Benthic animals

Benthic animals


Various benthic animals. 1. common whelk, 2. blue mussel, 3. Neptune whelk. 4. periwinkle, 5. Yoldia hyperborea, 6. sand gaper, 7. limpet, 8. scallop, 9. chiton, 10. common starfish, 11. green sea urchin, 12. brittlestar, 13. ragworm, 14. pectinaria worm,15. scaleworm, 16. skeleton shrimp, 17. isopod, 18. barnacle, 19. benthic amphipod

Illustration: Kristín Líf Valtýsdóttir

The diversity of marine life is most obvious on the bottom where animals have adapted to a multitude of niches and with extremely diverse looks and behaviour. Some are buried in sediment all their life, others are attached to stones or even other organisms, some are slow and sluggish, others roving and rapid. Feeding habits are also diverse. Many filter feed, that is they actively or passively sieve food particles or small organisms from the ocean. Others eat the bottom sediments (detritus feeders), graze on kelp (grazers), eat carrion (scavengers) or hunt other animals (carnivores). Although total biomass and biomass production is much higher in the pelagic environment, the diversity of life is much higher on the bottom.

This diversity is primarily because the bottom offers many more types of habitats that organisms can adapt to than the pelagic environment. The most diverse ecosystems in the ocean are on hard bottoms. In shallow waters these are usually covered with coral reefs in the tropics, but kelp forests in the temperate and Arctic waters. Below the sunlit zone, sessile animals such as sea anemones, sponges, hydrozoans, tunicates and many other animal groups take over these habitats from the benthic algae. Cold water corals can also be found in deep and cold waters, including the waters around Iceland.

Most of the ocean floor is, however, not covered with hard substrate, but is sandy or muddy. The large conspicuous, often sessile, animals are not common there, and in fact these habitats may at first look rather lifeless. This, however, is a deceptive impression as most of the benthic animals in these habitats live buried in the bottom. Many large polychaeta worms and bivalves occur there as well as a myriad of smaller species.

Many species of benthos are harvested and several of them are crucial the ecosystem. Important fish species such as haddock, catfish and most flatfishes primarily eat benthos. Many benthic animals, primarily bivalves, sieve the ocean and effectively clean it up. Others clean up the bottom by scavenging on dead organisms. Detritius feeders and other active diggers regularly move the bottom sediments around and therefore increase their oxygen content and overall productivity, much like earthworms on land. There are six main groups of benthic animals, described further below. It should be noted that many other less common groups of benthic animals exist, but most contain few and rare species.


Sponges (phylum porifera) are probably the least animal-like of all animals. They are in fact the most primitive of the multicellular animals. They lack all internal organs, a neural system and sensory organs and can almost better be described as a colony of single celled animals. They do, however, have specialized cells that have different roles within the sponge. Sponges cannot move at all and can look like mats or spongy balls. Some species are erect and branch out like plants. All sponges are filter feeders. Bathroom sponges were originally made from warm water sponges. However, this is impossible with Icelandic sponges as they contain a multitude of small needles that can badly irritate the skin. Sponges also defend themselves with chemicals and these have drawn the attention of the biotechnology industry.


Sea anemones are animals, not plants.

Photo: Erlendur Bogason


These are the second most primitive animals after the sponges. The cnidarians have a very simple construction, but contain some internal organs as well as a primitive neural system. The best known cnidarians are planktonic jellyfish, but well know benthic groups are corals, sea anemones and hydrozoans. Corals and sea anemones are closely related, but differ, mainly because corals are colonies of very small individuals, while sea anemones are large solitary individuals. Corals are usually associated with the tropics, but coral reefs also occur in deep waters in colder areas of the oceans, including Icelandic waters. Cnidarians are predators that eat whatever reaches their tentacles and they can handle. Cnidarians are not harvested in the North Atlantic. However, in the Far East some species are harvested and eaten.

Bristleworms or polychaets

Bristleworms are segmented worms, closely related to earthworms. However, they have quite diverse looks compared to the rather simple looking earthworms. All of them have appendages and bristles, but they vary enormously in size and shape. Some do not look like worms at all. Bristleworms are found all over the ocean bottom, and a few species are even planktonic. The feeding habits are also diverse; they are filter feeders, detritus feeders, scavengers and carnivores. Polychaets have not been harvested commercially in Iceland but have been collected for bait. Polychaets are important food for many fish species, especially flatfishes.


The three main groups of molluscs are bivalves (shells), gastropods (snails) and cephalopods (squids and octopuses). Feeding habits among these groups are quite different. Bivalves are primarily filter or detritus feeders, gastropods are algae scrapers or predators on sessile animals, cephalopods are active predators on fishes and other cephalopods. Many species within this group can grow quite large and are an important commercial catch in many parts of the world. Significant commercial species in Icelandic waters are scallop, ocean quahog, blue mussel and whelk. Cephalopods have occasionally been harvested in Icelandic waters.


Two crustaceans, a spider crab (Hyas spp.) in the upper left corner and a squat lobster (Munida spp) at the bottom.

Photo: Erlendur Bogason


Crustaceans are the most species rich group of marine animals and can be found in all marine habitats. Crustaceans in the ocean are comparable to insects on land which they are actually related to. A few crustacean species have also been able to invade land. Not a single species of insect, however, lives below the ocean surface. Benthic crustaceans are of various sizes. Many, such as amphipods and isopods, are insect-sized and are, as insects are on land, important for the benthic ecosystem as scavengers and grazers. Shrimps are intermediate in size. Many species of shrimp can be found around Iceland, but only one is abundant enough to sustain catches; northern shrimp, commonly referred to as just “shrimp” in Iceland. The largest are the true crabs and lobsters. Several species of crab can be found in Icelandic waters, but no species is abundant enough to sustain commercial catches. However, only one species of lobster lives in Icelandic waters, Nephrops lobster, which is an important commercial species. One group of crustaceans, the barnacles, is sessile; they are found in abundance on the seashore, but also occur deeper.


Not many echinoderm species occur in Icelandic waters compared to the above mentioned groups, but they are large and conspicuous animals, and therefore among the best known of marine invertebrates. The five groups of echinoderms have quite different habits but all have similar internal structure. Sea stars are probably the best known group. Although innocent looking, they are in fact voracious predators on other slow moving or sessile animals. Fishermen consider them a pest when they invade scallop beds. Brittle stars or ophiuroids are closely related to sea stars but smaller and more mobile. They mostly eat detritus or are filter feeders. Sea urchins are grazers or detritus feeders. Sea cucumbers are detritus or filter feeders. The last and least known groups are the filter feeding crinoids. Sea urchins and sea cucumbers have been harvested in small quantities in Icelandic waters.

References and further information

  • Gunnarsson, K., Jónsson, G., & Pálsson, Ó. K. (1998). Sjávarnytjar viđ Ísland (Marine resources in Icelandic waters). Reykjavík: Mál og Menning, 280 pp. (in Icelandic)
  • Sigvaldadóttir, E., Helgason, G. V., Guđmundsson, G., & Fauchald, K. (2000). Bioice: Benthic invertebrates of icelandic waters. Bulletin of Marine Science 67 (1), 673-674.

Hreiđar Ţór Valtýsson, University of Akureyri


ţetta vefsvćđi byggir á eplica. eplica vefforitunvefforitun - nánari upplýsingar á heimasíđu eplica.