Three ponds containing Biomphalaria glabrata, the snail host of schistosomiasis, and other aquatic snails were studied in Puerto Rico for over a year to determine birth, death. and growth rates of the snails. A sampling system involving an Emergy dredge was used to estimate the total number of snails in the ponds as well as the amount of vegetation. In addition, water temperatures and pond size were recorded at each biweekly sampling. The results showed that B. glabrata lived less than 200 days in the field, about a third of their life span in the laboratory. In contrast, the ampllarid snail, Marisa cornuarietis, had a life span of over 500 days in the field, providing a partial explanation for its domination of one of the ponds to the exclusion of B. glabrata. It was also shown that adult B. glabrata were highly resistant to desiccation and that water temperature was an important environmental parameter influencing the number of snails in a habitat by controlling the seasonal pattern of reproduction.
Formerly assistant chief, San Juan Laboratories. Present address: 7 Philip Lane, Foxboro, Massachusetts 02035.