Vieror S. Kennedy, Roger I.E. Newell. and Albert E Ebie, eds. 1996. The Eastern Oyster: CrdJJOJlrM lIirginrca. College Park. Maryland: Maryland Sea Granc College. xvi +734 pp. Chapter 13 Natural Environmental Factors SANDRA E. SHUMWAY INTRODUCTION Of all the abiotic factors that can affect me biol- ogy of an estuarine organism such as C. virgitliClI, me Cunrer (1957) wrote, "temperature is the most synergistic effects of temperature and salinity proba- important single factor governing the occurrence and hly have me most profound effects. As, pointed out behavior of life," a paine reiterated by Kinne (1970) by a number of researchers (e.g., Alderdice 1972; who stared mat, "with regard to life on earth, rem- Vemberg and Vernberg 1972), rwo or more environ- perarure is, next to light, the most potent environ- mental componenr." It can act directly on me organ- mental variables working in concert can have more ism as a factor affecting physiological performance, profound biological consequences than anyone of - and it can also be a factor in evolution of rhe species, chose facrors acring independently. acting as a selective force in speciation. Pryrbcrch Temperature or salinity affect virtually every as- (1928) stated that, "in the environrnenr of the oyster, pect of oyster biology including feeding, respiration, temperature is the most important factor as it con- utilization of food reserves, gonadal development trols, either directly or indirectly, me growth and re- and time of spawning, parasite-disease interactions, production of the organism." In the eastern oyster, predation rates, growth, and distribution. Tempera- Crassostrea virginica, as in many ocher organisms, me ture and salinity can affect rate functions, can be the p rocesses of reproduction, developmenr, and growth trigger that initiates a process, or can be a threshold arc intimately linked seasonally to climatic condi- factor such mac a particular temperature-salinity com- tions and to the availability of energy resources. In bination is necessary for continuation of specific pro- addition, thermal effluent may, in many instances, cesses. Further, effects can vary with specific stages of induce detrimental changes in reproduction, gameto- the oyster's life cycle. genesis, fecuudiry, and larval development. Conver- In addition to temperature and salinity variations, sely, heared effluents from power planes have been of oysters experience other environmenral factors such as benefit to oyster growers in their efforts to extend the variations in sesron concentrations, light, and pH. growing season. There is an extensive literature available on rhe ef- For estuarine species, salinity is also an irnportanr fects of all of these factors on marine invertebrates, and influenrial facror, limiring disrriburion of many and only chose studies that specifically pertain to C aquatic organisms (Gumer 1961; Wells 1961). Buder (1949c) suggesred that the single most imporranr fac- virgilJica arc discussed here. This review will elucidate [or affecting oyster populations is salinity. Salinity the physiological and behavioral mechanisms that al- variations in estuaries may be diurnal, seasonal, or spa- low me easrern oyster to survive and flourish under rial and changes may be gradual or abrupr. often harsh estuarine conditio us. 467