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The pelagic zone contrasts with the benthic and demersal zones at the bottom of the sea. The benthic zone is the ecological region at the very bottom, including the sediment surface and some subsurface layers. Marine organisms such as clams and crabs living in this zone are called benthos. Just above the benthic zone is the demersal zone.
The photic zone allows sufficient light for phytoplankton to photosynthesize. [6] A vast habitat for most pelagic fish, the epipelagic zone is well lit so visual predators can use their eyesight, is usually well mixed and oxygenated from wave action, and can be a good habitat for algae to grow. However, it is an almost featureless habitat.
Life for many marine organisms begins near the ocean surface. Buoyant eggs hatch into planktonic larvae that develop and disperse in the ocean for weeks to months before transitioning into juveniles and eventually finding suitable adult habitat. [97] The pelagic larval stage connects populations and serves as a source of new adults.
Oceanic nekton comprises aquatic animals largely from three clades: Vertebrates (phylum Chordata), particularly pelagic fish, cetaceans and sea turtles, form the largest contribution; these animals have endoskeletons made of bones and cartilages and propel themselves via a powerful tail and fan/paddle-shaped appendages such as fins, flippers or ...
Many organisms in the mesopelagic zone move up into the epipelagic zone at night, and retreat to the mesopelagic zone during the day, which is known as diel vertical migration. [8] These migrators can therefore avoid visual predators during the day and feed at night, while some of their predators also migrate up at night to follow the prey.
The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. The word abyss comes from the Greek word ἄβυσσος ( ábussos ), meaning "bottomless". [ 1 ] At depths of 4,000–6,000 m (13,000–20,000 ft), [ 2 ] this zone remains in perpetual darkness.
In the pelagic zone, dead fish and the occasional allochthonous input of litterfall are examples of coarse particulate organic matter (CPOM>1 mm). Bacteria degrade these into fine particulate organic matter (FPOM<1 mm) and then further into usable nutrients. Small organisms such as plankton are also characterized as FPOM.
Siphonophores are most often pelagic organisms, yet level species are benthic. [11] Smaller, warm-water siphonophores typically live in the epipelagic zone and use their tentacles to capture zooplankton and copepods. [11] Larger siphonophores live in deeper waters, as they are generally longer and more fragile and must avoid strong currents.