Marine Biology Terms Starting With D
Marine Biology Glossary: D
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Deep Sea
/ DEEP SEE / · Old English deop meaning deep and sae meaning sea
Deep Sea is the ocean region below 200 meters where sunlight cannot penetrate, pressure increases by roughly one atmosphere for every 10 meters of depth, and organisms must survive in permanent darkness with limited food supply.
Below 200 meters, the water column is divided into the mesopelagic zone down to 1,000 meters, the bathypelagic zone to 4,000 meters, and the abyssal and hadal zones below that. Temperatures in the bathypelagic zone hover near 2 to 4 degrees Celsius, and pressure at 4,000 meters exceeds 400 atmospheres. Most deep-sea animals depend on marine snow, a continuous rain of organic particles sinking from surface waters, as their primary food source.
At hydrothermal vents and cold seeps, chemosynthetic bacteria support entire food webs independent of sunlight, sustaining tube worms (Riftia pachyptila) that can exceed 2 meters in length.
The deep sea covers more than 65 percent of Earth's surface and represents about 95 percent of the total volume of the biosphere available to living organisms, yet fewer than 20 percent of deep-sea species are estimated to have been formally described by science.
Deep-sea life is rare everywhere. Hydrothermal vents, cold seeps, whale falls, and seamounts can host organism densities rivaling those of tropical coral reefs, concentrated wherever chemical energy or sinking organic matter accumulates.
Anglerfish (order Lophiiformes) in the bathypelagic zone use a bioluminescent lure dangling from a modified dorsal spine to attract prey in complete darkness. In some species, the lure is colonized by bioluminescent bacteria, and the light it produces can be detected by prey from distances of 1 to several meters in water where no other light exists.
Deep Sea Trench
/ DEEP SEE TRENCH / · Old English deop meaning deep and trench meaning cut or ditch
Deep Sea Trench is the deepest type of seafloor feature, formed where one tectonic plate subducts beneath another and pulls the ocean floor downward into a narrow, elongated depression that can exceed 11,000 meters in depth.
Trenches form at convergent plate boundaries, where the denser oceanic plate bends and descends into the mantle at angles that can exceed 45 degrees. Pressure at the bottom of the Mariana Trench, the deepest known trench at approximately 11,034 meters, reaches more than 1,000 atmospheres. Despite these conditions, hadal amphipods (family Lysianassidae) reach densities of thousands of individuals per trap haul, feeding on organic material that sinks from the surface.
Microbial communities in trench sediments process carbon and nitrogen at measurable rates, linking the hadal zone to global biogeochemical cycles.
The Challenger Deep in the Mariana Trench reaches about 11,034 meters below sea level. Mount Everest placed at its bottom would still be submerged by more than 2 kilometers, and the water pressure at that depth is roughly 1,086 times greater than atmospheric pressure at sea level.
Trenches are lifeless because pressure is too extreme for biology. Specialized bacteria, amphipods, polychaete worms, and snailfish (Pseudoliparis swirei) have been documented living and actively feeding at depths exceeding 8,000 meters.
The Kermadec Trench northeast of New Zealand reaches depths of about 10,047 meters and hosts populations of the hadal snailfish (Notoliparis kermadecensis). Researchers using baited camera landers have recorded these fish actively swimming and feeding at depths greater than 7,000 meters, demonstrating that vertebrate life persists even under extreme hadal pressure.
Demersal Fish
/ dih-MUR-sul FISH / · Latin demergere meaning sink down
Demersal Fish are fish that live and feed on or near the seafloor, using the bottom substrate as a primary habitat for foraging, shelter, or reproduction rather than spending most of their time in the open water column.
Many demersal species have flattened body forms, cryptic coloration, or downward-facing mouths that suit life on or just above sediment, rock, or reef surfaces. Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) forage across sandy and gravelly shelf bottoms, consuming polychaete worms, crustaceans, and small fish. Flatfish such as European plaice (Pleuronectes platessa) lie partially buried in sediment and ambush prey, while skates and rays use electroreceptors to detect invertebrates hidden beneath the seafloor.
Demersal species account for a large share of global commercial landings, and bottom trawling targeting these fish disturbs benthic habitats across millions of square kilometers of shelf each year.
Some demersal fish undertake long vertical migrations: Pacific halibut (Hippoglossus stenolepis) spawn at depths of 270 to 360 meters but juveniles migrate to intertidal and shallow subtidal zones, sometimes traveling hundreds of kilometers along the coast before returning to deeper adult habitat.
All fish swim constantly in open water. More than half of all known fish species spend most of their lives within a few meters of the seafloor, foraging on bottom-dwelling invertebrates and organic debris rather than pursuing prey through the water column.
Atlantic halibut (Hippoglossus hippoglossus) are among the largest demersal fish in the North Atlantic, reaching lengths of up to 2.5 meters and weights exceeding 300 kilograms. These flatfish lie on sandy or gravelly bottoms at depths of 200 to 600 meters, ambushing cod, redfish, and large crustaceans that pass within striking range.
Dissolved Oxygen
/ dih-ZOLVD OK-sih-jen / · Latin dissolvere meaning loosen and Greek oxys meaning sharp
Dissolved Oxygen is the molecular oxygen gas that diffuses from the atmosphere and from photosynthesis into water, where aquatic organisms extract it directly through gills, skin, or other respiratory surfaces to fuel aerobic metabolism.
Oxygen dissolves more readily in cold water than warm water: at 0 degrees Celsius, seawater holds about 14.6 milligrams of oxygen per liter, while at 25 degrees Celsius that capacity drops to roughly 8.2 milligrams per liter. Photosynthesis by phytoplankton and seagrasses adds oxygen to surface waters during daylight, while microbial decomposition of organic matter consumes it, sometimes creating hypoxic zones where concentrations fall below 2 milligrams per liter. The Gulf of Mexico dead zone, which forms each summer off the Louisiana coast, covers thousands of square kilometers where oxygen depletion forces mobile fish to flee and kills sessile invertebrates.
Salmon require concentrations above 7 milligrams per liter during spawning, making dissolved oxygen a direct constraint on where these fish can successfully reproduce.
The Black Sea below about 150 meters is permanently anoxic, meaning dissolved oxygen is essentially absent. Sulfide-producing bacteria dominate this zone, and no fish or most invertebrates can survive there, making it one of the largest oxygen-depleted marine environments on Earth.
Water's oxygen comes from splitting water molecules during respiration. Fish and other aquatic animals use molecular oxygen gas dissolved in water, the same O2 found in air, not oxygen atoms chemically bound within water molecules.
Respiratory System Fun Facts →Sockeye salmon (Oncorhynchus nerka) returning to spawn in British Columbia rivers require dissolved oxygen concentrations above 6 to 7 milligrams per liter to sustain the physiological demands of upstream migration. Stream temperatures above 20 degrees Celsius reduce oxygen solubility enough to cause measurable mortality in these fish before they reach their spawning grounds.