A Method for the Relaxation and Immobilization of Pulmonate Snails

Abstract

In studies on the biological control of schistosome-bearing snails, the inoculation of microorganisms into pulmonate snails proved to be exceedingly difficult. Methods available for relaxing mollusks were ineffective. Most of the chemical agents recommended in the past for relaxing mollusks, including chloral hydrate, cocaine, carbon dioxide, chlorotone, nembutal, and aqueous and alcoholic solutions of menthol (Berry, 1943; Hubendick, 1954; Lee, 1950; van der Schalie, 1954), were intended primarily to facilitate killing, fixing, and to provide specimens for dissection. Our snails rarely recovered after being exposed to concentrations required for adequate relaxation. It has been our experience that prolonged exposure to certain of these chemical agents, as required for adequate relaxation, disturbed the osmo-regulatory mechanism of the snail's tissues, permitting an increase of hemolymph in the foot and neck sinuses resulting in swelling of these regions. Injection into these distended regions often resulted in profuse hemorrhage. In the present study a method was developed which results in complete exposure of the anterior portion of the snail's body and in preventing its subsequent retraction until inoculation is completed. Two inherent difficulties associated with inoculating snails, (1) the intense withdrawal reaction to tactile and chemical stimuli and (2) the lack of resistance offered by their tissues to positive pressure, are successfully controlled. The technique is a two-step procedure incorporating both chemical and mechanical action. In the first step, the snail is partially narcotized to facilitate subsequent mechanical extension and immobilization. Narcotization is accomplished by immersing the snail in a 0.5 to 1.0 percent aqueous solution of urethan2 (ethyl carbamate). The concentration of urethan and the interval of exposure are dependent upon the species and size of the snail. We have found that a concentration of 0.5 percent urethan and an exposure of three to five hours were suitable for Australorbis glabratus and Helisoma anceps having a shell diameter of 10.5 to 15.5 mm. Australorbis having a shell diameter greater than 16.0 mm. were kept overnight (15-18 hrs.) in a 0.5 percent solution. In a series of experiments (Table 1) it was observed that Australorbis would recover from exposures of 48 hours at concentrations of 0.5 percent urethan. A proper degree of narcotization generally results in the protrusion of the preputium.