Implication of the visual system in the regulation of activity cycles in the absence of solar light: 2-[I-125]iodomelatonin binding sites and melatonin receptor gene expression in the brains of demersal deep-sea gadiform fish

Relative eye size, gross brain morphology and central localization of 2-[I- 125]iodomelatonin binding sites and melatonin receptor gene expression were compared in six gadiform fish living at different depths in the north-east Atlantic Ocean: Phycis blennoides (capture depth range 265-1260 m), Nezumi a aequalis (445-1512 m), Coryphaenoides rupestris (706-1932 m), Trachyrincu s murrayi (1010-1884 m), Coryphaenoides guentheri (1030 m) and Coryphaenoid es (Nematonurus) armatus (2172-4787 m). Amongst these, the eye size range w as 0.15-0.35 of head length with a value of 0.19 for C.(N.) armatus, the de epest species. Brain morphology reflected behavioural differences with well -developed olfactory regions in P.blennoides, T.murrayi and C. (N.) armatus and evidence of olfactory deficit in N. aequalis, C. rupestris and C. guen theri. All species had a clearly defined optic tectum with 2-[I-125] iodome latonin binding and melatonin receptor gene expression localized to specifi c brain regions in a similar pattern to that found in shallow-water fish. M elatonin receptors were found throughout the visual structures of the brain s of all species. Despite living beyond the depth of penetration of solar l ight these fish have retained central features associated with the coupling of cycles of growth, behaviour and reproduction to the diel light-dark cyc le. How this functions in the deep sea remains enigmatic.