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Biosphere as Place

Ocean: Benthic Biomes One

  Structure and Flow in Benthic Biomes
  Continental Shelf Benthic Biomes
  Warm Water Coral Reefs
  Shelf-edge Sponge Reefs
  The Abyssal Plain
  Cold Water Corals







Ocean Life Zones are based on depth from the surface and how much light they receive. They are not defined by a bottom, which seems strange to land dwellers These zones are three dimensional volumes of saltwater, for the most part in eternal dark. There are no landmarks. But the ocean does have bottoms, from inches to miles below the surface.The seafloor is vast and mostly unexplored. Sea floor communities of life are called the benthos. The biomes they occupy are called benthic.

The bottom of the ocean is enormous; a collection of the largest biomes on the planet. The Continental Shelf extends from the shore to an average of 50 miles out to sea. It is relatively shallow, down to about 500 feet. Then the seafloor drops off steeply at the continental slope, where the continental plate ends and the true sea floor begins. Eventually, ocean depth increases until it can be measured in miles. The seafloor there is called the Abyssal Plain.

A graphic may give some sense of the ocean's nature:

Graphic courtesy Wikipedia


The continental shelf is an underwater part of the continental plate. During Glacial Periods it is dry land. Our civilization exists in the Holocene Interglacial Period, which began 11,400 years ago.

Structure and Flow in Benthic Biomes

The first key to benthic habitat is structure. The second key is flow.

Every physical interruption in a sameness, whether on a sandy bottom, estuary muds, or the vast abyssal plains miles down, will provide for an eruption of life, because it will provide multiple places to live with some degree of protection.
In the photo, a stone is colonized by gray sea squirts, a sponge, sea stars and many other lives.

Physical structure, from a pebble 1/2" tall to a mound, tends to create flow. Flow of any sort allows filter feeders, from mussels to sponges to corals, to sift food from passing water. Structure at ecotones (edges between ecosystems) tend to have so much flow they are called currents. At the edges of continental shelves there are dropoffs toward the abyss. Such abrupt changes also create flows.

The most important kind of flow to living organisms in saltwater are upwellings, when water moves upward from nutrient rich sediments in the dark (where nutrients cannot be used) to the lighted plankton zone on top. Upwellings often occur where water meets land: offshore winds combine with currents to move colder water up to replace warmer water blown offshore. Upwellings are seasonal. Large phytoplankton blooms occur at upwellings.




graphic courtesy of Geosciences, U. Arizona

In the deep ocean, strong upwellings occur at sea mountains, creating oases of life above.

In the satellite photo above
, almost the whole Baltic Sea is covered by an algae bloom.

Another way to think about Benthic Biomes as distinct from depth or
light--based Ocean Life Zones is that they occur at interfaces, or meeting points, between water and seafloor. Such dramatic differences in habitat create edges, which are places of energy. The intersection of water and earth means a feeding ground for carnivores, which means that omnivores and detritus feeders also get to eat.

As you recall, an ecological niche is both a place to live and a way of making a living—an address and a job. On the continental shelves, every kind of structure is likely to harbor many kinds of life—boulders and gravel dropped from an iceberg ten thousand years ago, a sunken log, a shipwreck, channels carved by currents—even non-chemical debris from our civilization--create many niches in which life thrives.

Below on this page you will find several kinds of benthic hotspots. Science has so recently been able to explore the ocean deeps that it seems as if every month a totally new ocean feature is discovered. We learn of whale falls, cold seeps, carbonate mounds, coldwater coral reefs and mud volcanoes, and wonder what other marvels will be soon discovered. The deep abyssal plain covers 60% of Earth's surface; by 2007 only about 3% had been explored.

At the offshore ends of the continental shelves there is a steep dropoff called a continental slope, which carries the seafloor and sediments downhill from the shelf. Most of these sediments are not from river runoff. They are thousands of years old, from the melting of glaciers that carried volumes of soils, clays, gravels and boulders scoured from the land as the ice traveled. These old sediments tend not to be covered over by more recent river deliveries.

Where there is a sudden change in topographical structure, like the continental slope (often cliff-like), there will be ocean currents. These currents usually flow horizontally, but when structure such as canyons interrupt, they change in unpredictable ways, which include upwellings. The slope surface is given vertical structure here and there by glacial dropstones which are habitat for many animals. Sediment channels from long ago also groove canyons in the slope.

From the bottom of the continental slope the seafloor slopes down gradually to the abyssal plain. This deeper, less steep slope is called the Continental Rise.

