A method for spatial and temporal assessment of gastropod grazing intensity in the field: the use of radula scrapes on wax surfaces

Abstract

The feeding apparatus of many marine molluscan herbivores leaves distinctive marks on the surface of dental wax. This method can be used in the field to assess the spatial and temporal patterns of gastropod grazing on rocky shores. Among the common gastropod grazers of intertidal habitats on the Isle of Man, distinctive rasping marks were made by docoglossan ( Patella vulgata.), rhipidoglossan ( Calliostoma zizyphinum. and Gibbula spp.) and larger taenioglossan ( Littorina obtusata.) grazers. Our technique for the field deployment of wax surfaces is simple, inexpensive, and permits a realistic placement of the wax surface in the environment. This placement is achieved by casting the wax into small discs (14 mm diameter) and setting them into pre-formed holes in the rock surface. By quantifying either the number of discs scraped or the area of the wax surface scraped, patterns of grazing intensity (defined as areal extent of the surface grazed in a given period) can be assessed over a variety of spatial and temporal scales. To illustrate this method and refine its use, we recorded the grazing patterns of the limpet Patella vulgata for periods from 12 h to 17 days. The optimal period of deployment depended on the specific habitat, but we often found periods of 1–14 days to be appropriate. Regular arrays of discs also demonstrated that grazing intensity was spatially variable at a scale of 0.25 m, and that grazing intensity increased throughout the late winter and spring. This method provides a cheap and direct measure of feeding intensity that is directly relevant to understanding the effect of grazing molluscs on algal communities. Moreover, it can be used over a variety of spatial and temporal scales.