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The oceanic zone is often subdivided into five layers of differing depth. The three uppermost layers include the epipelagic zone (depth 0-200 m), the mesopelagic zone (200-1000 m), and the bathypelagic zone (often 1000-2000 m). Although the open ocean is not as productive as coastal waters, the epipelagic zone is the most productive layer within the open ocean, supporting a diverse array of predators that feed on the plankton that teem near the sunlit surface. With increasing depth, water temperature decreases, currents slow, salinity and pressure increase, and food availability declines. The mesopelagic zone receives very little sunlight, but many of the animals in this layer generate their own light. Bioluminescence is a dim, usually blue, light that can be used to attract mates, deter predators, or lure potential prey. Some of the most bizarre looking fishes live in this zone. The depth range of the bathypelagic zone can vary, depending on the definition used. Like the mesopelagic zone, this layer is home to fishes (e.g., catsharks, myctohpids) and numerous organisms that feed on "marine snow" (falling organic debris) or each other. In some cases this zone may extend to the seafloor.
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