SYMBIONTS. — From the Greek σύν (= together) and φλοι (= life). This term, introduced by A. de Bary in 1879, denotes the intimate and constant association of two animal or plant organisms of different species, capable of creating mutual relationships between them that allow them to achieve more favorable living conditions.
When both components of a symbiosis (symbionts) derive equal benefit from their association, one speaks more precisely of mutualism or mutualistic symbiosis. From this condition, one gradually passes to others,
COURTESY OF DR. ENRICO VANNINI — Section of the thallus of a lichen, consisting of the symbiosis between the cells of an alga (in black) and the filaments of a fungus.
SYMBIONTS. — Section of the thallus of a lichen, consisting of the symbiosis between the cells of an alga (in black) and the filaments of a fungus.
COURTESY OF DR. VANNINI — Section of the thallus of a lichen, consisting of the symbiosis between the cells of an alga (in black) and the filaments of a fungus.
Among animals, the advantages gained by the symbionts generally consist in defense against predatory organisms or in greater ease in obtaining food. A well-known example is that of a group of soft-bodied crustaceans (hermit crabs), which find protection by living inside empty shells of gastropod mollusks and which often form symbioses with coelenterates (sea anemones) well provided with stinging cells, attached to the shell of the mollusk. The stinging batteries of the anemones constitute a defensive weapon for the hermit crab; on the other hand, the anemones derive benefit from the hermit crab’s leftover food and, above all, from the opportunity to be passively transported during the latter’s wanderings, thus becoming vagrant instead of remaining sedentary as they normally would. Similar symbioses, with similar reciprocal advantages, are established in sponges of the genus *Suberites*, which implant themselves on the exoskeleton of crabs of the genus *Dromia*; or also in certain small fish, which live among the tentacles of anemones or jellyfish, thus defending themselves from attacks by their own predators, while at the same time, through their movements, they facilitate the arrival of food particles suspended in the water to the mouth of the coelenterate.
A separate mention must be made of symbioses between protozoa and metazoa. The case of termites is cited, whose intestines host a considerable number of flagellates, indispensable for the digestion of the wood on which these insects feed; and that of ruminants, whose stomachs harbor certain species of ciliates, necessary for the digestion of the cellulose in the cell walls of plant cells.
Finally, numerous and important are the symbioses between animal and plant organisms (Buchner). In these cases, the plant symbiont is generally a microscopic form belonging to the groups of fungi, algae, or bacteria. Among the most widespread examples, one may recall that of various protozoa or metazoa (sponges, coelenterates, annelids, mollusks, etc.), whose cells exhibit a yellow or green coloration due to the presence, in the cytoplasm, of unicellular algae (zooxanthellae, zoochlorellae), which benefit from the protection offered by the animal and use, for their photosynthetic function, the carbon dioxide eliminated by the latter, while the animal, at least to a certain extent, can exploit both the oxygen and the starch produced by the alga.
More complex are the cases indicated by Pierantoni under the name of hereditary physiological symbiosis. These are constant and indispensable mutualistic associations in which a lower plant (yeast or bacterium) lives in association with an animal that, through various mechanisms, transmits the symbiont to its descendants. In many insects, such a symbiosis has nutritive functions. Thus, the female of *Icerya purchasi* has, on either side of the intestine, a special tissue (*mycetome*) filled with blastomycetes, which find their nourishment in the animal cells and digest carbohydrates for the benefit of the insect, which feeds on them. The transmission of the blastomycetes to subsequent generations is ensured by their passage from the mycetome to the oocytes contained in the ovaries. Similar phenomena have been observed in aphids. In certain cases, the production of vitamins indispensable to the host by the symbiotic plant seems to be demonstrated.
Other cases of physiological symbiosis between animals and bacteria are connected with phenomena of bioluminescence (Pierantoni, Buchner). Thus, in the luminous organs of various insects, the presence of bacteria has been noted, which participate more or less directly in the biochemical processes of light production. Numerous luminous bacteria have also been found within the luminous organs of marine animals (cephalopods, tunicates, fish). Not all cases of bioluminescence, however—especially frequent in abyssal faunas—seem attributable to phenomena of symbiosis with bacteria.
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