/. mar. biol. Ass. U.K. (1991), 71,1-10 Printed in Great Britain COMPARATIVE LOCOMOTOR ACTIVITY PATTERNS IN THE PORTUNID CRABS LIOCARCINUS HOLSATUS AND L. DEPURATOR P. ABELLO*, D.G. REID* AND E. NAYLOR School of Ocean Sciences, University College of North Wales, Menai Bridge, Gwynedd, LL59 5EY Present addresses: *Institut de Ciencies del Mar, Passeig Nacional, 08003 Barcelona, Spain •Department of Agriculture and Fisheries for Scotland, Marine Laboratories, Victoria Road, PO Box 101, Torry, Aberdeen, AB9 8DB The free-running locomotor activity rhythms of freshly-captured swimming crabs Liocarcinus holsatus and L. depurator have been studied in constant conditions in the labora- tory. L. holsatus captured in the intertidal zone of sandy beaches showed strong circatidal activity rhythms with maximum activity at high tide. L. holsatus captured in the sublittoral zone had a clear tendency to show circadian activity rhythms with highest activity during day-time hours. L. depurator occurred only sublittorally and showed circadian activity patterns with highest activity during the night. Exposure to hydrostatic pressure cycles of tidal amplitude and periodicity, entrained strong circatidal activity rhythms in previously arhythmic L. holsatus. This activity pattern also showed a marked circadian component. Exposure to the same regime entrained a circadian rhythm, but not a circatidal rhythm in L. depurator. In the sublittoral zone L. depurator is active mainly during the night, whereas L. holsatus, is active mainly during the day. This may constitute a behavioural mechanism for minimizing competitive interactions between these two sympatric crabs. INTRODUCTION The portunid crabs Liocarcinus holsatus (Fabr.) and L. depurator (L.) are found com- monly on sandy and muddy-sandy bottoms of the shallow continental shelf around the British Isles (Ingle, 1983; Clark, 1986). Both species occur sympatrically in the subtidal zone, but L. holsatus is also found in the very low intertidal or immediate sub-tidal zones of sandy beaches from late spring to early autumn. In such closely-related species (Ingle, 1980; Manning & Holthuis, 1981), which are of similar size and feeding habits (Choy, 1986; Abello & Cartes, 1987), and which widely coexist (Clark, 1986), one might expect a high degree of interspecific competition. One possible mechanism for reducing interspecific competition is the evolution of different activity patterns. The present study was therefore designed to investigate the endogenous rhythmic locomotor activity patterns of the two species in relation to the tidal and diurnal cycles, in order to detect any possible behavioural mechanisms that could be of adaptive value in reducing interspecific competition between these two partially sympatric crabs.