deepsea

Deep sea refers to the portions of the ocean beyond the reach of sunlight, where photosynthesis is negligible. Definitions vary, but it is commonly taken to begin around 200 meters and extends through the mesopelagic (roughly 200–1000 m), bathypelagic (1000–4000 m), abyssopelagic (4000–6000 m), and hadal zones (deeper than 6000 m).

Conditions in the deep sea are extreme: high hydrostatic pressure, near-freezing temperatures, and permanent darkness. Food is scarce and mostly arrives as marine snow that sinks from above, with additional input from carcasses and chemolithoautotrophic ecosystems near hydrothermal vents and cold seeps. Many deep-sea communities rely on detritus rather than sunlight for energy.

Biological communities are diverse, including fishes, cephalopods, crustaceans, echinoderms, and microbial mats. Adaptations common in deep-sea life include slow metabolism, enlarged mouths and stomachs for scarce prey, specialized eyes or loss of vision, and bioluminescence for camouflage, prey attraction, or communication. Some groups show gigantism, while others remain very small. Reproductive strategies vary, but many species exhibit low growth rates and long lifespans due to the environment.

Habitats span abyssal plains, mid-ocean ridges, trenches, seamounts, and vent and seep communities. Hydrothermal vent ecosystems host microbes that derive energy from chemical reactions, supporting complex animal communities independent of sunlight.

Exploration and study of the deep sea rely on deep-submergence vehicles, remotely operated vehicles, and advanced mapping technologies. Challenges include pressure, darkness, cold, and logistical costs. Deep-sea research informs biology, geology, and climate science, and raises considerations regarding conservation and sustainable use of deep-ocean resources.