THE ENTRAINMENT OF DEEP-WATER BENTHIC SPECIES TO MONOCHROMATIC LIGHT INTENSITY CYCLES MIMICKING INCREASING DEPTHS: THE CASE OF THE NORWAY LOBSTER, Nephrops norvegicus (CRUSTACEA: DECAPODA)

*Corresponding autor e-mail: jjosechiesa@unq.edu.ar Introduction: The entrainment of deep-water benthic species represents a not yet understood aspect of marine chronobiology. Species with wide depth range of distribution, encompassing shelves and slopes show entrainment for monochromatic 480 nm blue light, since this is the only wavelength invariantly present down to the inferior limit of the twilight zone (1000 m depth). Different populations experience entrainment to light-dark cycles (LD) of markedly different intensities. The Norway lobster, Nephrops norvegicus , inhabit shelves and slopes, is territorial spending its life close to the same burrow. Field and laboratory studies show that animals emerge with a timing that is depth dependent: at night on shallow shelf, at crepuscular hours on lower shelf, and at daytime on the slope. Methods: In this study, our aim was to assess the entrainment capability of Nephrops in the laboratory, using two monochromatic (480 nm) light-dark cycles (LD) of intensities mimicking the shelf and the slope. With a new IR actograph prototype endowed with a burrow, we could measure rhythms in doorkeeping behaviour (DK) as well as proximaland distal emergence (PE and DE, respectively). Two groups of animals were exposed to LD of 0.1 lux (N = 8) mimicking the slope, and to 10 lux mimicking the shelf (N =10) (LD1). Animals were then transferred to constant darkness (DD), and then re-exposed to the same LD (LD2). Results: At 0.1 lux, all behavioural activity was diurnal and entrainment stronger, since similar activity levels appeared either confined during photophase at LD1, the expected photophase at DD, as well as during the re-imposed photophase at LD2. Conversely, under 10 lux a weak entrainment was found with no dominant diurnal or nocturnal DK activity at LD, or with aparent nocturnal PE and DE activities due to exogenous masking by light. Conclusion: Present data indicate that the determination of diurnal/nocturnal niche in Nephrops is a process which depends on light intensity, reflecting slope and shelve depths in the field, due to balance in circadian/masking modulation of behaviour. Publication History Article published online: 16 June 2026 © 2009. Brazilian Sleep Academy. This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/) Thieme Revinter Publicações Ltda. Rua Rego Freitas, 175, loja 1, República, São Paulo, SP, CEP 01220-010, Brazil