Tolerance of the Snail Thais Floridana to Waters of Low Salinity and the Effect of Size

Abstract

The snail Thais floridana is one of the prominent molluscs of the Gulf coast. It is well known to oyster fishermen there as a predator of oysters. The size of this animal is considerably smaller in the vicinity of Pensacola than it is west of the Mississippi; also, the architecture of the shells is typically different. Thais floridana floridana Conrad from Santa Rosa Sound is a smooth-shelled form, whereas Thais fioridana haysae Clench, obtained in Grand Bayou and from Sugar House Reef, Barataria Bay, La., is characterized by a double row of protruding knobs about the largest whorl of the shell. The identification of these two geographical varieties was kindly made by Dr. Paul Bartsch of the United States National Museum, from specimens sent to him. Snails from various localities were used together in these experiments in order to obtain a complete range in size. There did not seem to be any specific difference in their reaction to the experimental conditions. In view of the fact that salinity is one of the important ecological variables of confined coastal waters, such as obtain in Santa Rosa Sound and Barataria Bay, a study was undertaken of the effects of salinity changes, particularly from high to low salinity, on the snail Thais. The significance of the study is twofold. First, there is the consideration that animals living in or near the tidal zone must in one way or another exhibit a high degree of tolerance to salinity changes in order to survive. A study of behavior in relation to this environmental factor is therefore of general importance to the great animal majority which makes the littoral zone the most populated region of the sea. Secondly, inasmuch as the snail under investigation is an important enemy of oysters, and in view of the fact that grounds where both the oyster and the snail live are normally subject to dilution with river water, a study of the salinity tolerances of Thais may have practical value in determining the location of planted oyster beds. This hypothesis is based, of course, on the supposition that the oyster itself possesses a higher range of tolerance than does the snail. All information now available (communication from Dr. A. E. Hopkins) indicates that such is actually the case. The work here reported was done at the U. S. Fish and Wildlife Service Laboratory at Pensacola, Florida, during the first half of the year 1942. For the use of the laboratory and for extending opportunities for studies in the field, I am deeply indebted to the Director, Dr. A. E. Hopkins. Material was collected locally; and from Barataria Bay by courtesy of Mr. J. N. McConnell of the Department of Conservation, who kindly allowed the use of the facilities of the patrol boat Louisiana for the purpose.