CHAPTER XII

COMMENSALISM, SYMBIOSIS, PARASITISM, AND OTHER FORMS OF ASSOCIATION

The associations we have thus far traced depend upon community of blood. But organisms enter into relationships of various kinds with other species besides their own. A very important part of the environment of every organism consists of other organisms. Different species in their efforts to get on in the world have had to adapt themselves to each other, and these adaptations have often entailed far-reaching modifications of structure and habit.

In the kind of association known as commensalism, organisms take up the same abode, or one may live upon or within the other, without either one living at the other's expense. There are many kinds of worms, crustaceans, and other animals that live within the cavities of sponges, receiving there protection and such food as they can abstract from the water. Certain species of small fishes live within the digestive cavity of the large sea-anemone Discosoma, and probably take advantage of some of the food captured by their protector. There are other little fishes that swim about among the stinging tentacles of the Portuguese man-of-war. A common illustration of a commensal, or messmate, is furnished by the little crab Pinnotheres which lives within the shells of oysters, mussels, and other bivalved molluscs. The crab is not parasitic upon the molluscs, although it may deprive its host of some of its food. A more

FIG. 139-Fierasfer acus in the act of entering the intestine of a holothurian. (After Grassi.)

curious case of commensalism is that of the female of the crab Hapalocarcinus marsupialis which takes up its abode upon a coral, and eventually comes to be almost surrounded by a limy inclosure secreted by the coral polyps. The crab being thus made a prisoner subsists upon the food drawn in by the action of her appendages. Another singular domicile is chosen by the small fish Fierasfer which inhabits the terminal part of the intestine of a sea cucumber. The form of the fish, which is long and narrow, is especially adapted to its environment. The tail is narrow and tapering, which is a very unusual form for the tail of a fish, but it is obviously correlated with the creature's custom of entering its habitat tail first.

There are several animals that attach themselves, temporarily or permanently, to others, but without inflicting any injury upon their benefactors. The remora has a peculiar adhesive disc on the top of its head whereby it attaches itself to a larger fish or the bottom of a boat, and thereby secures transportation without efforts of its own. The whale barnacle Coronula is always found attached to whales, and may become partly overgrown by the skin. Another barnacle, Tubicinella, lives almost completely inclosed by the whale's skin. The chief advantages gained by the barnacles probably consist in the greater amount of food secured on account of the whale's movements.

Cases of symbiosis, or associations in which each species derives some advantage from the other, are very common. Not improbably the association of the hermit crab Pagurus with the anemone Adamsia is a case in point. The anemone attaches itself to the coiled shell which is occupied by the crab, and derives the advantage of being carried about from place to place. The crab probably enjoys a certain amount of protection on account of the stinging cells of the anemone. If the anemone is removed, the crab hunts about for a new anemone which it endeavors to tear loose and place upon its shell. Another kind of hermit crab carries an anemone attached to the surface of its larger pincer. Apparently the crab makes active efforts to place an anemone in this position, for, after molting the skin, the crab tears off

the anemone from its discarded covering, places it upon its pincer, and carries it about as before.

The association of ants and aphids are in several cases symbiotic. The eggs of the corn-root aphids are protected in ants' nests during the winter and then carried to appropriate plants in the spring. The ants profit from the arrangement by deriving the sweet honeydew from the aphids. Among the numerous species of creatures inhabiting the nests of ants and termites there are many that supply their hosts with exudations secreted by various glands, and receive in turn food which their hosts regurgitate or otherwise provide.

Many kinds of Protozoa, hydroids, corals, anemones, and worms, form symbiotic associations with unicellular green algæ. The green Hydra (H. viridis) differs from the common brown species (H. fusca) in harboring minute algæ in the cells of the entoderm. If these same algæ are taken in by the brown Hydra they are digested, but, owing to some specific peculiarity of their proper host, the algæ engulfed by the entoderm cells of the green Hydra find a favorable habitat for growth and multiplication. Under the influence of light, the CO2 given off by the Hydra is employed by the alge in the synthesis of their carbohydrates, and the oxygen liberated as a result of photosynthesis may be utilized by the Hydra. The mutual services performed on a large scale by the plant and animal kingdoms are thus reproduced in miniature within the limits of a single small organism. The algae occur in the egg of the green Hydra, and are thereby passed on in the process of sexual reproduction from one generation to the next.

A similar symbiotic relation between one-celled algæ and a small flatworm, Convoluta, has been studied by Keeble, who finds that in Convoluta roscoffensis the animal may be entirely dependent upon its algæ for its supply of food. In its early stages this species may devour minute plant and animal organisms, but when its contained algæ increase in number it ceases to take in solid food. For a long time it may live in filtered sea water upon the photosynthetic products of its plant cells.

Finally, in old age, the animal digests its helpful symbiotic companions, loses its green color, and dies. Without its algæ it is unable to live.

If the Convoluta is kept in the dark its algæ decrease in number and finally most of them are digested. The animal grows smaller and paler, but if it is restored to sunlight the algæ multiply, and when this occurs the animal may regain its normal size. Unlike those of the green Hydra, the symbiotic algæ of Convoluta are not transmitted through the egg, and the young larvæ raised in filtered water so as to escape infection were observed to be generally devoid of green cells. If the colorless larvæ were transferred to ordinary sea water they were found to become infected by green cells within a few days. While the algae as such could not be cultivated independently of the animal, it was found that they gave rise to numerous, free-swimming flagellate organisms which form the means of infecting the young

worms.

In Convoluta we have two kinds of organisms which have become mutually dependent to such a degree that neither can complete its life history without the other. The larval worm that does not become infected dwindles and dies. If it is fortunate enough to ingest some algæ it takes on a new lease of life, and "becomes instead of a microscopic, transparent object, a visible green organism." Keeble showed that the algae were able to assimilate urea, and that they probably subsist in part upon the nitrogenous waste matter of their hosts. They probably furnish their host with sugar, fat, and nitrogenous matter in a form suitable for assimilation into animal tissue.

It has been found that certain tissues of the bodies of higher invertebrates, especially crustaceans, insects, and arachnids, contain numerous bacteria, and sometimes yeast cells and fungi, which have been considered to stand in a symbiotic relation to the cells of their host. The constant association of these organisms with specific tissues, the absence of any indication injurious effects that can be ascribed to them, and their regul transmission within egg cells from generation to generation

lend support to the theory that they play some useful rôle in the life of the organism.

Symbiosis is more common among plants than among animals. One whole group, the lichens, containing many families and numerous genera and species, is composed of organisms consisting of fungi and algæ. In the lichen, the algæ are closely enveloped in the network of threads of the fungus. As the fungi, being devoid of chlorophyll, are unable to build up their substance

FIG. 140-A part of a lichen, Xanthoria parietina: 1, germinating spore, sp, of fungus, whose branches have become applied to two cells of an alga, a, a, 2, a mass of fungus threads and algæ, a, a, from the tissue of the lichen. (After Bonnier.)

through photosynthesis, they resemble the bodies of animals in their relations to the algæ. Each partner to the association derives an advantage from the other. While the algae are generally capable of leading an independent life, the fungi usually do not thrive unless they become associated with algæ.

What are commonly called fungi represent an extensive and miscellaneous assortment of plants that have, as it were, fallen from grace in the sense that they have lost their chlorophyll, and have usually become dependent upon organic matter of some sort for their nutrition. Most of them are saprophytes, or