SPECIATION IN THE FISHES OF THE GENUS MENIDIA

Abstract

The genus Menidia, which includes the commoni silversides of the Atlantic and Gulf coasts of the United States, presents several aspects of interest for an evolutionary study. Members of this genus have repeatedly entered fresh water from the sea:-two full species, MU. audens and M. ertensa, have evolved from such invasions, and there are rather numerous examples of less-marked fresh-water differentiation within M. beryllina. Two species, M. beryllina and M. mitenidia, occur together along the Atlantic coast from Massachusetts to Florida, thus permitting a comparative analysis of geographic gradients. Furtlhermore, these same two species h-ybridize in Florida. Thus, all gradations from inicipient speciation to full specific differentiation, with and without hybridization, are found. According to the most recent treatment (Schultz, 1948), there are only four species in Mc enidia; it is these four that are dealt with here. MW. teunidia is a saltwater f6orm ranging from Nova Scotia to Florida. Al. beryllina occurs in salt, brackish, and coastwise fresh water from Massachusetts to Vera Cruz, Mexico. M. aztdens is found in fresh water in the lower Mississippi Valley, and Il. exteusa is confinled to Lake Waccamiaw, a fauLlnistically isolated lake in North Carolinla (Hubbs and Raniey, 1946). The data to be presented h-ere are focused oni AI. beryllina, for the stock of this species seems to have givenl rise to MA. audens and Al. ertentsa in onle direction an-d hvbriclizes with Al. wnenidia in anotlher. The chief characters wlhiclh lhave beeni used in differentiating the formiis of the