Lexikon

zoobenthos are the animal organisms which live on, in, or near the bed of any surface water: stream- and riverbeds, seabed, the sediment of lakes, tidal pools, foreshores, both in the surface of the bottom-sediment and the deeper layers. In the sea and in the oceans many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column.

Because light does not penetrate surface waters and deep ocean-water, the energy source for benthic ecosystems is often organic matter from higher up in the water column which drifts down to the depths. The dead organic matter, the decaying matter is the nutrient source of the benthic ecosystem: many of the benthic animals are scavengers or detrivores.

Zoobenthos generally live in close relationship with the substrate bottom; many such organisms are permanently attached to the bottom. The superficial layer of the soil lining the given body of water, the benthic boundary layer, is an integral part of the benthic zone, as it influences greatly the biological activity which takes place there. Examples of contact soil layers include sand bottoms, rock outcrops, coral, and bay mud.

By location we distinguish between epibenthos, living on top of the sediment, and hyperbenthos, living just above the sediment.

According to the size of the organisms we differentiate between

- macrozoobenthos: larger, more visible, animals, greater than 0.5 mm in size, such as worms, crabs, clams, sediment-living fishes, insects;

- meiozoobenthos: tiny benthos that are less than 0.5 mm but greater than 32 µm in size, such as nematodes, foraminiferans, water bears, gastrotriches and smaller crustaceans such as copepods and ostracodes.

- microzoobenthos: microscopic benthos that are less than 32 µm in size. Some microanimals: ciliates, amoeba.

Sources: http://en.wikipedia.org/wiki/Benthos
http://lakes.chebucto.org/ZOOBENTH/BENTHOS/i.html