What is benthos?

Benthos is a common name for organisms that live on the bottom. This includes thousands of plants and animals of various sizes and shapes. All of them belong to different groups, differ in lifestyle and nutrition. In addition to habitat, they have little that unites. The classification of benthos, photos of its representatives and their description can be found below.

Aquatic inhabitants

Residents of marine and freshwater bodies are usually divided into three large groups: plankton, nekton and benthos. They include both vegetable and animal. The main characteristic for their classification is what layers of water they inhabit and what kind of lifestyle they lead.

Benthos is a community of organisms that inhabit the bottom or in its upper layers. Among them are microscopic bacteria, tiny crustaceans and worms, and giant clams, huge sponges and bottom fish. Animals that are included in it are called zoobenthos, and plants, respectively, are phytobenthos.

Necton is a group of actively swimming organisms that move independently, can resist currents and often move throughout the water column.This includes not only cetaceans, fish, and mobile mollusks, but also animals that lead a semi-aquatic lifestyle, for example, penguins, turtles, sea snakes, seals and walruses.

Plankton consists of various organisms, the main difference of which is inactivity. These are mainly gray-green algae, small crustaceans, swimming fish roe, larvae of animals, ciliates, radiolarians, pantopods. The size of some representatives can reach several meters, but most of them are still small. Plankton differs from nekton and benthos in their inability to overcome the power of currents. He just drifts in the water, succumbing to her unrest.

Features benthos

The benthos includes a huge number of living creatures, most of which inhabit the temperate and subtropical geographic zones. They mostly live in shallow water - in small rivers, lakes or near the shore. For example, in the area of ​​the continental shelves and coral reefs there are more than 200 thousand species of marine benthos.

Not everyone can adapt to cold, darkness and the pressure of great depths, therefore the number of bottom dwellers decreases significantly with increasing depth.The most deserted places are oceanic depressions, holes and rifts, but even there you can find life, especially near thermal springs.

Marine and freshwater benthos is food for many animals. Only fish eat about 2 billion tons of benthic mass annually. In addition, its representatives are important fishing targets. They are caught for sale, and some are grown on special farms. Lobsters, crabs, oysters, mussels, sea urchins and stars, purple algae, kelp, anfeltia and others are especially popular.

benthos this

Types and classifications

Representatives of the benthos are very heterogeneous and can differ from each other in almost all aspects. Some lead a sedentary, or attached, way of life, others wander, overcoming long distances. Some of them are buried in the ground or drill holes in underwater rocks, and some just lie at the bottom.

The lower layers of reservoirs are rich in all sorts of sediments from organic and inorganic substances - residues of tissues, skeletons, foreign secretions and sedimentary rocks. These "goodies" are often included in the diet of local residents, although there are some species that survive hunting. According to the way of feeding, the organisms of benthos are:

  • predatory;
  • filter feeders;
  • grunt eaters;
  • scrapers;
  • suspension eaters.

Species that live on the surface of the bottom are called epibenthos, those that dig deep, endobenthos. In size, they all share:

  • On macrobenthos - they differ with the naked eye and reach more than 1 mm in length.
  • Meiobenthos - organisms difficult to distinguish with the naked eye, whose size reaches from 0.1 mm to 1 mm.
  • Microbenthos - animals and plants less than 0.1 mm in size, for example, bacteria, protozoa, nematodes.


Unlike nekton, benthos does not always move freely in water. Some of its representatives are attached to a substrate and remain motionless during all or part of life.

A significant part of the sitting benthos is the plant. They are attached to stones and soil with the help of developed rhizomes or special organs - rhizoids. Sessile animals include corals, sponges, bryozoans, bivalves, sea lilies, some worms and crayfish. They are held in place by suckers, attachments, stems, tendrils or organs like rhizomes. Some settle inside the lime sinks, which are attached to the substrate.


