Digging in sand crabs (Decapoda, Anomura, Hippoidea): interleg coordination

Sand crabs (Decapoda, Anomura, Hippoidea) are highly specialised for digging into sand using their thoracic legs. Using video-recording and electromyography, we examined the digging leg movements of three species of sand crabs belonging to two families: Blepharipoda occidentalis (Albuneidae), Lepidopa californica (Albuneidae) and Emerita analoga (Hippidae). The digging patterns of all three species are similar. The ipsilateral legs 2 and 3 are tightly coupled and shovel sand forward from underneath the animal, whereas the movements of leg 4 are more variable, apparently stirring up sand and providing the purchase for rearward 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 legs 2 and 3, and both albuneid species switch gait from bilateral alternation to synchrony midway through digging. In E. analoga, right and left legs 2 and 3 always alternate. Legs 4 can cycle at about twice the frequency of legs 2 and 3, and they tend to move in bilateral synchrony during high-frequency leg movements (e.g. at the start of digging); their bilateral coupling becomes variable during low-frequency movements. Sand crab digging may have originated as a modified form of walking, but this behavioural innovation subsequently diverged in the sand crab superfamily.