The Rise is made of accumulated sediments from the continent; it makes a kind of apron or margin between the continent and the deep ocean. Rise sediments are small particles generally, clays and muds, so there are few interstices for life to exploit. The water at the rise is deep and not far above freezing cold. Most life on the rise lives on the sediment surface; some is dug in. Burrowing in such cold is energy costly, so animals can't do lots of it. Like woodpecker holes in trees, burrows here are recycled and used several times by generations of shrimps and worms. Burrows become a bit larger with re-use, which means more oxygen-rich water for the dweller.

Most of the benthos on the Continental Rise are deposit feeders on the surface. They are preyed upon by bottom-feeding (demersal) fish. There are more fish living near the bottom than in the cold lonely dark of the bathypelagic life zone.

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Continental Shelf Benthic Biomes

Continental shelves are extensions of the continental plates. They surround each continent. Some extend a few miles, some 250 miles. The average width is fifty miles. They create a rich marine habitat that receives some sunlight even at their average depths of 150 meters (490').

The continent's shelves are the ocean we know best. It is where the great fisheries are, it is where the plankton flourishes and the tropical corals and seagrasses grow, and not least, it is where we drill for offshore oil and gas. The North and Baltic seas are entirely on the continental shelf.

The productivity of shelf waters is partly the result of upwellings, surges of water from the depths to surface waters that are caused by coastal currents and offshore winds. These upwellings bring nutrients to the surface from sediments, where they have lain unavailable to the plankton. Upwellings result in explosive increases in the phytoplankton, which increases the zooplankton, and on up the food web. Shoals of fish congregate on the shelves when blooms occur.

The continental shelves are also where most shipwrecks lie. Because they add structure to the seafloor, the benthos responds as if wrecks were a new reef, which attracts fish and other macro-animals. Shipwrecks provide hard surfaces, which recruit larval filter feeders such as soft corals and sponges, which in turn create many micro-habitats, so wrecks become hotspots of biodiversity, especially where they lie on soft sediments, which have community assemblages quite different from the community that assembles on the wreck.

Detail, wreck at Edmonds underwater park
graphic courtesy Owen Caddy

Continental Shelf Rocky Shores



Rocks provide many ecological niches. Each irregularity, each crack, however small, is colonized by larval animals and by macroalgae. Rocks are often covered with red coralline algaes, which tend to be rough-surfaced. These bumpy surfaces are colonized by larval mollusks, which then graze on green algae which can overlay and eventually kill the coralline algae that first colonized the rock.

Sandy shores also offer multiple habitats, especially for meiofauna. Meiofauna are minute but not microscopic animals that live in tiny niches, such as those between grains of sand. They include nematode worms and copepods.

Sands are made of fairly uniform-sized grains. Spaces between sand grains are interstices; animals who live there are interstitial. The larger the sand grains, the larger the space available, so larger interstices mean larger animals.  Sands also support burrowing macrofauna, such as clams, snails, crabs, and worms.

Gravel is looser than sand; it’s made of small, water-smoothed stones and pebbles of different sizes. Only larger animals can dig in gravel, but if you’re tiny, you can just slip between the pebbles. Unlike fine sands or muds, gravels can offer flow. Flow means that small suspension feeders who filter food from water passing by can colonize the upper layers of gravel or attach to the tallest stone on the gravel surface.

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Continental Shelf Seagrass Meadows



Along coastlines, in estuaries and shallow bays, seagrass meadows provide both food and refuge to young invertebrates and most dramatically, to the dugong and manatee. Seagrasses, unlike the macroalgae "seaweeds" which tend to dominate coastal habitats, are flowering plants which evolved on land and returned to the sea, just like the manatee, seals, and cetaceans.

Dugong grazing in seagrass meadow off Australia. North American manatees are close cousins, but unlike dugong, use both fresh and salt water.

Seagrasses are coastal "canary" species; when they die out, we are warned: pollution has become too destructive. Seagrass meadows are threatened around the world. Coastal waters eventually receive all the runoff from cities and farmlands alike. This runoff includes hundreds of chemicals we use but seem determined to be ignorant about. As a result, North American coasts have lost around 50% of their seagrass in the past hundred years. Seagrasses are destroyed also by dredging and dumping, by boat motors and anchors, and generally by the pressures of vast human seaside development in recent years.

The ecological roles of seagrasses include stabilizing bottom sediments, filtering excess nutrients out of the water, and providing essential nursery habitat to young of many kinds, including flounder and sea bass, lobsters and shrimp, scallops and mussels. In addition, many small animals live on seagrass stems and leaves; tiny shrimps and crabs, invertebrates such as amphipods, and sponges. Their loss is a loss of biodiversity.

