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

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Total ammonia content (TAmm) and excretion (JAmm), 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 °C and 34% S) or in light (light-arrested, nonon-hatching embryos). The TAmmof the eggs increased from 250-300 nmol ind-1during the first 5-10 days post fertilization (dPF) to 400-500 nmol ind-1at the time of natural hatching in darkness. TAmmdid not start to decrease before 2-5 days after hatch, and yolk-sac larvae had a higher TAmmcompared to the light-arrested embryos of the same age and batch. JAmmincreased exponentially rom about 0.05 nmol ind-1h-1at 10 dPF to about 3 nmol ind-1h-1at hatch. Embryos undergoing hatching showed a significantly higher JAmmthan non-hatching, light-arrested embryos of the same age. The content of K+ in distilled water rinsed eggs declined from about 0.85 µmol ind-1at 2-4 dPF to about 0.15 µmol ind-1at hatch, while that of Cl-remained constant at about t 0.85 µmol in-1, and that of Na+increased slowly from 0.10 to 0.20 µmol 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 TAmmof 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 TAmmcomp ared 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.

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