A METHOD FOR SPATIAL AND TEMPORAL ASSESSMENT OF GASTROPOD GRAZING INTENSITY IN THE FIELD - THE USE OF RADULA SCRAPES ON WAX SURFACES

The feeding apparatus of many marine molluscan herbivores leaves disti nctive marks on the surface of dental wax. This method can be used in the held to assess the spatial and temporal patterns of gastropod graz ing on rocky shores. Among the common gastropod grazers of intertidal habitats on the Isle of Man, distinctive rasping marks were made by do coglossan (Patella vulgata L.), rhipidoglossan (Calliostoma zizyphinum L. and Gibbula spp.) and larger taenioglossan (Littorina obtusata L.) grazers. Our technique for the field deployment of wax surfaces is si mple, inexpensive, and permits a realistic placement of the wax surfac e in the environment. This placement is achieved by casting the wax in to small discs (14 mm diameter) and setting them into pre-formed holes in the rock surface. By quantifying either the number of discs scrape d or the area of the wax surface scraped, patterns of grazing intensit y (defined as areal extent of the surface grazed in a given period) ca n be assessed over a variety of spatial and temporal scales. To illust rate this method and refine its use, we recorded the grazing patterns of the limper Patella vulgata for periods from 12 h to 17 days. The op timal period of deployment depended on the specific habitat, but we of ten found periods of 1-14 days to be appropriate. Regular arrays of di scs also demonstrated that grazing intensity was spatially variable at a scale of 0.25 m, and that grazing intensity increased throughout th e late winter and spring. This method provides a cheap and direct meas ure of feeding intensity that is directly relevant to understanding th e effect of grazing molluscs on algal communities. Moreover, it can be used over a variety of spatial and temporal scales. (C) 1997 Elsevier Science B.V.