Meet three creatures in the seagrass meadows:

amphipod eating algae
pipefish hunting
small sea star
hunting on seagrass stem

Most marine animals pass through several life stages; larval stages of both fish and invertebrates are favorite foods of predators, and therefore depend for species survival on refuges such as seagrass meadows. Seagrass is one of the primary producers that begin food chains, so it is food as well as habitat. Many small organisms colonize seagrass stems and blades, and many others rely on them for food.

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Continental Shelf Hard Bottoms

Continental shelf biomes are often characterized as hard-bottom and soft-bottom; the hard are bottoms of small stones and gravels; soft-bottom biomes are mud sediments, clay muds and fine sandy muds. Animal assemblages on hard bottoms are typically filter feeders and a large meiofauna (meiofauna are minute but not microscopic animals that live in tiny niches), while soft bottom animals are typically deposit feeders, such as sea cucumbers. Many deposit feeders burrow, and some burrowers, such as shrimps, also swim up into the water column at night to feed.

Rock and gravel bottoms, while not the most common, offer many niches for small animals in the interstices of the gravel. Rock offers solid substrate to sessile animals that require strong attachments.

Here are a few creatures of rock and gravel bottoms on the continental shelf:

plumose anemone
on rock
eating squid eggs on rock
anemones on boulder
nudibranchs eating
sea strawberries

eel winds through
dead seaweeds

flounder hides itself except for periscope eyes
sandy bottom, a skate , sponges,
a fish hiding photo credit USGS
muddy sands: a flounder lies in wait,
sea stars photo credit USGS
dropstone:lobster, hermit crab, sea stars.
sea squirts photo credit USGS
boulders: sea star, fish, coralline algae
photo credit USGS
Continental Shelf Soft Bottoms


The bulk of the seafloor on the continental shelves is made of sediments. Some are sandy, some are clay, most are deposited muds. Animals that live on the surface of those sediments, as well as those that dig into the sediments, depend primarily on deposited dead material. They are called deposit feeders. But everywhere there is a bit of structure--a pebble, coarse gravels, junk, boulders, wrecks--there will also be suspension feeders that filter out food as if drifts by, for suspension feeders need flow to thrive.

The most common deposit feeders are sea cucumbers, snails, crabs and sea stars. They live above assemblages of burrowing worms of many kinds, and burrowing snails and mussels. The sediment muds and sands are also filled with protozoans and procaryotes, which are food for many minute animals called meiofauna.

Bryozoans filter feeding
Photo credit Beth Okamura
deposit feeding sea cucumber on mud flats
ostrocod meiofauna (enlarged)
burrow of shrimp in mud

Suspension feeders include soft corals, sponges, stalked barnacles, and colonial sea-squirts. Where structure is low but currents strong there are huge colonies of tiny bryozoans, or moss animals. Currents are often strong where land and water meet, so island waters often are surrounded by living structure.

Below are a few scenes from soft bottoms of the continental shelf:

amphipod tubes in muds
photo credit USGS
lobster preys on rock crab
photo credit USGS
mud shrimp burrows-in
photo credit USGS
fish in burrow in clay mud
photo credit USGS
spider crabs gather to mate
photo credit USGS
moonsnail digs in to hunt
photo credit USGS

From estuary salt marsh to seagrass meadow, from rock and gravel bottoms to fine sediment muds, from the bright plankton layer to the top of the twilight zone, from shores to the dropoff at the edge of the continental shelf, the shelf benthic biomes are diverse and productive.

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Warm Water Coral Reefs

Like a necklace around the planet, on both sides of the equator, coral reefs have created stunning biodiversity.

By far the most well known hotspots on the continental shelves are the warm water coral reefs. These structures offer myriad three-dimensional habitats that vary by height from the bottom and depth below surface, by how much sun and shade are received, by burrow depth, by overhangs, and so forth. Burrows in coral can be made by wave action breakage, and by animals that bore holes in calcium carbonates such as bivalve shells and coral skeletons. For example, sulfur sponges bore slowly into dead coral and cycle calcium into the water while creating new places for animals to shelter.

The reef is topographically complex. Much like a rain forest, it has many layers, and areas of shade. Because of the structural complexity, thousands of species of fish and invertebrates live in association with reefs, which are by far our richest marine habitats. In Caribbean reefs, for example, several hundred species of colonial invertebrates can be found living on the undersides of plate corals. It is not unusual for a reef to contain several hundred species of snails, sixty species of corals, and several hundred species of fish. Of all ocean habitats, reefs have the greatest development of complex symbiotic associations. These tropical reefs are the benthic biome most familiar to people in general. Because they occur just below the ocean surface, no deeper than 150', they lie within the teeming plankton zone of life.

