Many animals use multi-jointed limbs to move through their environment. In theory, the many degrees of freedom allowed by multiple joints should make multi-jointed limbs difficult to control (Whiting, 1984; Turvey et al. 1982). How central pattern generators (CPGs) coordinate the movements of multi- jointed limbs is somewhat less well understood than the production of rhythmic motor output to single joints and the coordination among multiple limbs. Nevertheless, the control of multi-jointed limbs will result from an interplay of extrinsic factors, such as sensory cues and reflexes (e.g. Cattaert et al. 1993; El Manira et al. 1991a,b; Müller and Clarac, 1990), and intrinsic factors, such as centrally generated motor programmes (e.g. Chrachri and Clarac, 1990; Bässler, 1993; Jamon and Clarac, 1997). Decapod crustacean locomotion is well suited for studying intra-limb coordination, and walking has been investigated extensively. Decapods are relatively large, which facilitates movement analyses; their legs have several joints, which are typically simple hinges (Lochhead, 1961); and decapods have a diverse set of locomotor behaviours, both within and among species. Movements of the two most proximal joints, the thorax–coxa and coxa–basis joints, are important in the kinematics (Ayers and Clarac, 1978; Clarac et al. 1987; Barnes, 1977; Jamon and Clarac, 1997; Macmillan, 1975) and proprioceptive regulation of decapod walking (Head and Bush, 1991, 1992; Sillar et al. 1986; Skorupski and Sillar, 1988). Movements of the merus–carpus joint are also important in some cases, notably in sideways walking (Ayers and Clarac, 1978; Barnes, 1977; Clarac et al. 1987; Jamon and Clarac, 1997), although their contribution to proprioception is less well investigated. The remaining three leg joints typically make smaller movements than the other joints (Barnes, 1977). Sand crabs use their multi-jointed legs and ‘tail’ for digging. Forward-going power strokes by legs 2 and 3 shovel sand from underneath the animal. Leg 4 pushes the rear end of an animal down into the sand, increasing the purchase of the other legs (Faulkes and Paul, 1997c). Rapid movements of the tail liquefy the sand, enabling the animal to descend rapidly into sand (Faulkes and Paul, 1997b). Digging leg movements are similar to (and, we hypothesise, evolutionarily derived from) walking leg movements in other decapods: both are locomotor behaviours using the legs. The intra-limb coordination of sand crab digging interests us for several reasons. First, the legs of the sand crabs are behaviourally specialised, which offers the possibility of studying different motor outputs in serially homologous limbs in a single animal. Second, digging 2139 The Journal of Experimental Biology 201, 2139–2149 (1998) Printed in Great Britain © The Company of Biologists Limited 1998 JEB1247 Sand crabs use their multi-jointed legs to dig into sand. Combined movement and electromyogram (EMG) analyses showed that the pattern of intra-leg coordination in the legs of two sand crabs of different families (Blepharipoda occidentalis and Emerita analoga) is similar in legs 2 and 3, but very different in leg 4. For example, the sequence of proximal joint movements in legs 2 and 3 is elevation, retraction, depression and protraction (similar to backward walking in most decapods), but the sequence of proximal joint movements in leg 4 is elevation, protraction, retraction and depression (similar to forward walking). The synergies are the same during leg movements in sea water and in sand, suggesting that the same motor programme is used in both situations. At the transition from sea water into sand, however, both the frequency and amplitude of the EMG potentials increase, and the phasing of the motor output to leg 2 (and presumably leg 3) changes from proportional (both power and return strokes co-vary with period) to return stroke constant (power strokes co- vary much more with period than do return strokes). The motor output to leg 4 remains intermediate between proportional and return stroke constant in sea water and in sand. On the basis of the segmental specialisation of the motor patterns for the legs, we hypothesize that sand crab digging may be an evolutionary mosaic of disparate ancestral locomotor behaviours. Key words: crustacean, evolution, kinematics, leg, locomotion, digging, swimming, crab, Blepharipoda occidentalis, Emerita analoga. Summary Introduction DIGGING IN SAND CRABS: COORDINATION OF JOINTS IN INDIVIDUAL LEGS ZEN FAULKES* AND DOROTHY H. PAUL† Department of Biology, University of Victoria, PO Box 3020, Victoria, British Columbia, Canada V8W 3N5 *Present address: Department of Biology, McGill University, 1205 Avenue Docteur-Penfield, Montréal, Québec, Canada H3A 1B1 †Author for correspondence (e-mail: dhp@uvvm.uvic.ca) Accepted 8 May; published on WWW 25 June 1998