Adaptive Radiation in Osmotic Regulatory Patterns among Three Species of Cyprinodontids (Teleostei: Atherinomorpha)

Comparisons of osmotic regulatory capabilities were made over a wide range of ambient salinities among three closely related cyprinodontids. Floridichthys carpio and Jordanella floridae were evaluated here and compared withprevious studies of Cyprinodon variegatus made by the senior author. Floridichthys is typical of nearshore marine areas, Jordanella of fresh waters, and C. variegatus ofsalt marshes, but in isolated instances the three species exist sympatrically. When groups ofacclimated individuals were used, Floridichthys showed bestplasma osmotic regulation at salinities from 435 to 2, 030 mosm/kg, while tolerating ambient salinities of 15 to 2,610 mosm/kg. Jordanella showed bestplasma osmotic regulation at ambientsalinities from 145 to 1,160 mosm/kg and tolerated salinitiesfrom that of fresh water to 2,320 mosm/kg. Cyprinodon variegatus showed goodplasma osmotic regulation over a wider range of ambient salinities than either of the other two, from fresh water to 2,030 mosm/kg, and tolerated ambient salinities from fresh water to 3,625 mosm/kg, greater than either of the others. All three species, regardless of present habitat, retain significant adaptive characteristics of euryhalinity.