These shallow coral reefs are all located in warm tropical waters, many of them on slowly eroding islands that were the tops of volcanoes. An atoll is a ring shaped group of coral reefs; the island that was once in the center of the ring has eroded away. Several Pacific Ocean island groups are atolls. The center of an atoll is a lagoon of shallow water with reduced wave action; this calm water enables a host of living communities.
The coral reefs in our minds teem with flashing synchronized fish and shrimps and anemones, all brilliantly colored. It's all true, for now.

Reefs are built slowly, over eons, by colonies of coral polyps which  grow calcium carbonate skeletons which are left behind as individual polyps die, and the next generation of corals builds their skeletons on top of the old ones. Corals are not the only reef builders. Other organisms on the reef also secrete calcium carbonate. Algae that deposit calcium carbonate are called coralline algaes. (Long, long ago, before corals existed, coralline algae built the very first ocean reefs.) Now, coralline algae glue the reef together, and help it resist wave action.

Coral animals pass through several life stages: eggs float away from the reef, hatch into tiny swimmers that look like tiny jellyfish, mature while drifting in the plankton, and eventually sink to the benthos to begin being polyps that stay put. Reef polyps are colonial; their stomach cavities are all connected. Food can be passed from one to another.

Over thousands of years, these reef structures become tall and complex, filled with nooks and crannies, shaded and lighted, each in turn filled by an animal with or without a backbone (moray eel, octopus, shrimp, anemone). The vertical height of the reef creates communities (more niches, more lifestyles) based on the variation in available sunlight as water gets deeper. Real estate is valuable, and there is competition. Algae want light, and try to overgrow the original tenants. Some parts of reefs are shadowed, which enables even more lifestyles. Near the top of the reef, wave-action creates other options resulting in even more kinds of assemblages.

These coral animals of warm-water reefs live in symbiosis with photosynthetic algae called zooxanthellae (zoo-zan-thel-A), which are one-celled dinoflagellate algae that live inside the coral’s body, mostly in the tentacles. The tentacles capture plankton to feed on, but sunlight indirectly provides, through the partner, about 95% of the polyp's food. The zooxanthellae receives nitrogen (in the form of ammonium) from the coral polyps, which feeds the algae and allows it to photosynthesize. Interliving of corals and zooxanthellae  allow shallow reefs to grow quickly in lighted hours compared to darkness, over twice as fast.

At left, octocorals open their tentacles to feed. Each tentacle is well-supplied with symbiotic photosynthesizers.

Just as terrestrial animals and plants depend on each other’s waste products to live in the great exchange of oxygen and carbon dioxide, the coral polyp and its interior algae symbionts live on each other’s waste, or by-products.

A host of other animals, especially colonial animals, live on coral reefs, including soft corals, called soft because they do not leave behind the stone skeletons that create reefs, but most do secrete many hard tiny “needles” called spicules. These spicules, over time, also add to the reef.

One group of sponges, the glass sponges, also contribute structure to the reef. Their latticework skeletons of silica break down and contribute to reef growth.

Many sponges live on the reefs, along with many snails, shrimps, sea slugs, bivalves, limpets, worms, sea urchins and starfish, not to mention hundreds of kinds of fish. So many species depend on the shallow reefs that their number is bewildering, and that is just the macrolife (visible to us).

On a patch reef the size of a living room one New Zealand study counted  one thousand individual fish of seventy species. A large meiofauna thrives; there are also many kinds of algae, mostly microscopic, and a huge number of protozoans, and of course, untold billions of procaryotes.

Symbiosis: Interliving characterizes all reef life. Nowhere else in the oceans is there more symbiosis than in warm coral reefs. Biodiversity creates cooperative opportunities for living. Some tiny crabs live inside clams their whole lives. Some sea urchins partner with a brittle star. Similarly, certain shrimps live on the arms of other brittle stars, almost invisibly camouflaged. There are many small crevices in the reef, where the inhabitants live off each others' leftovers. One little crab (trapezia) that lives within the branching arms of a soft coral, attacks with its pincers anything that tries to eat the coral. It can drive off crown of thorns starfish and parrotfish (see Trapezius photo below).

Symbiotic sea slug with algae living in its tissues. image ©Australian museum

Some sea slugs eat photosynthetic algae and keep the chloroplasts alive in their own outer tissues, so photosynthesis helps feed them with sunlight. Other sea slugs “harvest” zooxanthellae from their food. Some sponges live symbiotically with a macro red alga.

