The Relation of Sodium and Water Balance to Survival in Sea Water of Estuarine and Freshwater Races of the Snakes Nerodia fasciata, N. sipedon and N. valida

Dunson, William

Subspecies of the Florida banded water snake (Nerodia fasciata) vary markedly in their tolerance to sea water. The freshwater race N. f. pictiventris differs physiologically from the estuarine races N. f. compressicauda and N. f. clarki in several important ways. When placed in sea water it has a higher body water influx and efflux, a higher body sodium influx, and its skin is more permeable to water and sodium. It is likely that the high rate of sodium influx, immediately after placement in sea water, is the primary factor leading to drinking of sea water and subsequent death. Thus the distinction between freshwater and estuarine races is not simply behavioral, as was previously believed, but is dependent also on physiological differences. Other freshwater snakes studied (N. sipedon, Regina septemvittata) also had greater rates of water influx than found in marine or estuarine species. The queen snake (R. septemvittata) has the highest rates of sodium influx and water exchange. Immediately after immersion in sea water, water exchange is primarily through the skin and sodium uptake through the mouth. Intergrades between N. f. pictiventris and N. f. compressicauda had the seawater tolerance of the estuarine race. It appears that these estuarine subspecies are in the process of evolving into true marine species. They may not have a salt gland, but they are capable of surviving long periods in saline habitats. A similar evolutionary development may be occurring among coastal populations of the Mexican water snake N. valida.

The Relation of Sodium and Water Balance to Survival in Sea Water of Estuarine and Freshwater Races of the Snakes Nerodia fasciata, N. sipedon and N. valida

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