Egg Development of a Chydorid Cladoceran, Chydorus Sphaericus, Exposed to Constant and Alternating Tempeatures: Significance to Secondary Productivity in Fresh Waters

Abstract

Eggs of the chydorid cladoceran, chydorus sphaericus, were exposed to five constant temperatures of 10°, 15°, 20°, 25°, and 30°, and three 12.12 h diel thermal cycles of 10°:20° (daily mean [x] of 15°), 15°:25°, (° = 20°), and 20°:30° (x = 25°). Diurnal temperature fluctuations decreased the duration of development and, consequently, accelerated the developmental rate an average of 10% over the constant thermal regimes that were equal to the daily means of the alternating schedules. Both the Edmondson (1960) indirect method of calculating duration and the direct method were equally accurate. Both methods revealed durations that varied inversely with temperature and rates that varied directly, and disclosed a curvilinear relationship of temperature to duration and rate of development. The reduced duration was shown not to be caused by ineffective acclimation procedure, erroneous mathematical analysis and the assumed linearity of the rate—temperature function (Kaufmann effect), or pulsed egg laying. This supports the conclusion that hatching time actually was reduced and points to the occurrence of an unexpected physiological alteration of the developing embryo. Duration reductions, caused by the alternating thermal conditions, appear to provide an adaptive energetic advantage of enhanced metabolic egg development to littoral Cladocera. To test the effect of reduced duration of egg development associated with alternating temperatures on the calculation of durations under fluctuating field conditions, four methods of determination were compared. The "egg—specific weighted mean rate," although the most accurate technique with about a 3% overestimate of actual developmental time, requires precise knowledge of the developmental starting times of each egg, which has, until now, prevented its application in field studies. Because of relative ease of field application and negligible predictive error of less than 5%, the "weighted mean rate" of calculating duration from averaged rates of selected time intervals from continuous field thermal recordings is the method of choice in field studies. Thus, even though a real physiological alteration accompanies dynamic thermal regimes as experienced in the field, the rate—summation method of estimating duration accommodates this durational reduction to well within acceptable levels of current field accuracy. For greater accuracy in predicting the duration of development and, consequently, the birth rate of field populations, it is suggested that water temperature be continuously monitored, diurnal position of the study population be determined, acclimation of test animals be conducted prior to experimentation, a rate—summation method be applied, and mean field temperature calculations be made over the smallest practical time intervals.