Larval development and metamorphosis in Atlantic halibut (Hippoglossus hippoglossus) : influences of nutritional, environmental and physiological factors

Abstract

The present work reflects investigations performed, in the frame of a Ph.D. thesis, on the larval development and metamorphosis in Atlantic halibut (Hippoglossus hippoglossus). The emphasis was placed on the influence of nutritional, environmental and physiological factors. Three important periods have been identified during the larval development of Atlantic halibut; namely, the pre-metamorphic stage (up to 35 days post first-feeding (PFF)), the metamorphic climax (from day 35 to 45-50 PFF) and the post-metamorphic stage. The pre-metamorphic stage is characterised morphologically by a change of larval body shape with a widening of the body. The period of metamorphic climax is initiated by a surge in thyroid hormone synthesis followed by the differentiation of the stomach and the accumulation of lipid in the liver. Meanwhile, larvae fed enriched-Artemia instead of copepods, showed a delayed appearance of morphological changes associated with the shift from pelagic to demersal life-style. Their ability to digest and accumulate lipids was also delayed until the time at which the stomach became functional. Associated with this, the recruitment of rods into the retina was depleted as shown by a larval rod/cone ratio of 1.32 compared to 2.53 for the copepod-fed larvae. The ultrastructure of the outer segments of the retinal photoreceptors was characterised by the poor stacking of the membranes; and the retinal pigment epithelium revealed signs of lipid deficiencies such as an increase in appearance of phagosomes and the absence of lipid droplets. The use of Artemia HUFA-rich enrichments helped to improve the larval performances, especially in terms of growth and stress resistance. However, the occurrence of pseudoalbinism remained widespread. The modifications of the light environment used to rear the larvae could not be linked with an improvement in larval pigmentation. Nevertheless, the use of blue light (475nm) improves the survival and the food intake during part of the pre-metamorphic stage. The present results support the importance of HUFA in the diet of larval fish. However, it is suggested that the larvae are unable to fully digest Artemia at a time when their digestion relies solely on the pancreatic enzymes. It was observed that the exoskeleton of Artemia was not fully digested by larvae whose stomach was not functional. Consequently, the hypothesis under which only the food accessible via the anus and mouth of the Artemia was digestible, was raised, therefore questioning the adequacy of long term enrichments. Overall, the present study strengthened existing knowledge of the physiological, environmental and morphological parameters affected and affecting the halibut larval metamorphosis. The most prominent experimental evidence further emphasised the nutritional importance of HUFA especially for the development of the retina and presumably the whole nervous system.