Green turtle being cleaned. With their hard beaks, parrot fish nip off every hitchiker on the turtle's shell and skin

tiny wrasse fish do dental cleaning and are not swallowed

Colorful boxer crab carries anemones
Trapezius crab defends soft coral host against crown of thorns starfish
photo credit Roger Steene

The boxer crab (above) carries small sea anemones in its claws for defense. When a predator comes around the crab waves the anemones so the predator will be stung and leave. The anemones eat bits of dropped crab food.  Another anemone user is the anemone hermit crab, which carries anemones around on the shell it lives within.

Some little fish shelter safely inside sea cucumbers and come out to feed in the relative safety of night. Many little creatures share homes, often burrows. Many crustaceans hitch rides on other animals: crabs and shrimp take slow rides on sea cucumbers.

Many symbioses are behavioral. Clownfish living within sea anemones are well known. Certain little fish called wrasses set up “cleaning stations on the reef, and clean the teeth and gills of larger customers, who do not swallow them. Cleaner shrimps also set up cleaning stations for fish.

cleaner shrimp at work
Photo © Steven N. Norvich

At cleaning stations, wrasses and shrimps eat gill and scale parasites and also glean scraps of food from inside the mouth. Imagine how very long it has taken for these partnerships to safely form.

Warm water corals are all in peril from human activity. Warming seas take their toll by causing the coral polyps to expel their symbiotic algae, on whom they depend for health. This is called coral bleaching, and in many cases it causes the corals to die. But Australian researchers have found that some warm water corals can recruit additional species of algae as symbiotes, species which can apparently tolerate warming better. This may look more hopeful than it is, since the ocean shallows are predicted to become 3 degrees Celsius warmer than present, by 2050, which is more heat than any current symbiote alga can tolerate.

Reefs are also being destroyed by excavation for building materials, over-collecting of specimens for aquariums, and irresponsible tourism.

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Shelf-edge Sponge Reefs

The deeper outer part of the continental shelves of North America were sometimes ploughed-up by icebergs calved from the glaciers that repeatedly covered the continent. When an iceberg is still barely afloat, it scrapes on the bottom, and ice edges make effective plows.

Satellite photo of iceberg gouges off western Canada. Each gouge is
a canyon several kilometers long
photo courtesy USGS
actual photo of iceberg off Greenland to show how much is not visible

The bottoms of glacial icebergs are filled with rock and sediment carried from inland, so when the icebergs melt, quantities of stone drops to the seafloor. The outer shelf tends to be furrowed or grooved by the ice, with large boulders called dropstones lining the top edges of the furrows. Many dropstones occur on the continental slopes as well, and many on the abyysal plain nearest the shelves. These ocean places are largely unchanged from when they were made a blink of geologic time ago, only 14,000 years.

Outer shelves tend to have currents fast enough to provide food for filter feeders such as soft corals, sea squirts, and sponges. Sponges like to attach to a hard substrate, so sponge reefs have grown up on dropstones. British Columbia, Canada and the University of Stuttgart have led the way in researching sponge reefs.

The skeletons of a group of sponges called glass sponges
are the foundations of these reefs. The sponges slowly trap sediment, and when they die, the sediment and the glass skeletons are both the reef's foundation and its slow construction. Dead sponge skel-etons are also places where new colonies settle and grow, which helps to "glue" the reef together into one.

Below are a few inhabitants of a sponge reef off British Columbia:

octopus asleep
on sponge reef
photo courtesy Manfred Krautter
hatchling rockfish hiding on a sponge
photo courtesy Manfred Krautter

stunning heterochone
glass sponge
photo courtesy Manfred Krautter
columnar sponges
on sponge reef
photo courtesy Manfred Krautter
gorgon's head brittle star
on sponge reef
photo courtesy Manfred Krautter
crab climbs on sponges
photo courtesy Manfred Krautter
Heterochone glass sponges are major builders of these sponge reefs
photo courtesy Natural Resources Canada

Sponge reefs are less complex than warm water coral reefs. They occur in deep waters, so they do not benefit from feeding on plankton. However, a sponge reef off the Washington coast relies on a coldwater seep of methane, which feeds a dense population of bacteria.These bacteria are chemoautotrophs, self-feeders using methane to make food. The glass sponges sweep the bacteria in through their pores and eat them. This is a new ecosystem, like most deep benthic hotspots, powered by the planet from below the crust.

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The Abyssal Plain

At the bottom of Continental Rises lie the abyssal plains, by far the largest biomes on earth. Such plains are vast and relatively empty, but the flats are interrupted here and there with structure that creates a variety of hotspots of life and diversity. This plain is very cold and completely without light. More daunting to life, the ocean's deepest waters are low in dissolved oxygen.The sunlit surface is miles above, and the weight of all that water creates enormous pressures.

