Reviewed by: BD Editors

Definition of Benthos

The benthos is comprised of all the organisms that live at the bottom of a body of standing or running water. The layer that the benthos occupy is called the benthic zone, which is the lowest layer of a lake, sea, stream, or river. This zone, of course, ranges from the shallow depths where water meets land, to extreme depths that humans have not yet been able to explore. Benthos, therefore, vary greatly, and can either be observed crawling, burrowing, swimming near the bottom, or staying attached to the substrate. Many tend to rely on food sources that sink down to the bottom, such as inorganic matter and dead organisms, but the benthos also feed on one another. The zone above the benthic zone where organisms that swim or float – the pelagos – are found is called the pelagic zone.

Note that the phytobenthos, zoobenthos, and benthic microflora in the benthic zone are the equivalents of primary producers, consumers, and decomposers, respectively.

Types of Benthos

The benthos utilize all areas at the bottom of a body of water, and they can be categorized into three groups based on their habitat.


These are the organisms that have the ability to swim and live near the bottom but are not attached to it. Rock cods are hyperbenthic fish.


Epibenthos spend their lives attached to the floor, on rocks, or on shells and they include sponges.


Instead of living on top of the seafloor, these organisms have become adapted to live within the sediments, often creating underground tunnels. An example of endobenthos is the sand dollar.

Roles That Benthos Play

The benthos has a purpose in keeping balance in the environment. Filter-feeders, for example, like mussels that live in the benthic zone, play an essential role in keeping bodies of water healthy by cleaning them up from pollutants and waste as part of their feeding process. Many benthos in shallow waters also rely on dead organic matter as their source of nutrition, breaking it down and recycling it. This makes them very important for nutrient-cycling and returning nutrients to the environment in useable forms. Certain benthos are also useful to scientists in the way that can indicate the health and quality of the water. For example, a decline in the number of caddisflies, which are very sensitive to pollutants, can indicate an increase in pollution and waste in the water.

Examples of Benthos

Deep-Sea Anglerfish

As the name implies, these anglerfish live in the depths of seas where it’s cold and dark. Some of them emit light from the spine attached to their heads in order to attract prey in the dark bottoms. As we can see in the picture below, the tip, where the bioluminescence occurs, is in perfect position to allow the fish to capture their prey. Once an organism gets close enough to the light source, the anglerfish closes up its large mouth, creating a cage with its teeth. Also, because these organisms live in harsh environments, when a male anglerfish matures, it spends the rest of their life as a parasite attached to a female anglerfish, ensuring that reproduction of this species continues to occur. These adaptations are what allow anglerfish to survive at very low depths in the water.



These are deep-sea organisms that look like eels and rely on dead bodies of organisms that sink to the bottom for food. When a large animal, such as a whale dies and its body sinks to the bottom, hagfish burrow into the body and consume the flesh. This is the way that these organisms recycle organic matter that reaches the seafloor. Hagfish have also adapted to breathe through their skin when burrowed in the mud, and though they have very simple eyes and are almost blind, hagfish have an exceptionally strong sense of smell and touch, allowing them to find food in the dark. Lastly, in order to protect themselves, hagfish can produce a slimy substance that helps them fight off predators.


These are photosynthesizing flowering plants, so they are only found in the photic zone, where sunlight can reach the bottom. Seagrasses benefit the environment by stabilizing the seabed with their roots. They also provide shelter and food for some organisms, can slow down waves and reduce erosion of land, and like land plants, they help reduce the amount of carbon dioxide present in the environment.

  • Nekton – Aquatic organisms that swim freely through water, like whales, jellyfish, and turtles.
  • Neuston – Small organisms that inhabit the surface of aquatic habitats, such as fishing spiders and floating plants.
  • Plankton – Small aquatic organisms that float or drift with the water current, like algae and some worms.
  • Profundal zone – the deep-water layer of a standing body of water, like a lake or a pond, where the temperate is significantly colder than the surface layer.


1. Which of the following groups does the sea dollar belong to?
A. Pelagos
B. Epifauna
C. Endobenthos
D. Sea anemones

Answer to Question #1
C is correct. Sea dollars are endobenthic and burrow inside the seabed.

2. Which of the following applies to benthos?
A. They all live in deep waters.
B. The reason they live at or near the sea floor or lake bottom is because no predators exist there.
C. They help with the cycling of nutrients.
D. They all have very sharp teeth.

Answer to Question #2
C is correct. Benthos break down animals flesh and inorganic matter and return nutrients back to the environment.

3. Which of the following is true about anglerfish.
A. They are parasites that feed on their host’s bone.
B. They resemble eels.
C. Once the female has matured, it attaches itself to a male anglerfish.
D. They have a very large mouth.

Answer to Question #3
D is correct. Anglerfish have a very large mouth and sharp teeth that help them catch their prey easily.

Cite This Article

MLAAPAChicago Editors. "Benthos." Biology Dictionary,, 13 Dec. 2016, Editors. (2016, December 13). Benthos. Retrieved from Editors. "Benthos." Biology Dictionary., December 13, 2016.

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