AMMONIUM EXCRETION BY THE OPHIURID OPHIOTHRIX-FRAGILIS AS A FUNCTION OF SEASON AND TIDE

Ammonium excretion of a dense population (approximately 1500 individuals m-2) of the ophiurid Ophiothrix fragilis (Abildgaard) was measured in the Dover Straits (French coast) between May 1989 and March 1990: the excretion rate varied from 4.8-mu-g N g-1 dry wt h-1 in November to 12.8-mu-g N g-1 dry wt h-1 in June. Mean individual ammonium excretion, E, was E = 0.019.t + 1.26 (where E = mu-g N individual-1 and t = time in min; r = 0.80; N = 81). Variations in the ammonium excretion rate during a tidal cycle appeared to arise from variations in the duration of the suspension-feeding activity of O. fragilis, which was governed by the strength of the tidal current. During short-term starvation, excretion was low (E = 0.009.t + 1.47; r = 0.91; N = 17), increasing with increasing length of starvation [E = 4.62.1n t - 2.5; r = 0.95; N = 17], as observed for other echinoderms; this could be due to catabolism of tissue. The daily ammonia flux from this O. fragilis population to the water column was estimated at 41 mg N m-2 d-1.