For a long time humans believed there was no life in the ocean deeps. This untested belief persuaded people to dump every possible kinds of trash into the deeps because it was obvious that nothing could live down there anyway. By 2007, only about 3% of this vast biome had been even superficially explored. But driven by markets, the fishing industry, using deep bottom trawlers, which rake the bottom with huge chains, has undoubtedly destroyed much more than 3% of abyssal habitat that took millions of years to grow.


The average depth of the abyssal plain is around 11,000 feet, or two miles. The abyssal plains comprise some 60 percent of Earth's surface. The abyss is mostly flat, but is peppered by sea mounts, islands, and deep rifts (called trenches in the Pacific).The Pacific versions of mid-ocean ridges look more like rounded hills than mountains. They are called the Pacific Rise. Many Pacific seamounts and islands are dormant volcanoes. A few are active. The Atlantic has many seamounts as well. The Pacific is estimated to contain 50,000 undersea mountains, with an estimated total of 100,000 seamounts in all the world's oceans.

The Atlantic is partitioned north-south by the Mid-Ocean Ridge, a chain of mountains that extends 10,000 miles. These mountains, with a rift valley between,mark the boundaries of tectonic plates, which are slowly separating, and making new seafloor with molten magma welling up in the boundary rift. Only in the past few decades have we discovered hydrothermal vents at the ridge; hot water shooting up into cold water like black smoke, and tightly clustered diverse life forms all around them, famously, tubeworms (see below).

Ecology of Abyssal Benthos

 Abyssal organisms have year-round reproduction, small broods, very slow growth from low nutrient intake, and very long lives. Another result of extreme cold and enormous hydrostatic pressure is a very slow rate of bacterial decay. This means that organic remains from the surface or below lie on the sediment surface for a long time, which in turn enables scavengers to live and cycle nutrients one more time.

In the abyssal benthos, the only food, with a few exceptions, is what falls down dead from the surface. It drifts down slowly, for miles. On the way down everything organic is colonized by bacteria in the water column, most often by luminescent bacteria. So food descends from upstairs as bright little specks in the dark, part dead plankton and part living bacteria.

hatchet fish waits in total dark for any feeding opportunity.

Larger food rarely makes if to the depths. Every mesopelagic and bathypelagic predator is waiting for a dead fish from above, or a live fish encountered by chance. The predators often entice prey with their own luminescent bait organs, bright with the same bacteria.

Abyssal food chains typically begin with phytoplankton detritus, rather than with living producers. This is also true of the mesopelagic and bathypelagic life zones, for in the absolute dark, there are no photosynthetic producers. There are, however, in specialized hotspots of life detailed below, some amazing chemosynthetic producers.

A benthos community of the abyss will likely include echinoderms in numbers: sea cucumbers, sea stars, basket stars and brittle stars. Various crabs, dead white in the darkness, will also be present. All these sediment surface animals are either deposit feeders, eating what falls from above, or predators, eating each other. Most of the abyssal plain is floored by thick sediments; in northern and southern lattitudes sediments are around 500 feet thick, while near the equator they thicken to around 1,700 feet. These sediments are inhabited throughout by anerobic bacteria and archaea, for there is no oxygen to be had.

The surface layers of the sediments host a thriving community of worms and isopods, plus a great many small but not microscopic invertebrates known as meiofauna (tiny organisms, neither macro nor micro.) Up to 230 species of shelled protozoans called foraminifera (forams for short) have been found in the top centimeter of sediments in tropical waters, showing huge diversity. An abundance of nematodes (roundworms) live in the abyssal sediments too, as well as many additional protozoans. Crustaceans are represented by tiny harpacticoid copepods
(left, photo by MBARI).

Every worm that tunnels in the sediment changes it by passing the sediment through its body. Burrow construction by crustaceans or bivalves and snails digging down also play a part in mixing the sediments. This mixing process is called bioturbation.

Some near-shore abyssal sediments are composed largely of dusts blown from land, but many abyss sediments are made of the tiny shells (tests) of planktonic diatoms (algae), foraminifera, and radiolarians (protozoans), all microscopic.

Another source of sediments are ocean currents that carry fine clay particles from land that disperse throughout the oceans, and eventually settle in the abyss, so slowly that the deposit may be as low as 1 mm. per 1,000 years.

