Fig. 1. Schematic diagram of the apparatus used for monitoring shell gaping of the vent mussel B. azoricus consisting of (a) a pair of electric sensors attached to the edge of the shell that permits free movements of the animal; (b) experimental closed-system aquarium housing 20 animals; (c) signal amplifier and (d) computer interface that uses the signal analysis system Signalview (Real Time Devices USA, Inc., 1994) to produce the output (e), the `mussel actograph'.