Marine life

Marine organisms are divided into several groups based on where and how they live; the simplest division is:

Benthos: Organisms that live on the bottom, some are sessile while others may move around on the surface or dig into the sediment. This group includes both benthic algae and benthic animals. Note that many benthic animals such as sea anemones, corals, sponges and crinoids are sessile and are often confused with plants.

Plankton: Organisms that live in the water mass and have a very limited ability to move; they are therefore dependent on currents. This group is further divided into phytoplankton, mostly microscopic single celled algae and zooplankton which are usually small animals, but can range in size from single-celled protists to large jellyfish.

Nekton: Animals that have good abilities to move, for example fishes, squid and mammals.

Pelagic: This term is used for both plankton and nekton and refers to animals that live in the water column and are not dependent on the seabed. Sometimes there is a distinction between fishes that live in the upper layers of the ocean (epipelagic) or deeper down (mesopelagic).

Demersal: This is mostly used for fishes and other nekton that are associated with the bottom. Demersal fishes are also called benthic fishes or groundfishes. Fish species that are equally associated with the water column and the bottom are called benthopelagic.

This division is by no means clear as many organisms can be considered as belonging to two or more of these groups. Furthermore, many animals go through stages in their life history that are quite different with respect to morphology, behaviour and habitat. Adult cod is, therefore, clearly a demersal nekton, but the cod larvae are planktonic the first months of their life. Most benthic animals also have planktonic larvae.

The oceans are also divided into different areas where organisms live. The shallowest part is the seashore or the intertidal zone, the borderland between land and sea. Compared to the vast ocean this is only a narrow zone along the coast. Below the seashore is the continental shelf (sometimes called the subtidal or sublittoral zone). The continental shelf usually reaches down to 300 m depth, sometimes shallower, sometimes deeper. This is the area where most demersal fisheries are conducted. Below that is the continental slope down to about 1000 m depth. Limited deep sea fisheries are also conducted there.

Below that are the continental rises (also called the bathyal zone, at ca. 1000 to 4000 m depth) that finally merge with the abyssal plains (at ca. 4000 to 6000 m depth). The abyssal plains are in some places cut up with the deepsea trenches (below 6000 m depth). The fishes and other animals found on the continental slopes and below are usually strange looking and very different from “normal” marine creatures people know from shallower areas. Virtually no fisheries are conducted at these depths.

Hreiðar Þór Valtýsson, University of Akureyri