A few 'faces' of the Abyssal Benthos:

sea cucumber and small brittle star
photo courtesy MBARI
strange little amphipod from the deeps
photo courtesy MBARI
meiofauna ostracods from abyssal sediments (enlarged) photo courtesy MBARI
demersal rattail fish crowd scientists' chum bucket photo courtesy MBARI
gold sea urchin in the abyss
photo courtesy MBARI
sea cucumbers grazing sediments
photo courtesy MBARI
abyssal crinoid or sea lily
photo courtesy MBARI
abyssal amphipod (enlarged)
photo courtesy MBARI
monkfish has swallowed all but the tail
photo courtesy NOAA
two rattail fish on the flat abyssal plain
photo courtesy USGS
Some deep snails build towers of their eggs, perch on top until they hatch, then die and fall off. photo credit Yancy, Whitman College
A juvenile bivalve from the abyssal plain
photo courtesy of MBARI
skirted octopus,
abyssal plain
tanner crab,
abyssal plain
abyssal plain
sea cucumber, brittle star,
abyssal plain
photo credit Ben Wigham
deep ray,
abyssal plain
sea cucumbers,
abyssal plain

Photos such as the above can mislead; they seem to suggest that the abyssal plain is full of macrofauna. Large animals are rare, except in the hot-spot habitats we describe below. While microscopic lives are everywhere in abyssal sediments, they reproduce more slowly than in warmer and more nutrient-rich biomes. Meiofauna, small but not microscopic, are also widespread, but their numbers are small compared to those in biomes whose waters do not approach freezing.

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Mountains in the sea are still a fairly new idea. We have known for centuries that some islands are volcanoes rising up above sea level and falling off steeply downward. Suddenly, it seems, we now know that many thousands of undersea mountains break out of the abyss and rise over a mile toward the surface, but their crests remain in darkness, although a few rise into the euphotic region where photosynthesis takes place.

The tops of seamounts are often calderas, so fairly level surfaces are available to life. Like many volcanoes, most seamounts are roughly conical in shape. Seamounts rise steeply from the abyss to peaks below the surface; this sudden change of structure generates upwelling currents, which in turn support filter feeding life.

Recent estimates are 50,000 seamounts in the Pacific, and 100,000 in all the ocean waters. The tops of seamounts are almost islands: fairly shallow waters, substantial current flow, plankton downdrift much greater than in the abyss. Seamounts all seem to have great schools of mostly one fish species. What species depends on distance from the equator.

A chain of seamounts lies just off the continental shelf from Cape Cod, Massachusets. A satellite chart shows the three which have been briefly visited and explored.The continental shelf slope here looks more like a cliff.

Seamounts have been speculatively known only for 75 years, and it was not until the 1990s that fishing fleets discovered great seamount shoals of fish. Wherever discovered, the seamounts were immediately overfished and their benthic communities badly damaged. The crests of seamounts seem to be synonymous with coldwater coral reefs, for everywhere they are sampled, they are oases of life. Everywhere they have been trawled, however, they are coral and sponge rubble. And almost without fish.

Bear Seamount rise 6,000 feet from the continental slope, but its wide top is still 3,000 feet below the surface.

A few images of what lives on Bear and nearby seamounts:

chimara fish cruising the mount

grenadier fish patrols the basalt,
fan corals, anemone

octocoral polyps feeding, magnified
slickhead fish using a slick hunting trick, drifting in current, pretending sleep
Octocoral Metallogorgia on Kelvin Seamount @ 7,000' with a single brittle star.
NOAA explorers found many metallogorgia colonies on the seamounts, and each had a single brittle star, Asteronyx, in their branches. ©NOAA Ocean Explorer
cold coral community at 6500' deep on Nashville Seamount, 2005
Photo courtesy of DASS05_URI_IFE_IAO_NOAA

closeup of a Paragorgia coral with a sea star and yellow colonial zooanthids
All the seamount photos above are courtesy of NOAA Ocean Explorer.

Seamount coral reefs seem to be where many species of fish aggregate, for mating, and for feeding; the coral structure provides good hiding for eggs and hatchlings. Meiofauna provide food.

Some fish species use seamounts as landmarks in migrations. For instance, yellowfin tuna use various seamounts as navigation seamarks on their travels; they stay less than 5 minutes at Cross seamount near Hawaii. Hammerhead sharks use seamounts off Baja California and the Galapogos Islands to aggregate. They and other fish may navigate to selected seamounts by sensing seamount magnetic fields.

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Cold Water Corals

Coldwater corals are a stunning recent series of discoveries. Biologists have now found coldwater coral reefs in deep waters off 41 countries. These reefs grow slowly in dark cold water.

