Copepod equiproportional development: experimental confirmation of its independence of food supply level, and a conceptual model accounting for apparent exceptions

Two copepods - Metadiaptomus meridianus (Douwe) and tropodiaptomus spectabilis (Kiefer) - were reared from egg to adulthood at defined levels (50, 100, 250, 500, 1000 and 3500 mu g C I-1) of the flagellate alga Cryptomonas sp. in 0.45 mu m-filtered lake water, at 17 degrees C. Resulting estimates of total naupliar (D-n) and total copepodid (D-c) durations were used to test the predictions of recent theoretical models (Carlotti & Sciandra, 1989; van den Bosch & Gabriel, 1994) that equiproportional development (EPD), a theoretically feasible growth rule in copepods, is applicable at all food supply levels. Regression analysis of double log-transformed data indicated that D-c:D-n ratio values, a composite index used as a surrogate for individual life-stage ratios, were statistically independent of food supply level in the above taxa and in equivalent data obtained for Eudiaptomus gracilis in parallel experiments (Santer, 1994). Experimental support for the model predictions is accordingly provided. A graphical model of EPD in relation to food supply is also developed, and used to explain apparent exceptions to EPD arising from former observations (Hart, 1990) that D-c:D-n ratios varied systematically with food supply in the first two calanoids listed above, and in cyclops vicinus (Hansen & Santer, 1995). Recognition of these anomalies as apparent rather than real exceptions enhances the prospect that equiproportionality exists as a real, widely and generally valid growth 'rule' in copepods, at least non-diapausing species, with one proviso: dependence on food of suitable quality (obviously above the appropriate minimal energetic food threshold).