Measurements of respiration and excretion for the ctenophore Mnemiopsis mccradyi Mayer at 21 ~ at 4 concentrations of food (copepods: primarily Acartia tonsa) showed a marked effect of food availability on the metabolic rate.
Starved ctenophores respired at a rate of 6.0 ktg-at 02 h -1 g-1 dry wt, and excreted at 0.42/~g-at N-NH4 h -~ g-1 dry wt. Ctenophores fed at 500 prey 1-1 respired at a rate of 26.4~g-at O5h -1 g-1 dry wt and excreted at a rate of 1.8 ktg-at N-NH4 h -~ g-1 dry wt. Freshly collected ctenophores from Biscayne Bay, Florida, during November 1979, had intermediate metabolic rates. The atomic O:N ratio (based on ammonium alone) averaged 12:1 and was fairly constant for all conditions. The release of organic nitrogen was approximately equal to ammonium excretion, and urea was about 10% of the total. The elemental composition of carbon and nitrogen as a percentage of the dry weight varied directly with food availability. Total nitrogen turnover ranged from 8% per day for starved ctenophores, to 19% per day for the highest food concentration. Carbon turnover rates were higher. M. mccradyi exhibited a clear shift upwards in their excretion rate in response to the time they had been feeding, reaching a maximum after about 2 h. When ctenophores were removed from food, the metabolic rate declined exponentially with time, the most rapid change occurring in the first few hours of starvation. Comparisons of carefully measured metabolic rates for freshly collected ctenophores with experimental results such as these may help to establish the nutritional state and extent of food limitation in situ.