Benthos

Definition of Benthos: Benthos comprises organisms living on, in or closely associated with the bottom of aquatic environments. It includes animals (zoobenthos), photosynthetic organisms such as microalgae, macroalgae and seagrasses (phytobenthos), and benthic microorganisms. Organisms living on the seabed are called epibenthos; animals living within unconsolidated sediment are called infauna, while organisms living within sediment or hard substrate may more broadly be termed endobenthos. Mobile organisms living immediately above the bottom are called hyperbenthos. This is the common definition for Benthos, other definitions can be discussed in the article

Benthic fauna are an essential component of the marine ecosystem. Benthic organisms make a major contribution to the decomposition of organic matter and the cycling of nutrients in the marine environment. They not only fuel in this way the primary production in the water column but also constitute themselves an important food source for marine organisms at higher trophic levels. Many benthic organisms are ecosystem engineers that create ecological niches for themselves and for other benthic species by conditioning the substrate on which they live.

Benthic fauna are highly sensitive to changes in environmental conditions and can therefore fulfill a sentinel role. Because many benthic organisms are relatively sedentary and live for months or years, community composition integrates environmental conditions over longer periods than a single water sample. Changes in species composition, abundance and functional traits are therefore used to assess effects of eutrophication, oxygen deficiency, contamination, dredging, aggregate extraction, bottom trawling and habitat restoration. Such assessments must compare similar habitats, because grain size, salinity, depth and hydrodynamic disturbance also cause strong natural differences.

Benthic habitats

Fig. 1. Factors affecting benthic organisms in coastal ecosystems. Courtesy Satheesh and El-Sherbiny (2022)

Benthic communities can be distinguished according to the type of habitat they live in. Many benthic animals are so-called ecosystem engineers, which actively contribute to shaping these habitats (see Biogeomorphology of coastal systems). For example, mussel and oyster beds create hard, three-dimensional habitat and trap suspended material. Burrowing worms, bivalves and crustaceans mix and ventilate sediment, changing oxygen penetration, nutrient cycling and erodibility. Burrowing and feeding can destabilize the sediment surface, whereas tubes, reefs and mucus-bound surface layers can stabilize it. The mixing of sediment by organisms is called bioturbation; flushing of burrows with overlying water is called bioirrigation.

Major benthic habitats in the marine environment include rocky bottoms, sandy sediments, vegetated habitats such as seagrass meadows, kelp forests, salt marshes and mangroves, coral reefs, and the extensive soft-sediment habitats of continental shelves and the deep sea. Each habitat supports organisms adapted to its substrate, hydrodynamic conditions, light climate, salinity and depth. Compared to the pelagic zone, the seafloor presents a greater variety of physically diverse habitats that differ from each other in terms of depth, temperature, light availability, salinity, degree of immersion (tidal vs. subtidal), and type of substrate (Fig. 1). The great diversity of benthic habitats is reflected in the great diversity of benthic animal species. Most marine animal phyla occur on or in the seabed, and the number of benthic animal species is generally considered to exceed the number of pelagic animal species, although the total remains poorly known because many benthic species have not yet been described.

The dissolved oxygen (in combination with other factors such as organic carbon content, sediment size, and water depth) strongly influences the abundance and diversity of the benthos assemblage. Oxygen diffusion into the seabed is slow, especially in muddy sediments, and generally limited to the top millimeters in the absence of bioturbation. In organically enriched sediments with low oxygen concentrations, small opportunistic polychaetes (bristle worms) can become dominant. They tolerate low oxygen and sulfide, while their short life cycles and rapid colonization enable them to occupy recently disturbed or impoverished sediments. Burrowing and bioirrigation carry oxygenated water several centimeters or more below the surface, creating oxygenated zones around burrows within otherwise reducing sediment. The majority of benthic organisms are therefore found in the upper few centimeters of sediment near the sediment–water interface.

