Crab locomotion

Why do crabs walk sideways?

APPU J.

Manathavdy, Kerala

Crabs are chiefly marine, but some are terrestrial for long periods. Although they are capable of locomotion in all directions, crabs tend to move sideways. The swimming crabs have the last pair of legs flattened to form paddles.

The body of the forward walking crabs is slightly longer than it is wider in contrast to side-walking crabs, in which the width of the body is typically greater than the length.

Laterally walking crabs are characterized by certain features peculiar to their direction of movement. One feature is that legs on one side of the body pull, while those on the other side push. They accomplish these movements by using joints which allow flexion and extension. A second feature is that these crabs produce different gaits and phase relationships in leading and trailing legs. It appears that, mechanically, these laterally moving crabs may be ‘pulling’ with the leading legs to a greater extent than ‘pushing’ with the trailing legs.

However, forward-walking crabs may move differently. They may predominantly produce movements about joints which allow a leg swing forward and backward that is to promote and remote. Furthermore gaits and phase relationship of legs on opposite sides of the body may not differ in forward-walking crabs. This is because the contralateral legs can share equally the load of the body.

The angles of movement about the joints during antero-posterior movement will be greater in forward-walking crabs than the side-walking crabs. Similarly the angles of movement about the joints which permits lateral movement will be greater in side-walking crabs than the forward-walking crabs.

S. PALANIAPPAN

Editor, Research Journal of Biological Sciences

J.J. College of Arts and Science

Pudukkottai, Tamil Nadu