Effect of simulated tidal currents on the burrow emergence rhythms of the Norway lobster (Nephrops norvegicus)

Abstract

Light is the most important zeitgeber for the synchronization of biological rhythms in terrestrial organisms. In the sea, the light intensity progressively decreases, and tidal currents might control behavioural rhythms at disphotic depths. The Norway lobster, Nephrops norvegicus, is distributed from the upper shelf to middle slope areas. Its burrowing behaviour is under the control of a circadian system, and the effects of tidal currents have been inferred from catchability patterns. Male lobsters were collected from 100 m depth off the Ebro Delta, Tarragona, Spain (40° 39′N, 1° 13′E). Light intensity and water current cycles were simulated in the laboratory to investigate their combined effects on burrow emergence behavioural rhythms (June–July 2012). Periodic water currents (10 cm s−1) inhibited N. norvegicus burrow emergence to a degree dependent on the relative phase between light and water current cycles. The lobsters preferred to remain inside the burrow in the presence of water currents. However, when they were outside the burrow, they spent more time orientated downstream during darkness hours. Moreover, four of the 15 lobsters showed that a current could act as a putative zeitgeber for the circadian oscillator, but further experiments are needed to confirm this finding. These results indicate that tidal current is an important parameter to consider when interpreting fishery-dependent data and data from video surveys, not only N. norvegicus, but for other deep-water epibenthic species.