ALTERNATE SOURCES AND DYNAMICS OF VITAMIN-A AND ITS INCORPORATION INTO THE EYES DURING THE EARLY ENDOTROPHIC AND EXOTROPHIC LARVAL STAGES OF ATLANTIC HALIBUT (HIPPOGLOSSUS-HIPPOGLOSSUS L.)

The content of various forms of vitamin A were determined in developin g eggs and larval stages of Atlantic halibut (Hippoglossus hippoglossu s) both prior to and after the onset of first feeding. Almost all (95% ) of the vitamin A present in newly fertilized eggs was in the all-tra ns retinal form, the rest being all trans retinol and 13-cis retinol. No other major forms or precursors of vitamin A (carotenoids) were fou nd in the eggs. During the ensuing yolk-dependent development, until t he onset of first feeding (day 33 posthatch [PH]), the content of all- trans retinol increased steadily with a concurrent decline observed in all-trans retinal. Half of the increase in all-trans retinol occurred during the last 12 days before the onset of first feeding. Specific d issection and analyses of the eyes showed that at hatching, neither re tinal nor retinol was present. However, with continued development, pr imarily all-trans retinal, with smaller amounts of all-trans retinal a nd 13-cis retinal, were gradually incorporated into the eyes. Approxim ately 75% of the observed increase occurred between days 21 to 33 PH, which coincided with the period when the eyes became pigmented. Follow ing day 26 PH, when the liver anlage had become the major axis of yolk absorption, greater amounts of particularly all-trans retinal began t o be incorporated into the eyes, while both all-trans retinal and 13 c is retinal continued their incorporation, albeit at a slower rate. Aft er the onset of first feeding on natural zooplankton (>95% the calanoi d copepod Temora longicornis,) the trend established at day 26 PH cont inued throughout the studied exotrophic stages. The planktonic prey wa s found to contain high levels of carotenoids, but only trace amounts of all isomers of retinal and retinal. We argue that prior to the onse t of first feeding, vitamin A is derived from all-trans retinal stored in the yolk. This form may be more suited for storage in marine fish eggs and may also be readily incorporated in the developing retina. Af ter first feeding is established, vitamin A is derived from precursors present in the planktonic prey, mainly carotenoids. We also propose t hat the increased content of all-trans retinal in the eyes may form a labile source of vitamin A that can be readily be converted to the fun ctional aldehyde form, thereby allowing increased photo sensitivity co ntrol of retinal pigments during reduced light conditions. (C) 1998 El sevier Science Inc.