DIGGING IN SAND CRABS - COORDINATION OF JOINTS IN INDIVIDUAL LEGS

Sand crabs use their multi-jointed legs to dig into sand. Combined mov ement and electromyogram (EMG) analyses showed that the pattern of int ra-leg coordination in the legs of two sand crabs of different familie s (Blepharipoda occidentalis and Emerita analoga) is similar in legs 2 and 3, but very different in leg 4, For example, the sequence of prox imal joint movements in legs 2 and 3 is elevation, retraction, depress ion and protraction (similar to backward walking in most decapods), bu t the sequence of proximal joint movements in leg 4 is elevation, prot raction, retraction and depression (similar to forward walking), The s ynergies are the same during leg movements in sea water and in sand, s uggesting 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 m otor output to leg 2 (and presumably leg 3) changes from proportional (both power and return strokes co-vary with period) to return stroke c onstant (power strokes covary much more with period than do return str okes), The motor output to leg 4 remains intermediate between proporti onal and return stroke constant in sea water and in sand. On the basis of the segmental specialisation of the motor patterns for the legs, w e hypothesize that sand crab digging may be an evolutionary mosaic of disparate ancestral locomotor behaviours.