AMMONIA DYNAMICS IN RELATION TO HATCHING IN ATLANTIC HALIBUT (HIPPOGLOSSUS-HIPPOGLOSSUS L.)

Total ammonia content (T-Amm) and excretion (J(Amm)), and ionic conten t (K+, Na+, Cl-) have been studied in embryos and yolk-sac larvae of A tlantic halibut (Hippoglossus hippoglossus L.) maintained in darkness (6 degrees C and 34 parts per thousand S) or in light (light-arrested, nea-hatching embryos). The T-Amm Of the eggs increased from 250-300 n mol ind(-1) during the first 5-10 days post fertilization (dPF) to 400 -500 nmol ind(-1) at the time of natural hatching in darkness. T-Amm d id not start to decrease before 2-5 days after hatch, and yolk-sac lar vae had a higher T-Amm compared to the light-arrested embryos of the s ame age and batch. J(Amm) increased exponentially from about 0.05 nmol ind(-1) h(-1) at 10 dPF to about 3 nmol ind(-1) h(-1) at hatch. Embry os undergoing hatching showed a significantly higher J,mm than non-hat ching, light-arrested embryos of the same age. The content of K+ in di stilled water rinsed eggs declined from about 0.85 mu mol ind(-1) at 2 -4 dPF to about 0.15 mu mol ind(-1) at hatch, while that of Cl- remain ed constant at about 0.85 mu mol ind(-1), and that of Na+ increased sl owly from 0.10 to 0.20 mu mol ind(-1). Based on the measured ions, the re was an anionic deficit in the egg, especially during the first week of development. When exposed to increased ambient total ammonia (0-27 mM NH4Cl), no mortality occurred, and no significant increase was fou nd in the T-Amm of the Atlantic halibut egg before 9.2 dPF despite an estimated large inward diffusion gradient for the un-ionized ammonia s pecies (NH3). The delayed release of T-Amm compared to embryos and yol k-sac larvae of other marine teleosts, may relate to the deep-water sp awning of the Atlantic halibut, and the buoyancy regulation of the yol k-sac larvae.