Ecological barriers to gene exchange in the common water snake (Natrix sipedon).




Pettus, D.

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University of Texas at Austin


Natrix sipedon clarki and N. s. confluens are parapatric, interbreeding races of the common water snake. The former is typically limited to salt marsh habitats along the Gulf coast and the latter is found in a portion of the adjacent fresh water associations. An analysis of specimens collected along a transect perpendicular to the zone of contact between these two races, from Galveston, Texas, through the San Jacinto River estuary to Lake Houston, showed a gradient in morphotypes corresponding to a similar gradient in environments. Laboratory experiments demonstrated that individuals of the brackish water race possess a much higher tolerance to salinity than members of the fresh water population. The data suggest that N. s. clarki maintains its water balance by making use of water contained in the body of its food and water produced through oxidative metabolism if no fresh water is available. It apparently does not, however, drink seawater. A series of salinity discrimination and preference tests showed that individuals of both races preferred fresh water to sea water when they had only the alternatives of being partially immersed in one or the other. Skin permeability of the two races was tested with an osmometer. No water transfer through this structure could be demonstrated in either form. This leaves only the digestive tract by which water may enter the body. Differences in kidney structure were sought in a cursory histological investigation. No differences were found in this study; however, refinement of technique and more diligent searching might show such a conclusion to be premature. The dynamics of ecological barriers to gene-flow between N. s. clarki and N. s. confluens are discussed in relation to isolating mechanisms that are operating


87 p., Dissertation


common water snake, aquatic reptiles, marshes, salt marshes, fresh water, salinity tolerance, biotic barriers, genes