Crabs have a thick exoskeleton and one pair of claws or chelae. The claws in males are larger and sometimes, one is larger than the other and is primarily used for attracting a mate. Male crabs have narrow and triangular shaped abdomens when compared to female crabs which have more rounded abdomen. Crabs thus show distinct sexual dimorphism. Crabs walk sideways due to the articulation of legs. There are crabs that do walk forwards and backwards too, but the sideways gait is the most efficient.
Lobsters have strong but light weight exoskeleton and striated muscles enabling quick movement. Their articulated limbs allow bending at various points. They molt during their life cycle for growth and have ten walking legs of which front two are modified into giant claws. They have very poor eyesight and no nervous system. They walk slowly on the sea floor but are capable of swimming backwards by the curling and uncurling movement of their abdomen. They are very fast at this kind of locomotion which is adopted as an escape mechanism (caridoid escape reaction).