Experimental studies on food dependency of development times and reproductive effort (fecundity and egg size) of Tropodiaptomus cunningtoni in relation to its natural distribution in Lake Malawi

Abstract

The influence of food supply on postembryonic durations, egg production, and functional feeding responses (FFRs) of Tropodiaptomus cunningtoni was measured in relation to food supply under laboratory conditions at around 26.5°C. Growth and fecundity was determined in 20 μm-filtered lake water with seven levels of Cryptomonas sp. enrichment (0 to 2.5 mg C.l −1 ). FFRs were measured in GF/C-filtered lake water at six food enrichment levels (0 to 1 mg C.l −1 ). Naupliar durations (N 1 -C 1 ) showed little response to increasing food supply, whereas total (N 1 -C 6 ), and thus copepodite (C 1 -C 5 ) durations showed a continuous monotonic decline over the food range tested. Egg production continued to increase with rising food supply, although clutch size was only weakly related to food level. Individual egg volume declined markedly with clutch size, which ranged from one to eight eggs, under both laboratory and field conditions. Filtration rates of up to 8 and 28 ml.individual −1 .day −1 were measured in stage 3/4 nauplii and adult females. Maximal ingestion rates amounted to 37% and 27% body C.d −1 , and estimated metabolic maintenance demands of these stages were only met at food levels of 0.65 and 0.85 mg C.l −1 , far above those prevailing in the surface waters of L. Malawi. These experimental findings are discussed in relation to field data presented on aspects of the distribution of T. cunningtoni in L. Malawi, on which basis we suggest that naupliar development relies largely on lipid reserves, while copepodite growth and egg production by adults in situ may depend on deep chlorophyll maxima