Evidence for environmental sex determination in the American eel, Anguillarostrata

Males have predominated among migrating silver eels in the Annaquatucket Ri ver, Rhode Island, for at least two decades, with no significant variation in mean total length in either sex. Because the species is panmictic (rando m breeding), this consistency suggests environmental sex determination (ESD ). Most yellow (feeding phase) eels < 300 mm total length in the Annaquatuc ket are sexually undifferentiated, and in contrast to all other published s ex ratios, males greatly outnumber females (3:1) among differentiated yello w eels. Estimates of yellow eel population densities are 4-10 times greater than published values for other habitats. We propose that this crowding re sults in a long period of undifferentiation and the suppression of femalene ss. Published field and experimental evidence indicates that high populatio n density results in high proportions of males in Atlantic Anguilla, and th at low population density results in the predominance of females. This ESD may be adaptive, resulting in vast numbers of small males in coastal habita ts, relatively close to the spawing area, and much larger and more fecund f emales that occupy most of the available eel habitat.