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The benthos are animals that live on or underneath the sea bed, in what is called the benthic zone. They can be contrasted with plankton, which are free-floating. Some common benthos animals are various sea worms (especially polychaete annelids), seagrass (a type of flowering plant), clams, oysters, sea cucumbers (a type of echinoderm), brittle stars, sea anemones, sea stars, sea squirts (tunicates), nudibranchs (mollusks), and various shrimp and shrimp-like crustaceans, to name a few.
Along with the macroscopic benthos listed above are extremely abundant microscopic benthos, including water bears (tardigrades), nematodes (the most abundant multicellular animal on Earth), gastrotrichs, small crustaceans like copepods, foraminifera (common protists), diatoms, and assorted amoeboids, ciliates, and flagellates. Because light tends to be poorer on the sea bed than at the surface, and almost absent at depths below 200 m (656 ft), the foundation of the benthos food chain focuses more on dead animals and plants that fall from above than active photosynthesis.
Benthic organisms cover the entire surface of the oceans, though they are much rarer in locations off continental shelves. Some benthos are adapted to living close to the shore, even in the intertidal region, where they can sustain being out of water for hours on end thanks to special adaptations. Others, such as sea cucumbers, are adapted to living in the darkest depths of the oceans. The deep sea benthos is among the most unusual, and includes unicellular organisms over an inch across that are capable of leaving tracks, giant sea anemones up to 2 m (6 ft) across, and giant isopods (related to pill bugs) the size of house cats. Some of these benthic organisms are very poorly understood, and research into them is ongoing.
All the organisms in the world are completely dependent on benthic organisms to survive. These organisms stir up and consume the carcasses of animals that sink to the bottom, excreting them as feces, which dissolves into nutrients that gets carried back to the surface again in upwelling. In this fashion, carbon does not build up at the bottom of the ocean without getting carried back again. Otherwise, over millions of years all the world's carbon would be sequestered in the deep sea, leaving none for any form of life.
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