Photobiology in the twilight zone of the deep sea depends on faint light of two, or possibly three, origins:
sunlight, bioluminescence and some visible radiation near the bottom associated with hydrothermal vents. The deep
twilight zone also contains two quite distinct ecosystems: the vast open ocean pelagic regime far from the shore and
the bottom as well as the far less expansive benthic regime with quite different characteristic animals that live on, in
or near the sea bo10 Most of the whole ocean's benthic regime with a mean depth over 3000m is well below
the twilight zone, which eliminates sunlight as a light source there. Many of the most familiar deepsea animals
with their spectacular arrays of dennal light organs and remarkable eyes are from the pelagic 19, 25 The
less familiar benthic fishes and crustaceans sometimes have curious internal light organs powered by bacteria13 and
occasional incredibly modified eyes.30 With the exception of those on the fishing rods of most female deepsea
anglerfish, where the light is produced by symbiotic bacteria, all the numerous light organs of pelagic deepsea fishes
are generally believed to manage their own chemiluminescence independent of luminous bacteria.17
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