The feeding strategies of juvenile and adultBiomphalaria glabrata(Say) under simulated natural conditions and their relevance to ecological theory and snail control

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Abstract
Aspects of the feeding biology of the freshwater pulmonate snailBiomphalaria glabratawere studied under simulated natural conditions in the laboratory by examining crop and rectal contents, the mouth parts of the snail and its method of feeding when presented with a variety of potential food items. The results confirm the generalization that freshwater pulmonate snails, such asB.glabrata, subsist mainly by selecting decaying macrophyte tissue, colonial diatoms and smaller green algae in preference to living macrophyte tissue, filamentous green algae, or small adpressed forms of epiphytic diatoms and epiphytic bacteria. The generalist feeding strategy adopted byB.glabratacan be related to the morphology of the mouth parts and to its feeding behaviour. The adult snail has a total of about 4425 radula teeth (in 75 transverse rows each with 59 teeth) whose dimensions are of the same order of magnitude as diatoms, with narrow gaps in between, thus making the radula an ideal system for scraping the larger, colonial diatoms from the surface of freshwater macrophytes, but not the smaller adpressed diatoms or bacteria. The snails show considerable flexibility in the use of the mouth parts. Thus, the extent of the area grazed by the radula can be regulated by changes in the direction or velocity of the head movements. The latter may be either straight or pendular while the number of bites per sweep can be varied from two to nine. The dorsal mandible may also be used independently for scraping epiphytic algae. Living macrophyte tissue is cut by the backwardly directed power stroke of the radula, sometimes acting in opposition to the dorsal mandible, whereas the flaccid decaying macrophyte tissue can be ingested by a pumping action with minimal use of the radula. The costs and benefits of the various strategies are discussed. There are marked differences between the feeding niches of juvenile and adultB.glabrataas the former eat less living macrophyte tissue and fewer species of large diatoms but much larger quantities of decaying plant material than their adult conspecifics. These ontogenetic changes can be related to variations in the relative growth rate of the odontophore and their chemoreception niches. The ratio of the abundance of food items in the crop to the rectum indicate that macrophyte tissue is probably used almost completely but that the green algaeScenedesmusandRhizocloniummay pass through the gut undigested. Many of the diatom species are also resistant to digestion. The sizes of particles ingested vary from 0.026 to 0.78 mm2the latter being the largest the snails are capable of swallowing. As the snails prefer flaccid material it is suggested that controlled release formulations, designed to kill target snails selectively, should be of this consistency and the above size range. The results are discussed within the context of the hypothesis that the snails, the macrophytes, their epiphytic algae and the bacteria involved in decomposition are components of a module with six subsets (Paine 1980). Evidence is given that the linkage between each of the subsets are strongly mutualistic, and that they involve the release and use of dissolved organic matter. It is argued that these positive interactions must be taken into account when constructing models to explain stability and complexity of communities.