Tulane Studies in Zoology, Vol. 6 (Classic Reprint)

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Excerpt from Tulane Studies in Zoology, Vol. 6Fresh water turtles of the northern coast of the Gulf of Mexico are excellent materials for a study of speciation. Populations of species of several genera occupy approximately the same geographic areas and may have been exposed to the same modifications in drainage systems and geological changes in the Gulf coast.Turtles of the genus Sternothaerus are restricted to the United States and southern Canada. Three species (four forms) are found in the Gulf coastal plain and some adjacent areas, and presumably have undergone the major part of their evolution there. These four forms constitute the Sternothaerus carinatus complex. Because of its geographic position, a study of this complex utilizing a field approach with a laboratory analysis was feasible.Analysis of selected morphological characters as a means of evaluating relationships within the Sternothaerus carinatus complex was the first objective. The populations of each major drainage system were considered separately in order to gain a better understanding of geographic variation.The second objective, to obtain information on behavior, habits and ecology of these turtles, was accomplished primarily in the field, but a few laboratory experiments and observations were made.The third objective was to correlate the differentiation of the Sternothaerus carinatus complex with changes in river systems, stream capture, and changes in coastline elevation. This objective was difficult to attain because the geological history of the Gulf coast is inadequately known.The last objective was to compare the speciation pattern of the Sternothaerus carinatus complex with that discernible in the turtle genera Pseudemys, Graptemys and Trionyx that occur in the same area. The details of speciation in the last three groups have been worked out by other investigators, and the writer has had considerable field experience with many representatives of these genera.II. MethodologyCollection of samples. - Collections were made in most Gulf Coast rivers from central Texas to Florida and north into Arkansas and Tennessee (fig. 1). Most collections were made during the summer because this was the only time available for the extensive trips required for this type of study. Supplementary collections were made at other seasons in a few localities.Traps made of three-quarter inch seine netting yielded the greatest number of specimens. These traps were approximately 30 inches long, 12 to 14 inches in diameter with an open throat at each end. The traps were placed near stumps and brush in shallow, generally quiet-water areas of rivers. Trapping was the most reliable means of obtaining Sternothaerus.A second means of sampling was from brushpiles at night, using the method of Chaney and Smith (1950). This method, the best known for obtaining emydid turtles, is less effective for collecting Sternothaerus.A third method was successfully employed on two occasions. In the Suwannee River near Fannin Springs, Gilchrist County, Florida, Sternothaerus m. minor was obtained by backing a 7.5 horsepower outboard motor up to the bank near tangled cypress roots and accelerating the motor to stir up the water. This action attracted the turtles or washed them out of sleeping places under the bank. On June 7 to 8, 1953. 20 S. m. minor were taken by this method in about two hours. In the Wacissa River of northern Florida on June 5, 1953, Mr. Robert Webb and I swirled a boat in and out of weed beds in shallow water at night, and collected 34 Sternothaerus that came up beside the boat during a three-hour period. Whether the water current from the motor stirred the turtles to the surface or whether they were attracted by the sound of the engine could not be determined. In August 1954, the locality was revisited, the swirling technique employed, but no turtles were seen. Diving in the same area dur...