Monochromatic blue light entrains diel activity cycles in the Norway lobster, <i>Nephrops norvegicus</i> (L.) as measured by automated video-image analysis

Abstract

There is growing interest in developing automated, non-invasive techniques for long-lasting, laboratory-based monitoring of behaviour in organisms from deep-water continental margins which are of ecological and commercial importance. We monitored the burrow emergence rhythms in the Norway lobster, Nephrops norvegicus , which included: a) characterising the regulation of behavioural activity outside the burrow under monochromatic blue light-darkness (LD) cycles of 0.1 lx, recreating slope photic conditions (i.e. 200-300 m depth) and constant darkness (DD), which is necessary for the study of the circadian system; b) testing the performance of a newly designed digital video-image analysis system for tracking locomotor activity. We used infrared USB web cameras and customised software (in Matlab 7.1) to acquire and process digital frames of eight animals at a rate of one frame per minute under consecutive photoperiod stages for nine days each: LD, DD, and LD (subdivided into two stages, LD1 and LD2, for analysis purposes). The automated analysis allowed the production of time series of locomotor activity based on movements of the animals’ centroids. Data were studied with periodogram, waveform, and Fourier analyses. For the first time, we report robust diurnal burrow emergence rhythms during the LD period, which became weak in DD. Our results fit with field data accounting for midday peaks in catches at the depth of slopes. The comparison of the present locomotor pattern with those recorded at different light intensities clarifies the regulation of the clock of N. norvegicus at different depths.