A new tracking system for the measurement of diel locomotor rhythms in the Norway lobster, Nephrops norvegicus (L.)

Abstract

TheNorway lobster,Nephrops norvegicus (L.), is a deep-water burrowing decapod of highcommercial value. Diel variations in trawl captures are produced by population rhythms of burrow emergences related to day–night cycles. Rhythms seem to be different in males and females since catches showvariations in sex ratios depending on the season. Our hypothesis is that the diel rhythm of activity in this species can be distinguished in three different behavioural sets, the durations of which showgender-related modulation: door-keeping, proximal-, and distal-emergence from the burrow. Our aim is to detail the functioning of a new tracking system allowing the durations of these three behavioural components to be determined. Movement of animals was detected by subdividing aquaria into different zones by means of three rows of infrared-emitting and -receiving photodiodes in which blue light emitters were also integrated for the generation of light cycles. We recorded movement patterns in adult males and females (n = 20) exposed to a standard photoperiod regime (i.e., 12 h; monochromatic at 480 nm of 5 lx) over 12 days. Marked diel nocturnal rhythms were reported at all barriers, with activity peaks diffused over the night at the burrow entrance and located at the day–night transition at other barriers (i.e., crepuscular peaks that decreased in the next fewhours of darkness). Mean total activitywas significantly higher for females than males at the burrow entrance (i.e., door-keeping behaviour). Males had significantly higher activity at other locations (proximal- and distal-emergence behaviours).