EFFECTS OF FEEDING, FEEDING HISTORY, AND FOOD DEPRIVATION ON RESPIRATION AND EXCRETION RATES OF THE BATHYPELAGIC MYSIDGNATHOPHAUSIA INGENS

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Abstract
Groups of the large bathypelagic mysid Gnathophausia ingens were fed at different frequencies for at least three months in the laboratory, then starved for five weeks or alternately fed and starved over shorter periods of time. Oxygen consumption and ammonia excretion rates were determined before and after feeding and during starvation. Prolonged differences in the amount of food eaten prior to starvation affected the animals' initial responses to starvation. In the first 3 weeks, animals which had been more frequently fed maintained higher respiration and ammonia excretion rates relative to rates after this time. Animals fed less frequently maintained stable rates throughout the 5 week period of starvation. After a maximum of 3 weeks, starved individuals relied largely on nonnitrogenous energy stores, presumably lipids, regardless of feeding frequency prior to starvation. The high lipid content of G. ingens and the low metabolic rate of starved individuals are advantageous for life in the energy-poor deep-sea. We have observed transient postfeeding increases in respiration and excretion rates. Excretion rate (E, in micromoles NH3/h) increased with amount eaten (F, in mg ash-free dry weight of food) (E = 0.038F). Respiration rate (R, in micromoles O2/h) increased with excretion rate (R = 1.40 + 1.03E). Measurements of respiration and excretion rates using postdigestive individuals of G. ingens therefore underestimate average field rates by an amount proportional to food intake. The energetic effects of feeding on the metabolism of G. ingens are not negligible. We estimate that about 29% of the energy in the laboratory ration ingested by G. ingens is expended in the postfeeding increase in respiration.