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After establishing shore crabs, Carcinus maenas, individually in separate aquaria, we used a noninvasive infrared phototransducer to monitor their heartbeat rate continously before, during and after fights with intruder crabs. We confirmed that heartbeat rate is a reliable indicator of oxygen consumption and then used it to estimate indirectly the energetic cost of fights differing in duration and intensity, and its dependence on prior residence and relative size of opponent. Prior residence in aquaria significantly increased the probability that crabs would initiate fights against intruders. The majority of fights were resolved by aggressive contacts, display being used extensively only against smaller intruders. Fights between evenly sized opponents and between residents and larger intruders involved almost continuous aggression, whereas fights with smaller intruders involved several shorter bouts of aggression. Fight duration was weakly correlated with the relative size of opponents. Heartbeat rate, measured only in residents, was elevated above resting levels throughout fights, hence energy expenditure during fighting increased linearly with fight duration. Contrary to expectation, heartbeat rate was not significantly influenced by relative size of the opponent or by the intensity of aggression. After fighting, heartbeat rate usually returned to resting levels within 30-60 min, recovery taking longer in fights against larger intruders, when the fight was always lost. We propose that prolonged elevation of heartbeat rate in residents that had lost to larger intruders represented a state of alertness, adaptive against impending risks of resource loss or injury. Copyright 2000 The Association for the Study of Animal Behaviour.

We used noninvasive, infrared phototransducers to record continuously the heartbeat of common mussels, Mytilus edulis, experiencing successive phases of interaction with the predatory gastropod Nucella lapillus, from initial threat to attack and consumption. Coupling physiological monitoring with behavioural analysis allowed us to investigate in detail the responses of mussels to predation threat. Compared to values of normal feeding activity, heart rate increased significantly when mussels were in the presence of effluent from dogwhelks. When attacked by dogwhelks, mussels increased their heart rate further, together with the rate of valve gaping. Considering the heart rate as a reliable estimator of respiratory function, these cardiac responses might be a mechanism to compensate for increased energy demand in order to cope with predation hazard. If so, the theoretical importance of trading off energy balance against risk of predation is supported by our results. Cardiac and behavioural responses varied throughout the attack according to the penetration method adopted by dogwhelks. When mussels were attacked through a drilled hole, heart rate tended to increase and periods of cardiac pausing appeared close to the point of death. In contrast, mussels attacked by penetration between the valves showed decreasing heart rate throughout the attack, together with relatively earlier cessation of valve gaping and appearance of cardiac pausing. These differences clearly support the hypothesis that dogwhelks penetrating between the valves are able to induce muscular paralysis of prey by injecting toxins. Copyright 1999 The Association for the Study of Animal Behaviour.

We used an acoustic transducer to monitor the radular activity of dogwhelks, Nucella lapillus, drilling mussels, Mytilus edulis, in the laboratory and we examined the effect of dietary experience on prey-handling behaviour. For the first time, phases of inspection, penetration and ingestion could be distinguished directly, and consequently the prey-handling process analysed in detail. Dogwhelks with different field-based experience of mussels showed different handling behaviour. Those collected from a mussel-dominated shore more readily adopted the faster method of penetrating between the slightly gaping valves, instead of the slower method of drilling through the shell. Those collected from a barnacle-dominated shore took significantly longer to attack the mussel and then were unable to switch from drilling to penetrating through the gape between valves. Experience of specific prey in the field, by reducing handling time, could promote fitness by reducing exposure to environmental hazards. Laboratory attempts to train dogwhelks from the barnacle-dominated shore to use the gape penetration method failed, suggesting that functional constraints, such as injection of a relaxant when penetrating through the gape and/or genetically controlled behavioural traits, could limit the ability to learn handling skills. Copyright 1999 The Association for the Study of Animal Behaviour.