an animal or plant organism capable of surviving under substantial changes of environmental conditions. For example, eurytherms inhabiting the ocean littoral survive regular dry periods at low tide, extreme heat in summer, and chilling and sometimes even freezing in winter. Euryhaline animals inhabiting river estuaries tolerate substantial fluctuations in water salinity. A number of eurybathic animals inhabit areas with a broad range of hydrostatic pressure. Many terrestrial inhabitants of temperate latitudes are capable of surviving large seasonal fluctuations in temperature.
Some species have a higher survival capability because of their ability to tolerate unfavorable conditions in a state of anabiosis. These include many bacteria, the spores and seeds of many plants, adult perennial plants of cold and temperate latitudes, wintering buds of freshwater sponges and bryozoans, the eggs of branchiopods, adult arthropods of the phylum Tardigrada, and some rotifer. Hibernation fulfills a similar function for certain mammals. The oocysts of parasitic protozoans and the larvae and ova of certain nematodes tolerate extreme freezing and drying and are resistant to many toxins; all of these factors make it possible for them to retain their viability for a considerable time.
The larvae of some insects and crustaceans, such as dragonflies and land crabs, are aquatic, while the adults are terrestrial. Thus, the environmental conditions tolerated by a species vary greatly in different stages of the life cycle, although each stage tolerates a narrower range of conditions. This is also true of certain parasitic worms, which in different stages of their life cycle parasitize invertebrates, fishes, and mammals and live in the external environment.
Sometimes adult individuals can survive greater changes in environmental conditions than can the young of the same species, for example, certain aquatic invertebrates and fishes. Certain widely distributed species have various populations adapted to life in regions with various conditions; thus, the survival capability of a species as a whole is greater than that of a particular individual or of groups of organisms at a specific stage of development. Eurybionts are usually distributed far more widely than stenobionts.
REFERENCEOdum, E. Osnovy ekologii. Moscow, 1975 (Translated from English.)
G. M. BELIAEV