![]() ![]() Therefore, most organisms in the bathypelagic rely on the marine snow from regions higher in the vertical column. Since there is no light in the deep sea (aphotic), there is a lack of primary producers. In this way marine snow may be considered the foundation of deep-sea mesopelagic and benthic ecosystems: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source. However, most organic components of marine snow are consumed by microbes, zooplankton and other filter-feeding animals within the first 1,000 metres (3,281 ft) of their journey, that is, within the epipelagic zone. The "snowflakes" grow over time and may reach several centimetres in diameter, travelling for weeks before reaching the ocean floor. Marine snow includes dead or dying plankton, protists ( diatoms), fecal matter, sand, soot and other inorganic dust. Its origin lies in activities within the productive photic zone. In deep water, marine snow is a continuous shower of mostly organic detritus falling from the upper layers of the water column. In the deep ocean, the waters extend far below the epipelagic zone, and support very different types of pelagic fishes adapted to living in these deeper zones. Environment Scale diagram of the layers of the pelagic zone However, some modern deep-sea fish such as holocephalians are descendants of much older lineages, indicating that much earlier colonizations of the deep-sea by vertebrates may have occurred, although no fossil evidence of this is known. It has been speculated that deep-sea ecosystems may have been inhospitable to vertebrate life prior to an increased influx of nutrients into the ocean during the Late Jurassic and Early Cretaceous following the rise of angiosperms on land, which led to an increase in abyssal invertebrate life, allowing fish to in turn colonize these ecosystems. The Cretaceous origin for most modern deep-sea fish has been further affirmed with phylogenetic studies such as those of aulopiform fish, which indicate that many deep-sea lineages of these groups originated around this time. ![]() Prior to the discovery of these fossils, there was no evidence for deep-sea fish older than 50 million years in the Paleogene. The earliest known records of deep-sea fish are trace fossils of feeding and swimming behavior attributed to unidentified neoteleosts (referable to the ichnogenera Piscichnus and Undichna), from the Early Cretaceous (130 million-year-old) Palombini Shale of Italy, which is thought to have been deposited in the abyssal plain of the former Piemont-Liguria Ocean. The deep sea is also an extremely hostile environment, with temperatures that rarely exceed 3 ☌ (37 ☏) and fall as low as −1.8 ☌ (29 ☏) (with the exception of hydrothermal vent ecosystems that can exceed 350 ☌, or 662 ☏), low oxygen levels, and pressures between atm (between 2 and 100 MPa). Because this typically extends only a few hundred meters below the water, the deep sea, about 90% of the ocean volume, is in darkness. The epipelagic zone (0–200 metres (0–656 ft) deep) is the area where light penetrates the water and photosynthesis occurs. These zones make up about 75% of the inhabitable ocean space. The bathypelagic and abyssopelagic zones are aphotic, meaning that no light penetrates this area of the ocean. This area is also where nutrients are most abundant. The oxygen minimum layer exists somewhere between a depth of 700 metres (2,297 ft) and 1,000 metres (3,281 ft) deep depending on the place in the ocean. ![]() The mesopelagic zone is the disphotic zone, meaning light there is minimal but still measurable. However, characteristics of deep-sea organisms, such as bioluminescence can be seen in the mesopelagic (200–1,000 metres (656–3,281 ft) deep) zone as well. ![]() This means that they live in the water column as opposed to the benthic organisms that live in or on the sea floor. Only about 2% of known marine species inhabit the pelagic environment. Other deep-sea fishes include the flashlight fish, cookiecutter shark, bristlemouths, anglerfish, viperfish, and some species of eelpout. The lanternfish is, by far, the most common deep-sea fish. Deep-sea fish are fish that live in the darkness below the sunlit surface waters, that is below the epipelagic or photic zone of the sea. ![]()
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