12/10/2023 0 Comments Lantern fish reproductionMoreover, bioluminescent emissions may vary in intensity, duration, frequency, angular distribution and have an effective visual range limited to about 100–150 m. Deep-sea organisms have adapted to these particular conditions by producing bioluminescent emissions mainly in the blue–green range of the spectrum, although some species emit light of shorter or longer wavelengths. While daylight can be seen in all directions and covers the entire visibly useful light spectrum (300–800 nm) in the shallows, residual light becomes vertically symmetrical, and short and long wavelengths are rapidly absorbed with depth, only leaving blue–green light (around 480 nm in clear ocean) in deeper waters. In addition to changes in intensities, the spectral range and direction of light also vary greatly with depth, at least to start with. From around 800 to 1000 m in the clearest ocean, light from the surface is insufficient to be detected by deep-sea organisms, creating a dark and featureless background where bioluminescent emissions are the only sources of light. Here the euphotic zone transitions into the mesopelagic or twilight zone. Below 200 m, residual daylight is present in sufficient levels to allow visual tasks but not photosynthesis. While bioluminescence is found at all depths, downwelling light is predominant in the surface layers (upper 200 m in the epipelagic zone) and its intensity diminishes rapidly with depth though absorption and scattering. Briefly, the oceanic visual environment is driven by two main sources of light: the downwelling light, produced by the sun and stars, and reflected and scattered by the moon and sky and bioluminescence, produced by the organisms themselves. Light in the ocean, and particularly in the deep-sea, has been extensively reviewed elsewhere. In the marine environment, visual conditions are dependent on the light intensity, turbidity (amount of particulate or dissolved organic matter), type of habitat (benthic, pelagic, deep-sea), prey and predator relationships (size, colour), season, the organism's size and developmental stage and sexual communication. The visual system of different organisms generally reflects each species' ecology and behaviour by becoming adapted to their lifestyle and ambient environmental constraints. This article is part of the themed issue ‘Vision in dim light’. We discuss how the plasticity, performance and novelty of its visual adaptations, compared with other deep-sea fishes, might have contributed to the diversity and abundance of this family. This review covers the current body of knowledge on the visual system of one of the most abundant and intensely studied groups of mesopelagic fishes: the lanternfish (Myctophidae). As a result, the visual system of mesopelagic organisms has been pushed to its sensitivity limits in order to function in this extreme environment. This ambient light is, however, enhanced by a multitude of bioluminescent signals emitted by its inhabitants, but these are generally dim and intermittent. In the mesopelagic zone of the deep- sea, between 2 m, very low intensities of downwelling light remain, creating one of the dimmest habitats in the world. Ecological and behavioural constraints play a major role in shaping the visual system of different organisms.
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