Speciation in the Fishes of the Genus Menidia

The genus Menidia, which includes the common 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, M. au dens and M. extenso, 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. menidia, occur together along the Atlantic coast from Massachusetts to Florida, thus permitting a comparative analysis of geographic gradients.Furthermore.these same two species hybridize in Florida.Thus, all gradations from incipient 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 M cnidia; it is these four that are dealt with here.M. menidia is a saltwater form ranging from Nova Scotia to Florida."M, beryllina occurs in salt, brackish, and coastwise fresh water from Massachusetts to Vera Cruz.Mexico.M. audcns is found in fresh water in the lower Mississippi Valley, and M. extenso is confined to Lake Waccamaw, a faunistically isolated lake in North Carolina (Hubbs and Raney, 1946).The data to be presented here are focused on .71,1.bcryllina, for the stock of this species seems to have given rise to M. audens and M. extenso in one direction and hybridizes with M. mcnidia in another.The chief characters which have been used in differentiating the forms of the