DIGGING IN SAND CRABS (DECAPODA, ANOMURA, HIPPOIDEA) - INTERLEG COORDINATION

Sand crabs (Decapoda, Anomura, Hippoidea) are highly specialised for d igging into sand using their thoracic legs, Using video-recording and electromyography, we examined the digging leg movements of three speci es of sand crabs belonging to two families: Blepharipoda occidentalis (Albuneidae), Lepidopa californica (Albuneidae) and Emerita analoga (H ippidae), The digging patterns of all three species are similar, The i psilateral legs 2 and 3 are tightly coupled and shovel sand forward fr om underneath the animal, whereas the movements of leg 4 are more vari able, apparently stirring up sand and providing the purchase for rearw ard descent into the sand, The digging patterns of B. occidentalis and L. californica resemble each other more than either resembles that of E. analoga, In the albuneids, leg 4 cycles at the same frequency as l egs 2 and 3, and both albuneid species switch gait from bilateral alte rnation to synchrony midway through digging, In E. analoga, right and left legs 2 and 3 always alternate, Legs 4 can cycle at about twice th e frequency of legs 2 and 3, and they tend to move in bilateral synchr ony during high-frequency leg movements (e.g. at the start of digging) ; their bilateral coupling becomes variable during low-frequency movem ents, Sand crab digging may have originated as a modified form of walk ing, but this behavioural innovation subsequently diverged in the sand crab superfamily.