypic plasticity alone can account for all of the differences was not supported by three independent c...