Observations on the ecology, behaviour, swimming mechanism and energetics of the neustonic grapsid crab, Planes minutus

Extract

Planes minutus (L.) (Crustacea: Brachyura) swims well (forwards only) but has limited endurance; at 28°C crabs swam at the surface for 35–45 min before sinking. The legs of the neuston crab are flattened and rotated into the ‘advanced’ orientation of the legs to the cephalothorax, characteristic of swimming brachyurans. All legs are involved in swimming; pairs 1 and 2 mainly support the crab in the water column, pairs 3 and 4 provide forward propulsion too. In leg-pair 4, the plane of extension and flexion is close to the horizontal and the leg provides lift on the recovery stroke. Plumose hairs are present on the anterior borders of the legs, adding to the propulsive area. Stout spines line the anterior and posterior margins of the legs; they allow the crab to hang upside down from floating substrata, but are also used to hold food items. The chelae of Planes have cutting rather than crushing ‘teeth’. Crabs of the size studied (0·128−0·736 g wet wt) used oxygen (at 30°C) at about twice the rate when swimming (mean 1·014 ml O₂ g wet wt−¹ h¹) as did resting crabs (mean rate 0·495 ml O₂ g wet wt−¹ h¹). No oxygen debt is incurred during swimming. The mean crab density was 1·096 g ml−¹ (SD 0·060). Neustonic crabs would not move more than 5 cm away from their floating substrata to catch food. They show a novel ‘revving up’ procedure in which leg pairs 1 to 3 beat while the animal is attached to the substratum by spines on the tips of leg-pair 4. This allows much of the locomotor apparatus to overcome inertia and reach working speed, before an attack is carried out. Crabs ate postlarval and juvenile fish, euphausids, isopods, sea skaters and squid. Gelatinous animals were gener-ally avoided, though salps were captured and the stomach contents eaten. Planes will take prey as big as itself and stores food for later consumption.