Salinity as a Limiting Factor in the Distribution of Reptiles in Florida Bay: A Theory for the Estuarine Origin of Marine Snakes and Turtles
Salinity is hypothesized to be the major abiotic factor limiting the colonization of Florida Bay by estuarine reptiles, This premise is supported by the small number of species of reptiles found in the bay in comparison with fresh water, the distinct osmoregulatory specializations of the few estuarine specialists that occur there, and a remarkable cline in the ability to tolerate sea water found among modem-day estuarine and coastal reptiles. This latter cline in osmoregulatory abilities is believed to represent a model of the evolutionary stages through which pelagic snakes and turtles have passed in developing adaptations for life in the open sea. Florida Bay is an especially useful site for the study of such adaptations since it is the only known location in this hemisphere where three specialized estuarine reptiles are sympatrie: the American crocodile (Crocodylus aculus), the diamondback terrapin (Malaclemys terrapin), and the mangrove snake (Nerodia clarkii compressicauda). Small populations of freshwater turtles and the alligator also occur in tidal creeks along the northern shore. Recent advances in the study of turtles suggest that the single most important factor in determining tolerance to high salinity is the amount of sea water swallowed incidentally with food ingestion. This finding needs to be extended to other reptiles to test the hypothesis that fish eaters (such as snakes) that do not crush or bite chunks from their food have reduced incidental drinking. This could explain how the mangrove snake can survive in estuaries without a salt gland, whereas the sympatric terrapin possesses a sizeable lachrymal salt gland. We hypothesize that the following represent major transitional stages in the gradual evolution of marine snakes and turtles from freshwater ancestors: (1) utilization of behavioral osmoregulation to avoid salinities that cannot be directly tolerated; (2) a reduction in net salt uptake and water loss and in incidental drinking of sea water while feeding; (3) the first appearance of rudimentary salt-excreting glands; (4) hypertrophy of salt glands as dictated by rates of salt uptake, in concert with the development of a specialized external morphology suited for a pelagic life.
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Document Type: Research Article
Publication date: 1989-01-01
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