DENSITY, SIZE, AND CLUTCH OF TWO HIGH ALTITUDE DIAPTOMID COPEPODS

Population characteristics of two copepod species, Diaptomus shoshone Forbes and Diaptomus coloradensis Marsh, and both interspecific and intraspecific interactions are examined. Both species vary in size among years and among several high altitude ponds in Colorado. Larger individuals with larger clutches are found in ponds whcrc the density of developing individuals is low. Size differences can occur as a result of conditions in nauplii or copcpodid stages. Clutches OF both species vary in a given pond as the season progresses, but larger individuals carry larger clutches, and, at least in D. shoshone, first clutches are found on large individuals, There is little interaction between the two species, and temperature does not affect adult size. Size differences and clutch size differcnccs among populations of a single copepod species are widespread (Hutchinson 1967; Ravera and Tonolli 1956). Size differences have been attributed to genetic selection through loss rate due to lake outflow (Ravera and Tonolli 1956) or character displacement if some observed populations coexist with a similar species (Cole 1961; Hutchinson 1951). But most investigators have studied physiological reasons for size differences. McLaren (1965) suggested that copepod size is a function of temperature unless food is limiting; Mullin and Brooks (1970) felt that food quantity and quality arc the important determinants. The number of eggs in a clutch is generally believed to be a direct consequence of nutrition (Comita and Anderson 1959). McLaren ( 1965) believed that the largest individuals of a species are physically able to produce larger clutches although they do not necessarily do so. Smyly (1968) reported a general positive correlation between female size and clutch, but this relationship is not always found P avis 1961). Certainly clutch varies among populations and through time ( Hutchinson 1967 ) . Several high altitude ponds within a short radius of Rocky Mountain Biological l Supported by a Tufts Faculty Research Award. Laboratory (Crested Butte, Colorado) offer a unique opportunity to study such relationships in populations of two calanoids, Diaptomus shoshone Forbes and Diaptomus coloradensis Marsh. Because the growing season is short, there is only one synchronous generation of D. shoshone yearly, and the first generation of D. coloradensis is synchronous. There is considerable variation in density, size, and clutch size among populations in a given year and among years in a given pond. Because most ponds are small, reasonable estimates of relative density can be made, and environmental conditions may change dramatically and swiftly. I undertook this study to provide information bearing on several questions. First, is there an effect of population density during development on the adult size of a given species? Food is often in short supply in high altitude ponds found in cirques ( Russell-Hunter 1970) and in semidrainage ponds ( Pennak 1969). If food is limiting, there should be an inverse relationship between size of individuals and density during development. If there is no correlation between temperature and adult size, and if McLaren’s (1965) hypothcsis is correct, food may be limiting. Second, is there an advantage to large size? Other investigators think so (Hutchinson 1967), but in rapidly changing environLIMNOLOGY AND OCEANOGRAPHY 840 NOVEMBER 1973, V. lS(6) DIAPTOMID DENSITY, SIZE, AND CLUTCH 841 Table 1. Summary of transfer and enclosure experiments clone with Diaptomus shoshone Donor Recipient Time period Apparatus Purpose