Imagine the quiet deep sea, untroubled by wave or storm, and growing there, a half-inch a year, for thousands of years, a host of hard-coral animals and their structures. Many other animals are associated with them, especially soft corals and sea stars. Some gorgonian sea fans have been found that are larger than the submersible ROV Alvin. Cold corals have been carbon-dated at 8,000 years old.

Corals in the dark are still filter feeders, so they are found where currents will bring them food. Currents are dependable along continental shelf margins. The Rost reef, the largest cold-water coral reef so far known, is 25 miles long, (twice the size of Manhattan) and was only discovered in the year 2000. It forms part of a chain stretching from the tip of Norway across Europe to the coast of west Africa. Many deep water seamounts are rich with coral gardens. Seamounts occur from New Zealand near-coast seamounts to California to Ireland. Coldwater corals have been found as deep as 20,000 feet, under almost five miles of water. These corals are adaptable; they have also been found only 130 feet down. Cold corals thrive in waters between 39 degrees F and 55 degrees F.

Corals growing in the dark and cold have no symbiotic algae to help feed them, one reason they apparently take thousands of years to become extensive reefs.

There are many separate species of reef-building corals. The most common cold stony coral is Lophelia pertusa on right.
Over half of corals are deep-water species, and the rest form the shallow tropical reefs. Because of their depth and lack of photosynthesis, and the absence of plankton, the biodiversity of cold reefs is lower than that of tropical reefs, but still substantial, and for the deeps, amazing.

Deep corals grow where depth and structure allow. Seamounts are one kind of habitat.

Cold corals Desmophyllum dianthus and Lophelia pertusa on Nashville Seamount
Photo courtesy of DASS05_URI_IFE_IAO_NOAA

Carbonate mounds are another habitat. These are deepwater mounds created by gases seeping from the seabed. They provide hard bottoms, and elevations up to 90 feet. Carbonate mounds come in many shapes, from simple domes to long teardrops, depending on local currents.

Like warmwater corals, cold coral creates structural complexity, which creates opportunities for many kinds and sizes of animal. Larval animals are small, and require hiding places to survive. Reef building corals have a hard skeleton, so animals which need strong attachments colonize them.

rockfish , sea fan corals on a cold coral reef
deep king crab with feather stars
photo courtesy NOAA
7 foot elephant sponge with sea fans
photo courtesy NOAA
Conger eel, crabs, sea urchins, corals
photo by S.W.Ross, NOAA

lumbfish graze-hunting
Photo © Jago Team, Seewiessen

Acesta bivalve
Photo © Jago Team, Seewiessen

Munida "squat lobsters" square off for a fight in a sponge garden
Photo © Jago Team, Seewiessen (link)

iridigorgia soft coral grows in spirals and sways in currents like a dancer
Photo courtesy NOAA
bubblegum coral hosts brittle stars photo courtesy Alberto Lindner of NOAA
larval fish shelters in the octocoral
monkfish uses its pectoral fins like arms to lift in defensive stance
sea cucumbers, feather stars, basket stars, anemone, sponges and sea stars complete this colorful scene in the dark
cold coral scene with "squat lobsters" and feather stars
photo © lfremer & AWI

Go to Benthic Biomes 2

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Explore Further in Biosphere

Biosphere: Introduction
Biosphere as Place: Introduction
Biosphere as Ocean: Life Zones
Biosphere as Ocean Floor: Benthic Biomes One
Biosphere as Ocean Floor: Benthic Biomes Two
Biosphere on Land: Terrestrial Biomes
Biosphere on Land: Anthropogenic Biomes
Biosphere as Process: Introduction
Biosphere Process: Floating Continents, Tectonic Plates
Biosphere Process: Photosynthesis
Biosphere Process: Life Helps Make Earth's Crust
Biosphere Process:
Rock Cycle--Marriage of Water and Rock
Biosphere Process: Marriage of Wind and Water
Biosphere Process: Gas Exchange
Biosphere as An Expression of Spirit
The Ecological Function of Art
The Earth Goddess
The Tree of Life
The Green Man
Earth Art
Biosphere as Community
Biosphere Microcosm: Bacteria and Archaea
The Procaryote Domain
Biosphere Microcosm: Germs
Biosphere Community: The Eucaryote Domain
Biosphere Community: Protists 1: Algae
  Biosphere Community: Protists 2: Protozoa
Biosphere Community: Plants: What's New?
Biosphere Community: Plant Diversity--Major Groups
Biosphere Community: Plant Defense
Biosphere Community: Plant Pollination
Biosphere Community: Plant Seed Dispersal
Biosphere Community: Kingdom Animals
Biosphere Community: Kingdom Fungi
Biosphere Community: Six Great Extinctions
Return to Ecology Index