Structure and Size

Benthic organisms display a wide range of structures of varying size, life style and ecological roles. For the purpose of methodology benthic organisms are divided into various categories, and different criteria and terms have been coined to describe the various components of this spectrum. Thus in relation to size benthic organisms are designated as :

• microbenthos (<100mm ), comprising bacteria and protists
• meiobenthos (100-500mm), small metazoa and a few large protists (foraminifera)
• macrobenthos (>500mm) larger metazoa and
• megabenthos (very large) large crustaceans, molluscs etc.

This division according to size, and the above- mentioned limits is to some extent an arbitrary one, corresponding to scientists' need to refer to standard categories and to use the equivalent sampling gear. Nevertheless this type of classification relates also to the role that the different categories play in the ecosystem. For instance meiobenthic organisms which usually live among the grains in the sediment, eating mainly bacteria and having densities ranging from a few hundreds to a few million per square metre, have a completely different role than the large predator species of megabenthos living on the bottom surface with densities of less than 1 individual in 10 square metres.

Feeding types

However the most important division within benthic fauna is the one which is concerned with feeding types. Every species is a unique combination of morphological elements, internal anatomic structure, physiological peculiarities and ecological preferences. Yet it is possible to distinguish types of organisms which have some characteristics in common, by the way they acquire their food which in turn determines various other parameters of their lives.

The main benthic feeding types can be divided in two ways: what they feed on and how they feed . The first category can be divided into herbivores, carnivores and detritivores and the second into suspension feeders, filter feeders, detritus feeders, scavengers,predators

Herbivores

These are organisms which feed on plant material. They live mainly on plants or on the sediment surface, usually in shallow areas, and have special mouth parts (e.g. radula) which enable them to cut and chew vegetal material.

Carnivores/Predators

These are organisms which feed on live individuals of various sizes. They have well-developed sensory organs, which enable them to detect prey, they are highly mobile in order to pursue their prey and they have special mouth parts, jaws, teeth and extendible pharynxes to capture and consume their prey. Carnivores are found at all depths, on both hard and soft substrates.

Suspension feeders

These have appendages covered by mucus to which suspended particles of the water column are attached. They are then carried by means of the ciliae and antennae to the mouth. This category of organisms does not have much mobility and lives attached to the substrate; they prefer hard substrate and usually construct special hard cases or tubes into which they retreat when they sense danger. Very fine sediment is not a favourable environment for suspension feeders since it causes their feeding appendages to clog up. (See Figure 18. Systematic list of the main groups of benthic organisms.)

Filter feeders

These organisms find their food in much the same way as suspension feeders, the only difference being that they themselves create a water current towards the special food retention appendages by means of siphons or articulate appendages. Their ecological preferences do not differ in general from those of suspension feeders with the only important difference consisting in their ability to colonize poorer environments from the point of view of food.

Scavengers

These organisms can be considered as a sub-category of carnivores. They are of similar body construction and they feed on dead bodies or remnants of either benthic or water column organisms. They are found at all depths, they vary in size and belong to many taxonomic groups.

Deposit feeders

These organisms feed on organic material contained in the soft sediments of the sea bottom. The ways in which they acquire their food are quite diverse and range from a simple swallowing of the sediment and the digestion of the organic material contained in it, to the spreading of mucus on the sediment surface and swallowing those particles of organic material and or bacteria attached to it.

They are differentiated into selective and non-selective deposit feeders according to their ability to select the type and the size of the particles they swallow, and to surface and subsurface deposit feeders according to their ability to feed on particles on the surface or to exploit food located deeper in the sediment (though not deeper than a few cm). Deposit feeders are the most widely distributed organisms in soft sediments.

Recruitment

The recruitment process, i.e., the incorporation of young individuals to the adult population, is a very important issue for all marine organisms. As far as benthic organisms are concerned, however, it has an additional interest. Almost all benthic organisms have planktonic larvae that after some time settle on the substrate. This is in molluscs in the temperate zone approximately 6 weeks. In waters with tidal currents it follows that the parent population is apart from the colonization patches.The selection of the substrate is of vital importance in this process, since an appropriate choice enhances the chance of survival. That is why recognition mechanisms have been developed which allow the larvae to decide whether the substrate is appropriate or not. One of these mechanisms is the attraction by individuals of the adult population, the presence of which is a good indication (if not a proof) that the substrate can support the survival of individuals of the same species. Other development modes are direct development without a larval stage and brood protection

Microbes

Usually when talking about microbes we think of their ..pathogenic.. impact. It is true that in the marine environment there are pathogenic microbes which affect not only marine organisms but human beings as well. The latter category will be dealt with in the section concerning aspects of pollution.
However, apart from these pathogenic microbes, which are in any event particular types of microorganisms, there is a whole vast area of organisms of this kind without which marine ecosystems (and terrestrial ones as well) simply could not function.

These are the decomposing bacteria, i.e. microorganisms which acquire the energy they need through the oxidation of organic material. Without these bacteria the marine environment would be full of carcasses and excretions from marine animals, not to mention excretions of terrestrial origin discharged into the sea (usually known as domestic sewage) and the day to day oil pollution.. Without these organisms there would be only chemical oxidation to act upon the organic material and this of course would mean that the process would be very slow.

Most of these bacteria are aerobic, i.e., they need oxygen for the oxidation process, while a few are anaerobic, that is, they are capable of living without oxygen.

Different decomposing bacteria are involved in different parts of the chemical element cycles in the marine environment. For instance the bacterial species Nitrosomonas oxidizes ammonia into nitrite sand then Nitrobacter another bacterium, oxidizes nitrites into nitrates, Beggiatoa oxidizes H₂S in elemental sulphur while Thiobacillus transforms elemental sulphur in sulphates.

Mineralization is an important aspect of bacterial activity in the marine environment. Various essential elements are bound in organic compounds contained in "dead material". Bacterial activity transforms organic compounds into inorganic nutrients (nitrates, phosphates etc.) necessary for the development of phytoplankton.
Many marine organisms feed on the bacterial biomass including their mucus, i.e., on live bacterial cells of higher nutritional quality than some forms of dead organic material.

Temperature, pressure and radiation are important factors affecting bacterial activity. At very low temperatures bacterial decomposition is slow, it accelerates with rising temperature, while at very high temperatures (higher than 40^oC) a few bacterial species (thermophilic) can carry out the full cycle of their activities. High pressure is also an inhibiting factor and that is why at great depths only certain types of bacteria (barophilic) are encountered. Lastly, ultra-violet radiation has a lethal impact on bacteria, but this effect concerns only the water surface layer since as has already been mentioned, U.V. radiation is absorbed there.

Birds

Seabirds play a growing role in the investigation of marine ecosystems and food chain dynamics. Especially their interaction with marine fishes and the fishery are regarded of growing economic importance."Seabird" is a rather loose term traditionally used to cover those birds which obtain at least part of their food from the sea by travelling some distance over or under its surface. They typically breed on offshore or coastal areas like cliffs, dunes, skerries or remote islands. Some 274 species belong to seabirds, comprising mainly penguins, albatrosses, fulmars, petrels, shearwaters, pelicans, cormorants, skuas, ducks, terns and auks. Seabirds are characterized by longevity (high adults survival rate), high age at first breeding, slow reproductive rate and intense care for the offspring.

Seabirds are highly visible wide-ranging upper trophic level consumers that can indicate marine productivity and biotic interaction. Compared with fish, marine mammals and other marine animals that live primarily or exclusively underwater, seabirds are easy to survey, census and study. They are of high public interest and play a major role in raising public attention to environmental questions. Ë2Câþ@&Àl