DEEP-SEA BIOLOGY

PELAGIC ZONES:
"Pelagic" refers to the swimmers and drifters above the bottom; the main subcategories are plankton--organisms at the mercy of currents--and nekton--animals that can outswim currents. Pelagic zones are divided into layers (see Figure above, right):

--EPIPELAGIC: The familiar surface waters, where most of the energy input (sunlight) occurs, and food chains begin primarily with phytoplankton ("plant drifters"; mostly microscopic algae such as diatoms). Life can be abundant, if nutrients and sunlight are plentiful. By far the most studied habitat, it goes only to a maximum of 200m deep (maximum depth of light adequate for photosynthesis). Consider that in comparison to the average ocean depth at 4000m, going down to 11,000m!
--MESO-, BATHY-, ABYSSO-, HADO-PELAGIC: Below the epipelagic in the deeper zones, food chains are energy-poor (due to lack of sunlight) and usually begin with detritus and bacteria. Since no algae can grow in these zones, life gets sparser the deeper one goes. Most animals have watery, gelatinous low-energy muscles and other tissues, and sluggish, low-metabolism lifestyles. These features were originally thought to be ways to cope with low food energy--the "low-energy" hypothesis--but the features are also found in animals near Hawaii where there is more food available than in temperate zones. The alternative"visual-interactions" hypothesis proposes that the lack of light means that animals with eyes do not need robust muscles since they only need to be able to dart briefly in the darkness when a predator or prey come close enough to see.
These deep zones together form the largest habitat on earth and the least explored. The depths include:

• The Mesopelagic (=midwater or "twilight zone"), where there is still faint light but not enough for photosynthesis (about 200m-1000m). Bacteria, salps, shrimp, jellys, swimming (cirrate) octopods, vampire and other squids, and fish are typical; many are bioluminescent.
• The Bathypelagic and Abyssopelagic ("midnight zones"), which are even deeper (with depths not clearly defined). Finally, the deepest tectonic trench is about 11,000m, with those habitats sometimes called Hadopelagic. Examples of deep pelagic animals are shrimp and anglerfish, though no fish has ever been seen or caught below 8,400m. We are just beginning to study these deepest habitats; the MESOPELAGIC/Midwater page (link below) has some bathypelagic species and links.

BENTHIC ZONES:
"Benthic" refers to life on or in the ocean bottom; animals swimming just above are "benthopelagic."
-- The shallow zones are the Intertidal (between high and low tide levels) and the Subtidal (on continental shelves).

DEEP BENTHIC ZONES--Bathyal, Abyssal, Hadal:
--Bathyal zone: The continental slopes, usually in the range of 300-2000m.
--Abyssal zone: The general ocean bottom or abyssal plain (2000-6000m; average 4000m deep, and covering perhaps half of the entire planet);
--Hadalzone: The deep troughs and trenches --6000m to 11,000m max in the Mariana Trench; worldwide there are about 15 subduction trenches with hadal depths, and several deep non-subduction troughs. These have been little explored, bur are receiving renewed attention with our HADES and other programs, e.g., new camera/trap landers have recently been deployed by University of Aberdeen, Scripps Inst. Oceanogr., U. Hawai'i (2008 to present, including Kermadec and Mariana Trenches in 2014); and James Cameron's DEEPSEA CHALLENGE sub reached the Mariana Trench bottom in 2012. All the programs have yielded news-making results (new fish species, supergiant amphipods...see NEWS page and search for "trench" and news links at top of this page.)

The surface of most deep areas is mud and organic-mud mixture called ooze, though some bathyal slopes are rocky. Most life here must also cope with low food supply, and again most food chains start with detritus and bacteria. The most common abyssal fishes are the rattails or grenadiers and cusk eels; in the hadal zone, snailfish dominate. The most common non-microscopic invertebrates are echinoderms such as sea cucumbers, brittle stars, and amphipods; but there are many other animals such as sea anemones, sponges, molluscs, crabs, seaspiders (pycnogonids), burrowing worms of many types; etc. Echinoderms such as sea cucumbers have been seen at almost 11,000m depths!
SEAMOUNTS:
Rising from the seafloor all over the world's oceans, seamounts are hills and mountains that do not reach the ocean surface. There are thought to be over 30,000 of these, most of which are unexplored. Recently several expeditions have studied some of these, and are finding a large number of new species of animals. See Seamounts Online for information.

Manganese nodule

OUR STUDIES:
Go to our ANIMAL PAGES for bathyal and abyssal animals
and HADAL page for trench animals

SEAMOUNTS: See pictures from SEAMOUNTS at Seamounts Online

Seep tubeworm

Seep bacterial mats

Special Habitats--VENTS and SEEPS
These are exceptions to the rule that the deep sea is energy-poor. These unusual habitats are:

• Hydrothermal vents (deep volcanic hot springs), mainly along the midocean ridges (but sometimes shallow, e.g., near Iceland). Here hot (up to 400C) mineral-laden water provides abundant energy; and
• Cold hydrocarbon seeps, mostly along continental margins, where gases (methane--sometimes as methane-hydrate ice--and hydrogen sulfide) and oil seep out of the sediments and also provide abundant energy. By some estimates, there is more energy locked up in these methane ices than in all (other) fossil/hydrocarbon fuels combined.
  

NOTE to Researchers: We need help identifying species with a "*" on the above animal pages. If you can help with any of these below or on the linked animal pages, please send a message below, or to Whitman College at

We have some specimens, but some are deposited at the Field Museum in Chicago and loans of the material can be arranged through Janet Voight (voight@fmnh.org) or John Slapcinsky (Slapcin@fmnh.org).
Taxonomists might want to check out SCAMIT, a S. Calif. organization working to standardize marine invertebrate taxonomy.
See also the Ocean Biogeographical Information System: " ...(OBIS) is a web-based provider of global geo-referenced information on accurately identified marine species." Also, the Census of Marine Life site details this decade-long program to catalogue as many species of marine organisms as possible, with new species being discovered routinely.