Symbiosis (Gr., 'living together'), a biological term introduced by De Bary to denote certain kinds of physiological partnership between organisms of different kinds. Consortism is synonymous. As there are many kinds of organic association, it is convenient to restrict the term symbiosis to such intimate and complementary partnerships as exist between algoid and fungoid elements in lichens, or between unicellular Algæ and Radiolarians. In organic nature there is no isolation; no organism lives or dies to itself; there are countless vital associations, some very indirect and external—e.g. the mutual dependence of some flowers and insects—others very direct and intimate, as in the symbiosis of Algæ and Radiolarians. It often happens that two organisms live together without there being any apparent vital bonds between them; thus, Diatoms may be 'epiphytic' on Algæ; Algæ, lichens, mosses, ferns, orchids, &c. are often epiphytic on trees; many Algæ are 'epizoic' on animals—e.g. those which live among the hairs of sloths; and one animal may be epizoic on another, as sponges often are on zoophytes. Again, there may be external partnerships, such as those between pilot-fish and shark, or between beef-eater birds and wild cattle. These suggest cases of mutualism or Commensalism (q.v.), such as the partnership between certain hermit-crabs and sea-anemones. Probably the constant occurrence of colonies of the Alga Anabaena in the leaves of the aquatic plant Azolla is a similar partnership. Alike in symbiosis and in commensalism the partnerships are advantageous to both of the associated organisms, and are therefore to be distinguished from Parasitism (q.v.), in which the benefit is all on one side. It is useful to distinguish these different grades of association, but it cannot be pretended that the distinctions are rigid.
Apart from lichens, the partnership of unicellular Algae with Radiolarians is the best-known case of symbiosis. The partner Algae—known for a long time as ‘yellow cells’—used to be variously interpreted as reproductive cells, secretory cells, reserve stores, parasites, and so on; but the researches of Geddes, Brandt, and others demonstrated their algoid and truly symbiotic nature. They have a cellulose wall (except in the Acanthometridæ among Radiolarians), a nucleus, two pigments, of which one is at least closely analogous to the ordinary chlorophyll of plants; they are able to live and multiply after removal from their host or after its death; in sunlight they evolve oxygen and form starch; they multiply as do free unicellular Algae. From his experiments Geddes inferred that the starch formed by the Algae may be absorbed by the Radiolarians; that when they die the Algae are digested by their partners; that during life they absorb carbonic acid and nitrogenous waste from the Radiolarians and in turn liberate oxygen which may accelerate the vital functions of their bearers. It seems that the partnership is distinctly advantageous, for the Algae flourish and multiply, and those Radiolarians which are without Algae are few and much less common than the vast majority which exhibit symbiosis. Brandt’s results are for the most part in agreement with those of Geddes, though divergent on some points of details. The Algae may belong to a distinct genus (Zooxanthella of Brandt, Philozoon of Geddes), or may be simply the swarm-spores of various olive-green seaweeds.
Similar symbiotic Algae occur in some Foraminifers, in several Cœlenterates, especially otherwise colourless sea-anemones, and, according to Geddes, in some species of the Turbellarian Convoluta. Brandt maintains that in the fresh-water sponge (Spongilla) and in the fresh-water Hydra there are symbiotic Algae of the genus Zoochlorella, but Ray Lankester has shown to the satisfaction of most naturalists that the pigmented bodies in those animals are no more symbiotic Algae than are the green corpuscles in the leaf of a buttercup. Many marine sponges are infested by various kinds of Algae, but we do not know that they exhibit any real symbiosis.
In regard to some green Protozoa there is much dispute whether the green colour is due to chlorophyll bodies or to symbiotic Algae. Some forms—e.g. Stentor polymorphus, Coleps viridis, Ophrydium viride, and Vorticella chlorostigma—also occur in a colourless state. Some observers regard the bodies as Algae, others as chlorophyll corpuscles. ‘Zoochlorellæ,’ found in species of Paramœcium, Stentor, and Stylonichia, have been pronounced by good authorities symbiotic forms of the Protococcus-like Chlorella, which have also been discovered living freely. Thus it appears that, while many cases of symbiosis are indubitable, there are other cases in regard to which judgment must be for a time suspended.
See COMMENSALISM, EPIPHYTES, LICHENS, PARASITIC ANIMALS; O. Hertwig, Die Symbiose (Jena, 1883); Geddes, Nature, xxv. (1882); Brandt, Archiv. f. Anat. u. Physiol. (1882), and Mittheilungen Zool. Stat. Neapel, iv. (1883); Ray Lankester, Nature, xxvii. (1882).