A thermal gradient (ca 12 to 38 C), established in a trough containing a shallow depth of water, was used to study the behavior of Biomphalaria glabrata (= Australorbis glabratus), Bulinus truncatus, and Lymnaea palustris under uniform conditions of light. Twenty snails were used per trial and 10 trials constituted each experiment; groups of control snails were observed under similar conditions except for the absence of the thermal gradient. The location of snails in the trough was recorded 45, 60, 75, and 90 min after the start of each trial. Regardless of the initial pattern of placement in the gradient, normal, well-fed B. glabrata (8 to 10 mm diam.) generally avoided both thermal extremes, tended to avoid most of the cooler region, and accumulated with greatest frequency in zones approximating 27 to 32 C. These responses to the gradient were substantially unaltered when B. glabrata harbored immature or mature infections with Schistosoma mansoni, when large snails (15 to 19 mm) were used, or when near-total darkness was superimposed. However, well-fed B. glabrata congregated on wheat germ or lettuce when these foods were placed in a cool zone normally avoided by the snails. In the absence of food, B. glabrata which were starved prior to study, or which were well-fed but only 3 to 5 mm diam, became rather uniformly distributed in the gradient. Adult B. glabrata, L. palustris, and B. truncatus differed in their distributional responses to the thermal gradient; thus, B. glabrata were generally found in moderately warm loci, L. palustris tended to accumulate in the relatively cool portions of the gradient, and B. truncatus distributed themselves fairly uniformly throughout the gradient. The general findings may contribute to a better understanding of some of the influences underlying the localization of discontinuity of natural colonies of snails. Many aspects of the biology of normal and of trematode-infected snails are influenced by temperature. In most studies involving this parameter, snails have been confined at specified temperatures under conditions which precluded avoidance or escape. The following experiments were done to explore the behavior of Biomphalaria glabrata (= Australorbis glabratus), and of other snails, free to range in a thermal gradient. MATERIALS AND METHODS General Experiments were done in a temperature-controlled room (25 to 27 C) from which natural light was excluded. A bank of ceiling-level fluorescent lights, paralleling the long axis of the trough described below, provided an illumination of 50 to 60 foot candles at the surface of the trough. Snails used in most experiments were derived from the PR-2' strain of Puerto Rican B. glabrata maintained at 25 to 27 C on romaine lettuce. This strain originated in 1959 from one snail of the PR-2 strain (Pan, 1965). Except as noted, snails were 8 to 10 mm in diameter and were well supplied with lettuce until used. Different groups of snails were studied in successive sets of trials; some of the snails were reused on occasion, but only Received for publication 13 September 1967. * These studies were supported in part by Research Grant AI-00513 and by a Research Career Award from the NIAID, U. S. Public Health Service. after several days of normal maintenance had intervened. Certain experiments involved B. glabrata which had been infected, when small, by mass-exposure to miracidia of a Puerto Rican strain of Schistosoma mansoni (Chernin and Dunavan, 1962). In other experiments two additional species of snail were studied: Lymnaea palustris, 30 to 35 mm in length, maintained in this laboratory since 1958 (Michelson, 1961a); and Bulinus truncatus (? Liberia), 9 to 11 mm in length, maintained here since their receipt from Dr. Henry van der Schalie in 1963. These snails were liberally supplied with lettuce until used in experiments. Experimental conditions The temperature gradient was produced in a rectangular trough (88 by 10 by 7 cm) made of clear plastic (1/4 inch, Acrylite?). The trough (Fig. 1) was level and supported by rubber pads on a work bench. White blotting paper beneath the trough provided a uniform background. The central 70 cm of the trough was divided transversely into 14 numbered zones by parallel lines ruled 5 cm apart on the underside. Each end of the ruled area within the trough was bounded by a vertical stainless-steel screen (16-mesh per inch, 0.018 gauge wire) set transversely. In the space beyond each screen, a coil of 1/-inch aluminum tubing extended horizontally from wall to wall and rested on the floor of the trough. A circulating pump and lengths of rubber tubing conveyed hot water from a constant temperature bath through the coil set beyond zone 14; cold water was circulated through the opposite coil from an ice-water bath or from a refrigerated-heated bath and circulator (model 2093, Forma Scientific, Marietta, Ohio).
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Sep 20, 1992
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