Commensalism

Commensalism (from Latin con- together and mensa- table), or nakhlebnichestvo (literally 'table-sharing'), is one form of symbiosis, or cohabitation of two organisms of different species, wherein one of the commensals benefits from food, protection, or other advantages at the expense of another organism, which however suffers no damage or harm from this. The precise demarcation of commensalism from parasitism is difficult, as between the clearly expressed forms of these phenomena there exists a series of transitional forms. The relationships between the commensal and its host are extremely diverse: 1) the most primitive are those relationships when both organisms are merely in a certain spatial proximity to each other; 2) the commensal temporarily attaches itself to the host's body or permanently resides on it; 3) the commensal temporarily or permanently lives in the cavities of the host or in its internal organs. 1. Spatial proximity is often associated with sharing common shelter; the hermit crab (Eupagurus bernhardus) lives in empty shells of mollusks (Buccinum), into which it hides its soft abdomen; in case of danger, Eupagurus entirely hides in the shell. Very often an anemone Sagartia parasitica (see Mutualism) sits on the shell, and almost always the shell is overgrown with hydrozoans (Hydractinia echinata). The worm Nereidapsis fucata is a specific inhabitant of hermit crab shells; this nereid remains in the last whorl of the shell, and when the hermit crab devours mollusks, it comes out of its shelter and tries to snatch a piece of the prey. Interestingly, the hermit crab tolerates its commensal and does not touch it despite the latter's defenselessness. Various inhabitants of ant nests from arthropods are also commensals; such as for example the mite Le laps equitans, rove beetles (Hetaerius) and many others. It is characteristic that certain species of commensals inhabit the nests of specific ant species, which they also mimic. In the nests of the red ant (Formica rufa), the ants Formicoxenus nitidula and Solenopsis fugax can live in the position of commensals. This category of external inhabitants of ant nests are called synoikai, i.e. 'co-inhabitants of the house'; the latter feed on various decay, remnants of the host's food or the corpses of the latter, and the inhabitants of the ant nest are indifferent to such commensals. 2. Commensalism with temporary attachment to the host's body is observed in the remora fish (Echeneis remora), whose dorsal fin is transformed into a wide and long oval sucker. Thanks to this, Echeneis attaches to the skin of sharks, other large fish, turtles or to the hull of sea vessels, which allows the remora to passively move over large distances. When food scraps are thrown overboard, Echeneis moves away from the ship and devours them, after which it again sucks onto the hull of the vessel. The described form of relationships is classified under the category of paroikia, i.e. symbiosis of two animal species, when weaker species usually stay among or near stronger or better protected organisms, which gives the former certain benefits. Various coelenterate animals in the form of anemones, jellyfish, siphonophores possess the so-called stinging cells; upon touching such an animal, a thread of a poisonous substance is ejected from these cells, serving for protection. Among the tentacles of large anemones often stays a fish of the genus Trachichthys. If the anemone contracts, Trachichthys hides in its cavity; in both cases Trachichthys uses the anemone as shelter, providing reliable protection from attack by predatory fish. It is very probable that Trachichthys appropriates for itself part of the food consumed by the anemone. The elements of commensalism are present here. Trachichthys possesses immunity to the poisons of the corresponding coelenterates. Permanent attachment to the host's body is observed in cases of indifferent parasitism or parasitism of place. Colonies of bivalve mollusks mussels attach in masses to underwater stones and piles; sessile crustaceans barnacles (Balanus) also attach to animals, for example to mollusk shells or to crab carapaces; various hydrozoans also find shelter here and find place for the cases of tube worms. Such relationships are called by the term epoikia, which means habitation on the surface of an animal's body without harm to the latter, and the inhabitants themselves are called epizoans (from this category are excluded ectoparasites or external parasites). Besides cases of indifferent epoikia, there are examples of specific connections of epizoans with certain host species. Thus, the sessile crustacean Coronula diadema lives exclusively on the skin of whales; certain hydrozoans live only on the body of specific fish species, etc. The phenomena of epoikia are very difficult to clearly demarcate from external parasitism. The biting lice of mammals and bird lice feed on hair or keratinized scales of the epidermis and feathers, i.e. on already non-living parts of the host's organism. However, the holes gnawed by lice in the vane of feathers and especially the intense itching caused by these insects to their hosts force one to classify biting lice and bird lice as true ectoparasites, all the more so as certain species of them can also drink the blood of their hosts. 3. The habitation of commensals in the internal organs and cavities of the host manifests in diverse forms. Of temporary sheltering of commensals in the host's body has already been mentioned above. More specific relationships are represented by bivalve mollusks Pinna, Pecten, oysters, etc. and the various crustaceans (crabs, shrimp, etc.) inhabiting their mantle cavity. The crustaceans apparently feed on the same as the mollusks themselves, between the valves of which they find reliable shelter. Similar examples of such 'parasitism of place' can also be cited from other groups of animals. Thus, in the channels of freshwater sponges lives the larva of a net-winged insect Sisyra; the female of the crustacean Phronima lives in the gill cavity of salps; in a similar cavity of bivalve mollusks are found marine ribbon worms, and in the freshwater crab Telphusa as a rule live oligochaete worms (Epitelphusa catanensis). For such inhabitants of gill cavities, favorable living conditions are created in the running water, which is passed through the respiratory organs of the host. The digestive apparatus of certain echinoderms can on the same grounds be used by external inhabitants. In the hindgut and its protrusions in the sea urchin Strongylocentrotus gibbosus often live crabs Pinnaxodes chilensis (mainly females); in the cloaca and water organs of holothurians various crabs (Pinnixa and others) can be found. In the Mediterranean Sea, a small fish Fierasfer penetrates through the cloaca opening into the water lungs of the holothurian, where up to three, and under experimental conditions up to 7 fish accumulate. In the body of the holothurian, Fierasfer finds for itself blood and running water for breathing. This commensal feeds on small crustaceans, for which it hunts outside the body of the holothurian. Fierasfer can harm the latter if it destroys the wall of the water lungs and penetrates into the body cavity of its host. The cited examples of habitation of commensals in the bodies of hosts are classified as phenomena of entoikia, which means habitation inside another organism, but without harm to it. This last moment of entoikia and must distinguish it from endoparasitism. Already from the example of the holothurian and Fierasfer it is seen that there is no definite boundary between these two categories of phenomena. A characteristic example of the transition of entoikia to parasitism is the tiny catfish Stegophilus insidiosus, living in the gill cavity of large South American catfish Platystoma coruscans and feeding on blood sucked from the gills of the host. It is very probable that among non-pathogenic intestinal protozoa there are species that inhabit their hosts in the position of commensals (for example Entamoeba coli); when commenting on such facts, it is necessary to differentiate the phenomenon of entoikia not only from endoparasitism, but also from mutualism. By formal criteria, those parasitic worms whose harmlessness to their hosts would be exactly proven could also be classified as entoics. (This remark also applies to the normal bacterial flora of the body of higher organisms.) - In phylogenetic relation, commensalism in a number of cases is one of the phases of the evolution of parasitism, as the commensal that begins to harm its host already passes into the position of a true parasite.