Adults of Helisoma trivolvis were collected from five field sites in central New York state during August 1985. An hypothesis of feeding selectivity was tested by microscopic observations of diatom composition of snail stomach contents and of Aufwuchs mats upon which individuals had been feeding. Analyses resulted in the generic identification of 10,000 diatoms including 88 species from nine families. Although ingestion and trituration generally reduced diatom diversity among stomach contents, statistical analyses showed no evidence of feeding selectivity in this herbivorous gastropod mollusc. Historically, the radula has played an important role in malacological taxonomy for two reasons. First, within a species, the numbers, proportions, and shapes of radular teeth are unique like a fingerprint. Secondly, radulae may be extracted from preserved museum material and quantified to provide a basis for specific identification. As a result of extensive study in the past, there exists a vast literature of radular line drawings and of elaborate morphological description. Although this work constitutes a thorough documentation of radular diversity within particular groups of molluscs, little is known about the adaptive significance of general features of radular ribbon architecture or of the finer details of radular tooth shape. Although reports of statistically-significant levels of intraspecific variation in radular tooth biometry have been made by Berrie (1959) and others (Hunter, 1975; Katsigianis & Harman, 1973; Oberholzer et al., 1970) little is known about the biological significance of these differences (Kool, 1987; Smith, 1987, 1989). In parallel analyses of intraspecific variation in the radula of Helisoma trivolvis (Say, 1817) (see Smith, 1987, 1989), trials were performed to determine whether established differences in tooth shape could be correlated with feeding preferences among diatoms. Two hypotheses were considered: (1) that snails do not feed selectively among diatoms in natural Aufwuchs; and (2) that snails do show feeding selectively among diatoms. If support for the alternate hypothesis was found, then additional questions regarding the adaptive significance of radular morphology in this animal could be addressed. This study parallels other work on feeding in pulmonate snails (Calow, 1970; Thomas et al., 1985) and complements investigations of the adaptive significance ' Work supported by grants from the Senate Research Committee of Syracuse University, the Theodore Roosevelt Memorial Fund, and from the Treves and Carscallen Funds of Wabash College. The author thanks R. B. Hanna and A. C. Day, N. C. Brown Center for Ultrastructural Studies, State University of New York, College of Environmental Science and Forestry, for the use of SEM facilities. 2 Present address: Wabash College, Department of Biology, Crawfordsville, Indiana 47933, U.S.A. TRANS. AM. MICROSC. Soc., 108(4): 394-402. 1989. ? Copyright, 1989, by the American Microscopical Society, Inc. This content downloaded from 157.55.39.92 on Wed, 22 Jun 2016 06:05:51 UTC All use subject to http://about.jstor.org/terms VOL. 108, NO. 4, OCTOBER 1989 of radular form in the Basommatophora (Berrie, 1959; Hunter, 1975; Oberholzer et al., 1970; Smith, 1987). Results also supplement earlier descriptions of the diatom flora of New York state (Hohn, 1951). MATERIALS AND METHODS Tests of feeding selectivity were made at five sites during August 1985: Eaton Reservoir, Eaton, New York (EA), 75?42.27'W, 42?51.10'N; Ithaca, New York (IT), 76?22.96'W, 42?25.78'N; Meadowbrook Pond, DeWitt, New York (MB), 76?07.08'W, 43?01.59'N; Otter Pond, Cato-Meridian, New York (OT), 76?32.83'W, 43?09.52'N; and Silver Lake, Remsen, New York (RE), 75?08.19'W, 43?20.97'N. Snails were collected by hand, sacrificed immediately in boiling water, removed from their shells, and fixed in 7% formalin. Aquatic plants upon which snails had been grazing also were collected and preserved. Gut contents were quantified for snails taken from each of five populations. Processing involved excision of the digestive tract, removal of gut contents to a microscope slide, flooding the sample with 7% formalin, and sealing the mount with a coverglass and Euparol. Samples used to estimate food availability were prepared by vigorously shaking each of the five plant samples, removing the plant material, and allowing the remaining suspensions to settle. Subsequent inspection showed that few, if any, diatoms remained on plant surfaces after this treatment. Samples of approximately 0.5 ml of the dislodged Aufwuchs were placed on microscope slides, flooded with 7% formalin, and sealed with coverglasses and Euparol. Substrate diatom availability and snail gut contents were quantified by identifying 100 diatoms, taken from as many randomly located fields as necessary, from each of 10 Aufwuchs samples, and 10 gut samples. Because accurate generic identification of fragments could not be made, only undamaged frustules were quantified. Taxonomic authorities included Patrick & Reimer (1966) and Cleve-Euler (1951-1955).