Biology

Benthos

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Tubeworm colony (Lamellibrachia luymesi) in Gulf of Mexico cold seep. Source: Charles Fisher

Benthos is one of three major ecological groups into which marine organisms are divided, the other two being the nekton and the plankton. The benthos are organisms and communities found on or near the seabed. This includes those animals (zoobenthos) and plants (phytobenthos) living on (epifauna) or in (infauna) marine substrata as well as those that swim in close proximity to the bottom without ever actually leaving it. In terms of size, this is generally divided into three categories:

  • Meiobenthos, the organisms that pass through a 0.5 millimeter sieve;

  • Macrobenthos, those that are caught by grabs or dredges but retained on the 0.5 millimeter sieve;

  • Infauna, organisms that live in the sandy or muddy bottom sediments; and

  • Epifauna, organisms than live on, rather than in, the seabed.

Those in the latter category are usually larger.

Benthic life is subject to vertical zonation depending chiefly on filtered sunlight, moisture and pressure. This has led to the division of benthonic animals into two systems and seven zones. Proceeding from shallow to deep water, the first system is the phytal or littoral system, composed of the supralittoral, mediolittoral, infralittoral and circalittoral zones. The second system, the aphytal or deep system, is composed of the bathypelagic, abyssal and hadal zones.

Substrate types

The benthos has significant variation not only depending on ocean depth, but also upon the type of seabed. Some of the chief types of sea floor substrates are:

  • Sandy soft substrate
  • Muddy soft substrate
  • Hard substrate
  • Secondary hard substrate

The grain size is strongly influential on the packing density of seabed soils, so that the amount of interstitial space among sand or sediment grains affects the amount of dissolved oxygen avalable to benthic fauna, and thus is determinative as to which species can adapt to a given seabed. Another general rule is that most mud and sand bottom dwelling species are detritus feeders, the detritus being chiefly composed of plant material that has been partially decomposed by bacteria and fungi.

Depth zones

In general the organisms that live in the benthos of the epiphotic zone are those which may be primary producers, since sunlight is available, even if in small amounts. The fauna of this upper depth zone are adapted to low water pressures and generally warm temperatures, except in polar seas, where even epiphotic seas can be cold. In the deeper zones there are very few primary producers except for extremophiles such as certain bacteria who can metabolize sulfur based chemicals as an energy source.

Fauna of the mesopelagic and deeper zones must be able to withstand successively higher pressure and colder temperatures with greater depths. In addition these fauna are adapted to a dark realm, where they use smell, tactile mechanisms or sonar to locate prey and evade predation.

Ecology of sandy substrate

Nematodes represent a phylum of widespread and plentiful fauna occurring in sandy seafloors. They occupy interstitial spaces of sandy marine bottoms. Such fauna typically present as elongated and radially symmetrical with tapered ends. This threadlike, diminutive stature allows nematodes to navigate through the interstices by whiplike undulatory action.

Larger fauna found at the bottom of sandy seas include echinodermata such as the invertebrate sea cucumbers and the detritus eating sand dollars. Polychaete worms are significant annelids in sandy substrates. They have numerous setae (chitinous bristles) that allow traction along the sandy bottom; some polychaetes are found in relatively shallow seas, while others dwell in deep cold water trenches. Many sponge species are also found on sea floors composed of sand.

Ecology of muddy substrate

Mud seafloors compromise oxygen supplies, with metabolic wastes also being entrained in the mud, leading to to anoxic conditions that can produce hydrogen sulfide, severely limiting faunal survival. As in the case of sandy bottoms, microscopic organisms are found in the mud bottom. Some buried bivalves occurring beneath the anoxic zone employ siphons to extract nutrients and oxygen out of the water column above the mud.

Annelids are also found on mud bottoms; for example chaetopteridae filter feed using a mucous sac. Three piston-like parapodia push water through a tube. At the sac rear, the mucus containing embedded food is continually rolled into a pellet, and as this attains a critical mass, the fans cease beating, with the pellet being moved forward to the mouth via cilia action.

Ecology of hard substrate

Many of the fauna inhabiting hard substrates are characteristically near stationary. A common form of predation is to stretch tentacles into the water current above. There is a wide array of tenacle design and strength, manifesting the niche specialty of these predators and nature of the prey sought.

Hard substrate offers prime habitat for certain stalkless free moving crinoidea, which are suspension feeders of plankton, and therefore direct their oral aperture upward into the water column. The cirri of the aboral end suspend freely whilst in locomotion, but attach to the hard bottom when resting. The elements of their five starpoint arms serve not only as filtering organs but are also the locus from which juvenules depart from parents. The ciliated grooves along the arms convergeat the center of the oral surface to which trapped plankton are driven.

Ecology of secondary hard substrate

Secondary hard substrate is formed by consolidation of some combination of biotic and mineral debris, often consolidated into a hardened matrix by a variety of congealing mechanisms. Large amounts of micro-organic debris may have gradually accumulated on the seabed, for example foraminifera, or diatoms with silica wrapping, can combine with  disintegration of earlier rocks and volcanic ejecta, riverine runoff or glacial scrapings; Under certain circumstances, these sediments harden to create a congealed conglomerate, simulating normal hard substrate.\

Annelid tube worms may also secrete leathery or calcareous tubes that the worms inhabit, while these species extend feather-like tentacles to strain food. The tubes themselves are an alternate form of secondary hard substrate that provide a resting place for other very tiny sedentary organisms.

Further reading

Glossary

Citation

Baum, S., & Hogan, C. (2012). Benthos. Retrieved from http://www.eoearth.org/view/article/51cbed0f7896bb431f68f657

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