Ammonia excretion rate of Clytia spp. hydromedusae (Cnidaria, Thecata): effects of individual dry weight, temperature and food availability

Abstract

In order to estimate the in situ growth budget of Clytia spp. hydromedusae, ammonia excretion rates were measured for a variety of individual weights at 4 temperatures (15, 18, 21 and 25 "C), using 4 food concentrations (7, 15, 25 and 50 copepods 1-') Excretion rate increased with medusa dry wt and was directly correlated with temperature and food availability. Excretion values ranged between 0.04 and 0.936 pg-at. N-NH4 d -' ind:' Weight-specific excretion was independent of medusa dry weight ( W ) and ranged between 1.9 and 18.1 pg-at. N-NH, mg-' d.', according to experimental conditions. The daily nitrogen turnover (only considering ammonia) ranged from 6 % at the lowest temperature and food concentration to 33 % at 21 "C and the highest food concentration. The calculation of the growth budget, which represents the difference between quantity of nitrogen in food ingested and nitrogen lost through ammonia excretion extrapolated to the natural environment, showed high values during the spring, up to 55 pg N d-l ind:', and slightly negative values during the rest of the year.