FAULKES, Z.: Parallelism in digging behaviour in two distantly related decapod crustaceans

Walking and swimming are the most common forms of locomotion used routinely by decapod crustaceans, but some have evolved ways of locomoting through substrates, notably sand. Two such digging species, of different infraorders, are spanner crabs (Ranina ranina; Brachyura) and spiny sand crabs (Blepharipoda occidentalis; Anomura). Videotape and electromyogram analysis showed that digging by these species is similar, but Ranina has a wider locomotor repertoire than Blepharipoda: for instance, Ranina can walk forward on the benthos, whereas Blepharipoda (and other sand crabs) cannot walk. Similarly, although both Ranina and Blepharipoda dig into the sand using their legs and abdomen, leg movements of Ranina are less stereotyped than those of Blepharipoda. In both species, the second and third pairs of legs shovel sand forward from underneath the animals, while the fourth pair of legs circles in the opposite direction, forcing the posterior end of the animals down into sand. Both species show similar patterns of contralateral leg coordination: left and right legs move forward and backward alternately when above sand, but switch to bilateral synchrony when they begin to submerge into sand. The abdomen is also rhythmically active during digging in both species, even though the abdomen is relatively stiff in Ranina. Thus, Ranina is behaviourally intermediate to the highly specialized digging anomuran sand crabs (e.g., Blepharipoda) and more common walking decapod crustaceans (e.g., crayfish).