Transplant Studies With Physa anatina Lea (Gastropoda) in Andrews County, Texas

Abstract

Transplant studies and water analyses indicate that the absence of P. anatina in some stock ponds in the Southwest is due to unfavorable conditions rather than to failure of the snails to disperse. INTRODUCTION AND METHODS In portions of the southwestern United States there are numer- ous artificial stock ponds. In many cases the water supply for these ponds is derived from a subterranean well equipped with a windmill. Water is pumped into a metal or concrete open ground tank supplied with an overflow, allowing water to enter a dirt holding pond. Thus, there are two bodies of water, the tank and the pond, both of which are usually rather small. An average tank holds about 150 ft3 of water and a typical pond has a capacity seldom exceeding 20,000 ft3. Since there is a rather constant water supply, stock ponds of this type are permanent. Pennak (1958) emphasized the importance of these aquatic habitats as stepping stones enabling invertebrates to dis- perse with greater ease. Pennak further indicated that the biology of western stock ponds has been entirely neglected. From September, 1963, to September, 1964, a total of 49 such ponds were inspected in Andrews County, Texas. At least 20 were visited each month, with the exception of December, and all 49 were checked during November, March, and May. A total of 18 of the ponds were inhabited by Physa anatina Lea. Snails were found in both the tank and the pond at each of these locations. Since snails were not found in 31 of the ponds, an obvious ques- tion is whether the species is absent because it has not been trans- ported there or because the habitat is unsuitable for its establishment. All the ponds are located in mesquite pasture land having sandy soil, and from outward appearances the environments of all are similar. But since they are supplied by ground water, it is likely that the chemical characteristics vary considerably from pond to pond. To be uniform habitats, the water supply of each pond would have to be derived from the same underground strata or stream. Such is probably not the case since the ponds are spread over an area of about 625 square miles. To obtain information bearing on the problem, ten stock ponds that contained no snails were chosen for transplant studies. If the habitats were suitable, introduced snails would presumably thrive, suggesting that their previous absence from the ponds was due to a lack of dispersal. On the other hand, if the snails failed to survive, unfavorable habitats would be indicated, and natural introduction of the species would not be expected to result in its successful establish- 325