Myctophid vision: seeing and being seen in the mesopelagic zone

[Truncated abstract] The visual system of different organisms strongly reflects each species' ecology and behaviour by being adapted to their life style and ambient environmental constraints. In the mesopelagic zone of the oceans (200-1000m), only very low intensities of sunlight remain, creating a relatively dark environment accompanied by a multitude of bioluminescent flashes emitted by its inhabitants. As a result, the visual system of mesopelagic organisms has been under selective pressure to adapt to the ambient light and behavioural tasks of this extreme environment. This thesis aims to better understand mesopelagic fishes visual adaptations in relation to their environment and evolution history. Using a multidisciplinary approach, the thesis focuses on the visual system of one family of deep-sea fishes, the Myctophidae, taking into consideration 61 species, representatives of more than 50% of the recognised genera. Myctophids or lanternfishes are one of the most abundant groups of mesopelagic fishes in the world’s oceans and represent a good model for visual adaptation studies due to their diversity (~250 sp) and broad intra- and interspecific variation in the depths they occupy, as well as their migration patterns and the location of their luminous organs. The variability in relative eye size was first assessed within the family and the factors influencing interspecific differences investigated using phylogenetic comparative analyses. A great variability in relative eye size was found at all taxonomic levels, suggesting that this character is relatively labile. Variability in eye size within the family could not be explained by any of the ecological variables tested (bioluminescence and depth patterns), and appears to be driven instead by phylogenetic relationships. Since assessing visual capabilities using only eye size is limiting, the general morphology of the myctophid eye was investigated in detail. Myctophids possess several other visual adaptations for dim-light conditions, an aphakic gap, a tapetum lucidum and a pure rod retina with high densities of long photoreceptors, all of which were found to be highly variable between species. Two novel retinal specialisations, a fundal pigmentation and microtubular-like structures, were also described. Special attention was given to the first stage of retinal processing, the photoreceptors, by examining their size, arrangement, topographic distribution and contribution to optical sensitivity. Clear interspecific differences in visual specialisations, photoreceptor designs (length and diameter), density and sensitivity to downwelling light and bioluminescent emissions were found. Moreover, phylogenetic comparative analyses revealed several relationships between photoreceptor characteristics and the ecological variables tested. A new short-wavelength intra-ocular filter, a yellow pigmentation, present in the outer nuclear layer of the retina was found in several species and described. This specialisation appears to be species-specific, varying in location, shape, size and was found to be sexually dimorphic in two different species...