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.)

Abstract

The content of various forms of vitamin A were determined in developing eggs and larval stages of Atlantic halibut ( Hippoglossus hippoglossus) 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- trans retin al form, the rest being all- trans retin ol and 13- cis retin ol. No other major forms or precursors of vitamin A (carotenoids) were found in the eggs. During the ensuing yolk-dependent development, until the onset of first feeding (day 33 posthatch [PH]), the content of all- trans retin ol increased steadily with a concurrent decline observed in all- trans retin al. Half of the increase in all- trans retin ol occurred during the last 12 days before the onset of first feeding. Specific dissection and analyses of the eyes showed that at hatching, neither retin al nor retin ol was present. However, with continued development, primarily all- trans retin al, with smaller amounts of all- trans retin ol and 13- cis retin ol, were gradually incorporated into the eyes. Approximately 75% of the observed increase occurred between days 21 to 33 PH, which coincided with the period when the eyes became pigmented. Following day 26 PH, when the liver anlage had become the major axis of yolk absorption, greater amounts of particularly all- trans retin ol began to be incorporated into the eyes, while both all- trans retin al and 13 cis retin ol continued their incorporation, albeit at a slower rate. After the onset of first feeding on natural zooplankton (>95% the calanoid copepod Temora longicornis,) the trend established at day 26 PH continued throughout the studied exotrophic stages. The planktonic prey was found to contain high levels of carotenoids, but only trace amounts of all isomers of retin ol and retin al. We argue that prior to the onset of first feeding, vitamin A is derived from all- trans retin al 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. After first feeding is established, vitamin A is derived from precursors present in the planktonic prey, mainly carotenoids. We also propose that the increased content of all- trans retin ol in the eyes may form a labile source of vitamin A that can be readily be converted to the functional aldehyde form, thereby allowing increased photo-sensitivity control of retin al pigments during reduced light conditions.