The fixed lifestyle has a number of drawbacks: sedentary organisms cannot escape from the enemy, go in search of food and actively hunt, or change their habitat to a more favorable one. In this regard, they have acquired many useful tools. They feed by filtering water with plankton passing by. Some species have acquired clinging tentacles and poisonous parts of the body, which allow them to catch unwary animals and defend themselves from enemies.

Reproduced by budding, they expand their colonies and spread over the adjacent territories. Many of them live in colonies and can form extensive clusters. For example, corals sometimes stretch for hundreds of kilometers, and the limescale deposits in which they live form whole islands. Usongi and other species have a mobile stage of development. At a young age, they can actively move, traveling around the reservoir.

Free lying

Lying benthos - these are organisms that do not attach to the substrate, and are freely located on the bottom surface. As a rule, its representatives have a wide flat body and a masking color to match the color of the sludge and underwater rocks, which makes them invisible to prying eyes.

For protection and hunting, they can have a variety of outgrowths on the skin and the shell, emit a variety of scaring and poisonous secrets. Many of them do not have developed sense organs and limbs. For example, in scallops, the eyes are able to recognize only changes in lighting, and animals move with the help of jet thrust, which is created by abrupt closure of the shell valves.

lying benthos


Burying benthos is also called infauna. Its representatives live in a layer of soil or bottom sediment, in which they are completely immersed. There are no plants among burying benthos. It includes various gastropods and clams, crustaceans, round and ringed worms, holothurians, sea urchins, insect larvae and other invertebrates.

Some animals only dig a little into the ground, while others dig in it numerous passages and tunnels, which can greatly exceed their own size. Wrong sea urchins prefer to bury themselves in soft soil and sand. Sometimes for a shelter they choose not the rocks of the bottom, but algae, in the thickets of which they hide.Bivalve mollusks burrow into stones, soil, sand, silt with the help of a muscular leg. Often they live in a tidal zone near the coast and, when the water flows, they can stay out of the water for a long time.

mollusk shells


In contrast to burying benthos, drilling organisms do not live in soft soil, but prefer hard rock. They inhabit rocks, limestone, slate and granite rocks, wood and even the shells of other animals. Freshwater insect larvae live in clay or drill holes in plants, attaching to their leaves and stems.

They make moves thanks to hard teeth and crests on their body. Crawfats use for this purpose powerful mouth appendages, literally gnawing their way, and some mollusks secrete a secret that dissolves lime.

Drilling organisms feed on plankton, various particles floating in the water and sedimentary waters. A number of species consume substances in which they make moves. Eating them, they more and more go deep into breed, forming the real labyrinths.

sea ​​urchins


Crawling organisms belong to the rolling benthos and are ranked as epifaune.They freely move along the bottom of reservoirs in search of food, but they cannot swim in the water column. Sea urchins walk with needles or ambulatory feet. Polychaete worms move due to parapodia - skin-muscular growths, with which they cling to the substrate and pull the body forward. Starfish move with outpatient legs, which are additionally equipped with suction cups and allow them to cling to different surfaces.


Among the crawlers there are filter feeders, soil eaters and detritus, but there are also many predators. Almost all the starfish get their food by hunting. They use sea urchins and worms, with strong muscular arms they can open the shells of bivalve mollusks. Many crawlers are characterized by bilateral symmetry of the body, distinct from each other in the abdominal and dorsal parts.


Together with a group of crawling organisms, this type of organisms is usually referred to as mobile bentos. They have well-developed limbs, and they can move freely within the waters. In addition to invertebrate crustaceans and worms, this includes various bottom fish. Some of them spend their entire lives close to the bottom, others may occasionally rise to the surface.

groundfish flounder

Typical representatives of these animals are flounders.They have a flat, flattened top body with wide sides. The eyes, unlike most fish, are present not on the sides, but on the back. Flounders live in shallow water and are rarely found at depths greater than 50 meters. In search of food they swim near the seabed, and in case of danger they bury themselves in silt or sand.

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