NATURAL ASSOCIATIONS OF ORGANISMS

Depending upon their degree of intimacy, four types of associations may be recognized among organisms. The four types are: (1) association arising from similar requirements as to environment; (2) commensalism; (3) symbiosis; and (4) parasitism.

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Environmental Association. Animals and plants of different species will be found associated together in ecological communities or biocoenoses provided their environmental requirements with respect to physical and chemical conditions are much the same. The food relationships or predatory habits of the members must be such that their habits do not interfere seriously with each other. Much of the food may be brought to the community incidentally by currents and other water movements, but usually a restricted food cycle is established and many carnivorous animals prey upon each other. One species may dominate numerically owing to better means of protection or greater powers of survival or proliferation.

It should be noted that in a natural community the inorganic forces function to select a general type of fauna. All species that have not become adapted to the physiographic conditions, the range of salinity or temperature, will be promptly eliminated from the community. For instance, only organisms suited to live on muddy bottoms can become established in such habitats, and the same is true of rocky shores (Ricketts and Calvin, 1939, Pearse, 1939). However, since there is a great array of animals capable of living within the extremes of chemical-physical conditions of these environments, a competition for food and space is established among the members. It is here that some of the biotic forces come into play in determining the species or groups of species that will make up any given community. It has been found that certain species live most successfully only in certain combinations of associates (Petersen, 1913). The principal characteristic animal or animals supply the name to the community; thus there may be a Venus community or a Macoma community, depending upon the importance of these bivalves and the combination of animals associated with them. Communities are also sometimes named for the habitat, for example, they may be estuarine communities, exposed rocky coast communities, or wharf-pile communities.

The forces operating to mold the character of a fauna are thus a combination of the organic and inorganic factors, but the communities are more or less unstable since the forces so acting do not remain unchanged. Changes resulting from erosion of the substratum or from the degree or character of terrigenous deposition on a shore, for example, will result in a gradual succession of communities. It is obvious also that the organisms themselves, plants and animals, both pelagic and benthic, slowly bring about fundamental changes in the nature of the bottom, its macro- and microscopic structure, even to great ocean depths. Alterations may result from (1) burrowing activities of many forms such as worms, molluscs, crustaceans, and others that live in or upon the mud and sand, grinding and intermixing the sediments to a depth of a third of a meter or more; (2) boring into and eroding such harder structures as

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rocks, corals, shells, clay banks, and ledges as by several of the bivalves (Pholas, Saxicava, Lithophagus), crustaceans (Sphaeroma), sponges (Cliona), worms (Leucodore), and sea urchins, (3) accumulations into coral reefs, sediments, and oozes of the calcareous or siliceous skeletal remains of both pelagic and benthic forms; (4) accumulation of organic detritus and other organic derivatives. With these biologically produced changes, possibilities are created for yet other community changes, until a relatively stable climax may be reached wherein a dynamic equilibrium exists until the balance is disturbed by exceptional fluctuations in temperature, currents, and so forth. The precipitation of organic material to the bottom, in bodies of water lacking adequate ventilation through circulation of the water, leads to a biological climax wherein only anaerobic bacteria can exist. The Black Sea is an excellent example of this (p. 871).

Commensalism. A closer association than that above discussed is commensalism. Here two organisms live together, one at least being benefited and the other neither injured nor benefited. Certain small decapods are frequently found thus associated with bivalves, living within the mantle cavity. Scaleworms (polynoids) inhabit similar positions with limpets and chitons, and are also found between the rays or in the ambulacral grooves of starfish. Certain small fishes (gobies) live in the burrows of burrowing worms and crustaceans, and many small pelagic fish seek refuge among the stinging tentacles of large jellyfish and siphonophores. According to Wilson (1932) at least 80 per cent of the copepods belonging to the suborder Notodelphyoidea with 41 genera are commensals within ascidians. Certain barnacles (Coronula) and also diatoms (Cocconeis coticola) grow upon the skin of whales. The relative abundance of this diatom on the skin of whales is correlated with the length of time the animals have been in Antarctic waters, since this diatom does not thrive in the warm waters of lower latitudes. The yellow color imparted by these organisms to the ventral surface of the whale is thus an index of the fat condition, for it is in these waters that fatting takes place (Hart, 1935). Commensalism is very common among marine animals.

Symbiosis. Another type of yet closer association is found in the relationship known as symbiosis, wherein two or more organisms live together, this usually being obligatory and mutually beneficial. As illustrative of this mode of life may be mentioned the association between certain one-celled yellow (Zooxanthella) or green (Zoochlorella) algal plants and such animals as radiolaria, sponges, corals, sea anemones, and even bivalves and echinoderms. The algal partner, which in the sea usually is one of the Zooxanthellae, may live within the cytoplasm of the single cell as in the radiolaria; or within the cells intracellularly, or within the body cavities of multicellular forms. This type of relationship is common in the sea but its significance in the economy of the sea is not

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fully known. Doubtless the alga derives benefit by utilizing waste products from the animal, such as carbon dioxide and, perhaps, also nitrogenous wastes. The animal stands to gain by removal of wastes and by utilization of products of photosynthesis, for example oxygen, and is said to draw sometimes directly upon the plants for carbohydrates. The Tridacnidae, to which the giant clam Tridacna of the tropics belongs, are greatly modified for the housing of Zooxanthellae in the cells of the mantle tissues and for their final digestion in phagocytic blood cells which carry them from the mantle (Yonge, 1936). It has been suggested that the greatest depth to which some corals may grow is determined by the algal partner, but more recently it is believed that the corals do not derive food from the algae but instead live directly upon zooplankton organisms that come within reach of their tentacles (Vaughan, 1930, Yonge, 1940). Many reported cases of symbiosis may be commensalism or parasitism, since the reciprocal benefits derived are not easily ascertained.

It is probable that the algae symbiotic with such tropical planktonic animals as the Radiolaria represent an important flora which compensates in part for the scarcity of the free phytoplankton diatoms generally reported from high seas of tropical waters where the plant nutrients are low in the surface waters (Hardy, 1936).

Parasitism. In this type of close association one organism lives at the expense of another, known as its host. There are large numbers of both external and internal parasites in the sea. Even the diatoms do not escape; Chaetoceros, for instance, is host to Paulsenella, an unarmored dinoflagellate. Among the temporary planktonic parasites are unarmored dinoflagellates, many copepods, and the cercaria larvae of the flukes.