The locomotor toolbox of the spanner crab, Ranina ranina (brachyura, Raninidae)
Digging is not a well-understood form of locomotion, and it poses different mechanical problems than other forms of locomotion (e.g., walking). The spanner crab (Ranina ranina) digs into sand, primarily using its pereiopods. Above sand, pereiopod movement is variable, with four different movement patterns revealed by tip trajectories. The most common pattern of pereiopod movement above sand is the pattern apparently used for digging: pereiopods 2 and 3 shovel sand forward from underneath the animal, while pereiopod 4 shovels in the opposite direction, pushing the posterior end down into the sand. When above sand, each pair of pereiopods moves in bilateral alternation, but R. ranina switches gait to bilateral synchrony as it descends into sand. The abdomen is also rhythmically active during digging, despite being small and relatively stiff. Ranina ranina can also locomote forward on top of the substrate by punting with pereiopods 2 and 3. Although many aspects of digging in R. ranina are similar to those of digging by other crustaceans, particularly anomuran sand crabs, R. ranina has retained a wider behavioural repertoire.