Most of the ocean floor is deep and not a true habitat for animals. The continental shelf has the greatest biodiversity, with many plants and animals. The deep sea is mostly devoid of life, with ecosystems depending on organic debris and chemotrophic bacteria. The most abundant animal on the seabed is nematodes, and some fish and crustaceans can also be found in the deep ocean.
Contrary to popular belief, most of the seabed, known as the “open ocean,” is not true habitat for animals, just a place they pass by on their way to elsewhere. In most of the world, the ocean floor is very deep, with an average depth of 3,790 meters (12,430 feet). Nearly half of the world’s seabed is more than 3,000 meters (9,800 feet) deep. Collectively, the deep sea floor makes up about 71% of the world’s oceans, with shallow waters, such as continental shelves, making up 29%.
Animal life on the seabed is very different depending on whether you are talking about the continental shelf or the deep ocean. On the continental shelf is where the greatest biodiversity is found. Many plants and animals live here: kelp forests, huge amounts of algae, sharks, fish, crustaceans, brachiopods, bivalves, sponges, cnidarians (jellyfish and relatives), echinoderms (starfish, sea urchins and relatives), cetaceans (whales , dolphins , porpoises), and many others. Out of 38 animal phyla, all but one – the velvet worms – have a marine version. That’s why the field of marine biology is so vast.
Although continental shelves are technically the “seabed,” sometimes when the word is used, it specifically refers to the deep sea. The deep sea is mostly devoid of life, as the aphotic (lightless) zone of the ocean begins at a depth of 0.9 km (15,000 feet) and continues to the bottom. Since lightless regions cannot support photosynthesizing plants or bacteria, the ecosystem in the aphotic zone depends mainly on organic debris falling from above. Many animals that are in the aphotic zone are able to produce their own light, called bioluminescence.
Down to the bottom of the deep sea – an area of which humanity has explored less than 1% – the main ecosystems develop around seamounts, where animals tend to congregate, and rift valleys, where geological activity it can provide heat and chemicals to sustain a mini-ecosystem. Two famous seafloor ecosystems are those built around hydrothermal vents and cold seeps. None of these ecosystems depend on the sun for energy. The producers of these ecosystems are chemotrophic bacteria, which oxidize inorganic chemicals to produce energy.
The most abundant animal on the seabed is probably nematodes, also known as flatworms. These have an average size of about 1 mm and are found in the deepest ocean trenches, where the pressure exceeds a thousand atmospheres. Echinoderms (starfish and relatives) can also be found in the deep ocean, as well as fish such as sole and flounder. Crayfish were reportedly present at the bottom of the Mariana Trench, the deepest oceanic trench on the planet. One bizarre creature found on the ocean floor is the giant isopod, which looks similar to a wood louse, but can be a foot in size.
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