A single muscle moves a crustacean limb joint rhythmically by acting against a spring containing resilin

BACKGROUND: The beating or fanning movements of three pairs of maxilliped flagella in crabs and crayfish modify exhalent gill currents while drawing water over chemoreceptors on the head. They play an integral part both in signalling by distributing urine odours, and in active chemosensation. RESULTS: The rhythmical maxilliped movements start with maxilliped 3 followed after a delay of 15 to 20 ms in shore crabs by maxilliped 2 and then maxilliped 1, at a frequency of 18 to 20 Hz in crabs and 10 to 13 Hz in signal crayfish. The contraction of a single abductor muscle controls the power stroke (abduction) of each flagellum, which is accompanied by flaring of feather-like setae which increase its surface area. No muscle can bring about the return stroke (adduction). Release of an isolated flagellum from an imposed abduction is followed by a rapid recoil to its resting adducted position. The relationship between the extent of abduction and the angular velocity of the return stroke indicates the operation of a spring. Blue fluorescence under UV light, and its dependence on the pH of the bathing medium, indicates that resilin is present at the joint between an exopodite and flagellum, at the annuli of a flagellum and at the base of the setae. CONCLUSION: Resilin is progressively bent as a flagellum is abducted and resumes its natural shape when the joint recoils. Other distortions of the exopodites may also contribute to this spring-like action. The joint is therefore controlled by a single abductor muscle operating against a spring in which the elastic properties of resilin play a key role.