Sediment grain size is an important control on soft-bottom communities because it is related to bed mobility, permeability, oxygen supply and organic-matter content. Frequently disturbed mobile sands generally support fewer and more disturbance-tolerant organisms than stable sediments, whereas organically enriched fine sediment can support high abundance but reduced diversity when oxygen becomes depleted. In addition, fine sediments - clay particles in particular - retain more organic matter than coarse permeable sediments (see Coastal and marine sediments). Fine sediment soils can therefore support abundant deposit feeders. But there are also tradeoffs: fine-sediment beds are less oxygenated and adsorb toxic contaminants more easily.

Classification and description of benthos

Benthos can be classified in several independent ways: by body size, by feeding mode, and by position relative to the seabed—for example, as infauna living within the substrate or epifauna living on its surface.

Size classes

Megafauna

Megafauna are large benthic organisms visible in seabed photographs and often caught by trawls. They include flatfish, scallops, crabs, sea cucumbers, starfish and gastropods (slugs, snails). Megafauna represent many feeding types, including suspension feeders, deposit feeders, grazers, predators and scavengers. Predators keep grazers from depleting resources and are therefore crucial for maintaining the ecosystem.

Macrofauna

Macrofauna are smaller organisms that are retained on a 1 mm sieve. This size grouping includes polychaetes, crustaceans, bivalves, and many other invertebrate groups. Their abundance in coastal environments is highly variable depending on habitat characteristics and season, with ranges from 500 to 10,000 ind/m², while biomass ranges from 5 to 200 g/m².

Meiofauna

Meiofauna are animals that pass through a 0.5–1 mm sieve but are retained on a fine sieve, commonly 32–63 µm; the precise mesh sizes vary among studies. They occur in marine and freshwater sediments throughout the world and are generally concentrated in the upper few centimetres of the seabed. Meiofaunal communities are usually dominated numerically by nematodes and harpacticoid copepods, often with densities in the order of one million individuals per square metre. Other groups include ostracods, turbellarians, gastrotrichs, kinorhynchs and tardigrades.

Benthic foraminifera, although not animals, and small ostracods are also commonly collected in the meiofaunal size range. Because different species are associated with particular salinity, oxygen, substrate and water-depth conditions, their living assemblages are used in environmental assessment and their preserved remains in sediment cores are used to reconstruct past environments.

Microbenthos

Microbenthos comprise microscopic benthic organisms, including bacteria, archaea, protists and microalgae. These microorganisms drive much of the decomposition of organic matter in sediments and release nutrients such as ammonium and phosphate to the overlying water. These regenerated nutrients support phytoplankton production, creating an important coupling between benthic and pelagic ecosystems. Part of the nitrogen is also transformed or removed within the sediment through processes such as nitrification, denitrification and anammox. In oxygen-depleted sediment, anaerobic microbial processes use alternative electron acceptors such as nitrate, manganese and iron oxides, and sulfate. Sulfate reduction produces sulfide, which is toxic to many animals when it accumulates.

Crab

Starfish

Polychaete (bristle worm)

Gastropod Homalopoma sangarense. Courtesy Naturalis

Nematode Sabatieria longisetosa. Photo credit Daniel Leduc

The myodocopid ostracod Vargula hilgendorfii (sea firefly), famous for its bioluminescence. Courtesy R.J. Smith

Harpacticoid copepod. Photo credit Lodewijk van Walraven

Gastrotrich Photo credit Jasper Nance

Foraminifer Nonionella stella. Photo credit Isabel Mendes

Feeding modes

Below the depth reached by sufficient light for benthic photosynthesis, benthic communities depend mainly on organic matter produced in the water column and sinking to the seabed, together with detritus transported from coastal vegetation. Deposit feeding and suspension feeding are therefore common in soft-sediment communities.

Deposit feeders

Deposit feeders ingest organic particles and associated microorganisms from the sediment surface or from within the sediment. They are often abundant where organic matter accumulates, especially in muddy seabeds, sheltered tidal flats and deeper depositional environments, but they also occur in sandy habitats. Examples include lugworms such as Arenicola marina, sea cucumbers, many polychaete worms such as Capitella teleta, tellinid bivalves such as Limecola balthica (formerly Macoma balthica), heart urchins, and some amphipods and gastropods.

