Meagre’ s melatonin profiles under captivity: circadian rhythmicity and light sensitiveness Catarina C. V. Oliveira & Filipe Figueiredo & Florbela Soares & Wilson Pinto & Maria Teresa Dinis Received: 13 February 2017 /Accepted: 7 February 2018 # Springer Science+Business Media B.V., part of Springer Nature 2018 Abstract The present study reveals the first characteri- zation of the plasma melatonin rhythms of the meagre (Argyrosomus regius) under aquaculture conditions. Mel- atonin levels were monitored during a 24 h cycle under a photoperiod of 16 L:8D and under constant darkness (DD), respectively to characterize the daily rhythm of this indoleamine and to test its endogenous origin. Besides, to identify which light intensities are perceived as night or day by this species, the degree of inhibition of nocturnal melatonin production caused by increasing intensities of light was tested (3.3, 5.3, 10.5, and 120 μW/cm 2 ), apply- ing 1 h light pulses at Mid-Dark. The result for melatonin daily rhythm in plasma showed a typical profile: concen- tration remained low during all daytime points, increasing greatly during dark points, with maximum values at 16:00 and 22:00 h, zeitgeber time. Under DD conditions, the plasma melatonin profile persisted, with a similar acrophase but with a lower amplitude between subjective day and night periods, indicating this rhythm as being endogenously driven. Moreover, meagre seemed to be very sensitive to dim levels of illumination during the night, since an intensity of just 3.3 μW/cm 2 inhibited melatonin production. However, only the pulse of 5.3 μW/cm 2 caused a melatonin drop till daytime con- centrations. Thus, the threshold of light detection by the pineal organ was suggested as being located between 3.3 and 5.3 μW/cm 2 . Such results are an added value for this species biology knowledge, and in consequence to its adaptation to aquaculture conditions, allowing the im- provement of culture husbandry protocols. Keywords Argyrosomus regius . Pineal organ . Daily rhythms . Photoperiod . Artificial lightning . Melatonin inhibition Introduction There is an increasing interest among European aquacul- ture researchers and producers to expand this industry, in consonance with the strategy of the European Commis- sion, which intends to boost aquaculture. In consequence, new emerging finfish species have been identified based both on their biological and economical potential. The meagre (Argyrosomus regius) is a promising candidate for the diversification of aquaculture in Mediterranean countries (Diversify project, 7FP-KBBE-2013-GA 602131), recently becoming an appellative research mod- el. During the last few years, numerous publications Fish Physiol Biochem https://doi.org/10.1007/s10695-018-0478-0 C. C. V. Oliveira (*) : F. Figueiredo : M. T. Dinis CCMAR, Universidade do Algarve, Campus de Gambelas, Edifício 7, 8005-139 Faro, Portugal e-mail: ccoliveira@ualg.pt F. Soares IPMA, Avenida 5 de Outubro, 8700-305 Olhão, Portugal W. Pinto SPAROS, Lda, Área Empresarial de Marim, Lote C, 8700-221 Olhão, Portugal Present Address: F. Figueiredo Norwegian College of Fishery Science, UiT The Arctic University of Norway, 9037 Tromsø, Norway