The activity rhythm of berried and unberried females of Nephrops norvegicus (Decapoda, Nephropidae)

Females of the Norway lobster, Nephrops norvegicus are usually excluded from behavioural research based on the assumption that their reproductive cycle alters the results. A seasonal fluctuation in the sex ratio of commercial catches suggests that females do not emerge from their burrows when berried. Burrow emergence (i.e., catches) and feeding rhythms of berried and unberried females were compared with data on their endogenous locomotor activity rhythm. Catch data and the percentage of empty stomachs already published for a population dwelling on the continental slope (400-430 m) in the western Mediterranean were reprocessed focusing on data for berried and unberried females in October, a month that falls within the breeding season of N. norvegicus in the study area. Catches for both groups significantly increased at daytime, when both displayed the lowest percentages of empty stomachs (i.e., diurnal feeding pattern). To assess the occurrence of alterations in endogenous locomotor patterns depending on the presence of eggs, locomotor rhythms of berried and unberried females were studied under conditions of constant darkness in the laboratory. All individuals showed endogenous circadian and nocturnal locomotion. Taken together, these data suggest that feeding, burrow emergence, and endogenous locomotion rhythms are not inhibited in berried females. A reduction in the duration and range of emergence (i.e., doorkeeping) is hypothesized as the basis for the reported fluctuations in the sex ratio in commercial catches.