Suspension feeders

Suspension feeders remove phytoplankton, detritus and other particles from the water. They are often abundant where currents continually supply suspended food, provided that flow and sediment movement are not too strong for the organisms to remain attached or buried. Examples include mussels, oysters, cockles, barnacles, sponges, bryozoans, tube-dwelling polychaetes with feeding crowns, such as sabellids, and many ascidians.

Position relative to seabed

Benthic infauna

Benthic infauna is the assemblage of organisms that live within or partly within the sediments of the seafloor. These animals range from gastropods (slugs, snails), amphipods (shrimp-like crustaceans), polychaetes, and other invertebrates. Infauna dominate many unconsolidated soft-bottom habitats, from intertidal flats and beaches to subtidal shelf sediments. Although less common, infaunal species are also found in hard substrate communities, rock-boring bivalves being an example. Benthic communities, particularly the infaunal community, play an essential role in organic matter remineralization. In addition, infaunal species stimulate nitrogen cycling through burrow ventilation, particle reworking, bioirrigation and excretion. By enlarging the interface between oxic and anoxic sediment, they can promote coupled nitrification–denitrification and alter nutrient fluxes across the sediment–water interface (see Nutrient cycling). Burrows also create small oxygenated habitats within otherwise reducing sediment. Meiofauna may live in these oxygenated zones, while some larger infauna ventilate deep burrows or periodically migrate to the sediment surface.

Benthic epifauna

The benthic epifauna are animals living on or attached to the seafloor. Corals, mussels, barnacles, echinoderms (starfish), and sponges are examples of these organisms. They occur in almost all substrates, but are particularly abundant and diverse in rocky intertidal areas and coral reefs. Epifauna include suspension feeders, grazers, predators and scavengers and form important links between benthic primary production, suspended food particles and higher trophic levels.

Ecological role of meiofauna

Many meiofaunal organisms feed on bacteria, microalgae and organic particles on or between sediment grains. Some can be classified as deposit feeders, whereas others are grazers, omnivores or predators.

Grazing by meiofauna stimulates microbial growth and contributes to the decomposition of organic matter and the regeneration of nutrients. Their movement and feeding also redistribute sediment particles, organic matter and microorganisms over small distances within the sediment.

Meiofauna constitute an important food source for small bottom-feeding animals and juvenile fish. Harpacticoid copepods are particularly nutritious prey because they contain fatty acids required for fish growth. Meiofauna thus transfer part of the production of microalgae and microorganisms to higher trophic levels. Their high abundance and generally short life cycles enable populations to recover rapidly from predation.

Because meiofauna live in close contact with sediment throughout their life cycle, their abundance and species composition respond to changes in oxygen conditions, organic enrichment and contamination. Meiofaunal communities are therefore used, together with macrofauna and sediment characteristics, in assessments of environmental condition.

More details on meiofauna can be found in Meiofauna of Sandy Beaches.

Ecological role of phytobenthos

Microbial mat.

Microphytobenthos—mainly benthic diatoms and cyanobacteria—forms surface biofilms in illuminated sediments. Extracellular polymeric substances produced by these microorganisms can bind sediment grains and reduce erosion. Eutrophic coastal waters can also develop 'green tides': extensive accumulations of rapidly growing green macroalgae, particularly species of Ulva. Their decay can cause oxygen depletion and nuisance on shorelines. Macrophytobenthos includes attached macroalgae and seagrasses. Together with adjacent salt marshes and mangroves, these habitat-forming primary producers attenuate currents and waves, trap sediment and provide structurally complex habitat. They are a natural ally in protecting the coast against erosion and enabling shore accretion and extension, see Nature-based shore protection.

Related articles

Meiofauna of Sandy Beaches

Biogeomorphology of coastal systems

Rocky shore habitat

Coral reefs

Seagrass meadows

Mangroves

Salt marshes

Estuarine ecosystems

References

The main author of this article is Job Dronkers Please note that others may also have edited the contents of this article. Citation: Job Dronkers (2026): Benthos. Available from http://www.coastalwiki.org/wiki/Benthos [accessed on 28-07-2026] For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Dronkers J