Ammonia dynamics in relation to hatching in Atlantic halibut (Hippoglossus hippoglossus L.)

Total ammonia content (T-Amm) and excretion (J(Amm)), and ionic content (K+, Na+, Cl-) have been studied in embryos and yolk-sac larvae of Atlantic 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 nmol 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 did not start to decrease before 2-5 days after hatch, and yolk-sac larvae had a higher T-Amm compared to the light-arrested embryos of the same 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. Embryos undergoing hatching showed a significantly higher J,mm than non-hatching, light-arrested embryos of the same age. The content of K+ in distilled 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- remained constant at about 0.85 mu mol ind(-1), and that of Na+ increased slowly from 0.10 to 0.20 mu mol ind(-1). Based on the measured ions, there 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 found 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 species (NH3). The delayed release of T-Amm compared to embryos and yolk-sac larvae of other marine teleosts, may relate to the deep-water spawning of the Atlantic halibut, and the buoyancy regulation of the yolk-sac larvae.