Benthos

V. K. Verma¹, Dinesh Kumar² and B. J. Saud³

^{1 & 3}Research Associate, Central Inland Fisheries Research Institute, Regional Centre, Guwahati Assam

²Senior Research fellow, National Bureau of Fish Genetic Resources, Lucknow

Email: agnesv2008@gmail.com

The term benthos is derived from the Greek, meaning 'depths of the sea' and refers collectively to organisms which live on, in, or near the bottom of the sea. Benthos is also used in freshwater biology to refer to organisms at the bottom of freshwater bodies of water, such as lakes, rivers, and streams. Many organisms adapted to deep-water pressure, cannot survive in the upper parts of the water column.

Food Source

Light does not penetrate deeper water so; the energy source for deep benthic ecosystems is often organic matter. Dead and decaying matter sustains the benthic food chain; most organisms in the benthic zone are scavengers or detritivores.

The depth of water, temperature and salinity, and type of local substrate all affect the presence of benthic community. In coastal waters and other places where light reaches the bottom, benthic photosynthesizing diatoms can proliferate. Filter feeders, such as sponges and pelecypods, dominate hard, sandy bottoms. Deposit eaters, such as polychaetes, populate softer bottoms. Fish, sea stars, snails, cephalopods, and crustaceans are important predators and scavengers.

Benthic organisms, such as sea stars, oysters, clams, sea cucumbers, brittle stars and sea anemones, play an important role as a food source for fish and humans.

Classification- Benthic organisms can be classified on the different basis.

A- By substratum

The benthic community includes of a wide range of plants, animals and bacteria. They are classified into 3 categories

1- Infauna - organisms that live in the sediment

2- Epifauna - organisms which either attach to the bottom or substrate; move within the sediment; or that live on the sediment surface

3- Demersal - fish that feed on the benthic infauna and epifauna

B- By size

1- Macrobenthos

Macrobenthos are the larger in size (1 mm), more visible. For examples: polychaete worms, bivalves, echinoderms, sea anemones, corals, sponges, sea squirts and larger crustaceans such as crabs, lobsters.

2- Meiobenthos

Meiobethos are tiny benthos that are less than 1 mm but greater than 32 μm in size. Some examples are nematodes, foraminiferans water bears, gastrotriches and smaller rustaceans such as copepods and ostracodes.

3- Microbenthos

Microbenthos are microscopic benthos that are less than 32 μm in size viz. bacteria, diatoms, ciliates, amoeba, and flagellates

C- By type

1- Zoobenthos: The benthos those are animal origin called zoobenthos.

2- Phytobenthos: Benthos those are plant origin called phytobenthos.

D- By location

1- Epibenthos: Benthos living on the top of sediment.

2- Hyperbenthos: Hyperbenthos live just above the sediment.

Plankton (the organisms that float or drift within the water),

Nekton (the organisms that swim (powerfully) in the water), and

Neuston (the organisms that float on the water).

Benthic communities

Biologists refer to species dwelling on the sea floor at any depth as benthic organisms or 'the benthos'. They are also known as bottom dwellers. Fish are common in benthic communities in the mesopelagic and bathypelagic zones, but there are none on the deeper sea floors of the abyssopelagic and hadopelagic zones. Energy is in scarce supply here, deriving mainly from falling food that has made it past all the waiting mouths above.

Bacteria are probably the base for most benthic food chains, as they are able to release nutrients that other life forms cannot make use of directly. For example the armour-like exoskeleton of zooplankton such as krill contains carbohydrates locked up in the tough chitin or cellulose that only bacteria can break down. These nutrients can then be used by slightly larger organisms referred to as the meiofauna, which live in the fine sediment that covers most of the deep-sea floor. They in turn concentrate nutrients and make them available to larger organisms. The occasional bonanza, such as a dead whale, may provide food for decades.

Echinoderms

Echinoderms are the most obvious deep-sea species. They include starfish (sea stars) and brittle stars, sea urchins and sea cucumbers. Sea cucumbers may move by swimming or squirting water. Others walk on the sea floor using tube feet. In 1986 a new group of echinoderms, the Concentricycloidea, was discovered on wood in deep water off the New Zealand coast (although some taxonomists no longer consider them a new class).

Crustaceans of various kinds, polychaete worms and bivalve molluscs are also reasonably common. Some deep-sea invertebrates — most notably some isopods and pycnogonids (sea spiders) — are gigantic.

Fish

Among the benthic fish species is the strange tripod fish (belonging to the family Triacanthidae), which has long feelers on its pectoral and caudal fins. The ventral and caudal fins act like a tripod, holding the fish on the sea floor, while the pectorals are held sideways and detect any water movements caused by prey. These fish are virtually blind. Other bottom-dwelling species are fished commercially and end up on the table — grenadiers (or rat-tails), orange roughy, oreos and cusk eels (sold as ling). Less savoury characters include hagfish, brotulas, toadfish and snailfish.

References

Benthos. (2008). Encyclopedia Britannica. (Retrieved May 15, 2008, from Encyclopædia Britannica Online.)

Ryan, Paddy (2007). Benthic Communities Te Ara - the Encyclopædia of New Zealand, updated 21 September 2007.

Yip, Maricela and Madl, Pierre (1999). Benthos University of Salzburg.