We used noninvasive, infrared phototransducers to record continuously the h
eartbeat 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 be
havioural analysis allowed us to investigate in detail the responses of mus
sels to predation threat. Compared to values of normal feeding activity, he
art rate increased significantly when mussels were in the presence of efflu
ent from dogwhelks. When attacked by dogwhelks, mussels increased their hea
rt rate further, together with the rate of valve gaping. Considering the he
art rate as a reliable estimator of respiratory function, these cardiac res
ponses might be a mechanism to compensate for increased energy demand in or
der to cope with predation hazard. If so, the theoretical importance of tra
ding off energy balance against risk of predation is supported by our resul
ts. Cardiac and behavioural responses varied throughout the attack accordin
g to the penetration method adopted by dogwhelks. When mussels were attacke
d through a drilled hole, heart rate tended to increase and periods of card
iac pausing appeared close to the point of death. In contrast, mussels atta
cked by penetration between the valves showed decreasing heart rate through
out the attack, together with relatively earlier cessation of valve gaping
and appearance of cardiac pausing. These differences clearly support the hy
pothesis that dogwhelks penetrating between the valves are able to induce m
uscular paralysis of prey by injecting toxins. (C) 1999 The Association for
the Study of Animal Behaviour.