THE ORIENTATION OF TAIL-FLIP ESCAPE SWIMMING IN DECAPOD AND MYSID CRUSTACEANS

The orientation of tail-flip escape swimming in a range of adult decap od and mysid crustaceans is reviewed. In mechanical terms, tail-flip s wimming constitutes unsteady locomotion in which a single 'appendage', the abdomen, produces thrust by a combination of a rowing action and a final 'squeeze' force when the abdomen presses against the cephaloth orax. In small crustaceans, a symmetrical 'jack-knife' tail-flip is mo re typical. Tail-flip flexion is controlled by two giant-fibre pathway s, and also by a non-giant-neuronal network. The direction of thrust i n the sagittal plane, and hence the elevation, translation and rotatio n of the tail-flip are dependent upon the point of stimulation and on the giant-fibre pathway activated. The laterality of the stimulus also affects the orientation of swimming, which is directed away from the point of stimulation. In large decapods such as the lobsters Nephrops norvegicus and Jasus lalandii steering is produced by asymmetrical mov ements of various abdominal appendages, and by rotation of the abdomen about the cephalothorax. In slipper lobsters the flattened antennae p rovide steering surfaces. In smaller decapods, such as the brown shrim p Crangon crangon, and in mysids, such as Praunus flexuosus, steering is effected by a rapid rotation of the whole body about its longitudin al axis during the initial stages of tail-flip flexion. The effectiven ess of tail-flip swimming is considered in the context of predator-pre y interactions under natural conditions and in relation to artificial threats from fishing gear.