A field technique for estimating the influence of surface complexity on movement tortuosity in the tropical limpetcellana gratagould

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Abstract
Most studies of the movement patterns of intertidal herbivores assess overall displacement in an assumed homogeneous environment. In the present study, the impact of substrate complexity on spatial patterns of foraging in the limpet, Cellana grata, was investigated on a rocky shore in Hong Kong. Movement patterns were measured by photographing individual C. grata every 10 minutes over an entire foraging cycle. From the photographs, limpet positions were mapped onto the shore at low water and individual trails re-constructed in situ. Individual movement tortuosity (using a sinuosity index from a correlated random walk model and the fractal dimension of movement paths), mean speed, overall net displacement and trail length were measured. Surface profiles were recorded using a contour gauge and surface complexity described using fractal dimensions. The paths of C. grata became more tortuous as the complexity of the rock surface increased, suggesting that the limpet avoids moving and foraging directly over surface irregularities. Actual trail length was almost three times greater than the overall net displacement. This field technique estimates spatial movement patterns of intertidal gastropods in three dimensions and highlights the importance of detailed analyses of movement paths during complete foraging excursions, especially in energetic studies of foraging.