Further description of the development of the digestive organs in Atlantic halibut (Hippoglossus hippoglossus) larvae, with notes on differential absorption of copepod and Artemia prey

Despite the scientific and commercial interest that Atlantic halibut has received over the past decade, little is known of the ontogeny of its digestive tract during larval development. The provision of enriched Artemia as prey has been associated with incomplete metamorphosis while improved development has been achieved using wild plankton e.g., copepods. The aim of the present study was to investigate whether there was any variation during development in the ability of the larvae to digest and incorporate Artemia or copepods. Groups of larvae fed either Artemia enriched with an oil emulsion (Super Selco) and with a dried protist Schizochytrium (Algamac 2000) or with a marine calanoid copepod (Eurytemora velox) were sampled weekly from the end of yolksac resorption to the completion of metamorphosis. The sites of absorption were visualised by the presence of vacuoles in the epithelial cells of the digestive tract's mucosa. During the first 30 days of exogenous feeding, before the stomach had formed, there were small inclusions on the apical side of the rectal epithelial cells in both groups. Between 30 and 50 days post first-feeding, at the time when the stomach was differentiating with the appearance of gastric glands, and the four pyloric caecae were starting to elongate, numerous supranuclear vacuoles were observed in the mid- and hind-gut epithelial cells of the copepod-fed group. The rectal epithelial cells showed lipid vacuoles that filled their apical ends suggesting significant absorptive activity. By contrast, few vacuoles were present in the rectum of the Artemia-fed larvae. By the end of metamorphosis (up to 83 days post first-feeding), the copepod-fed larvae had an adult-like digestive tract with functional stomach and highly absorptive intestine. At the same age, the Artemia-fed larvae showed that the rectum was still the major site of absorption; this is a typical larval feature. Furthermore, observations of the gut's content confirmed the poor ability of the halibut larvae to digest Artemia in comparison to copepods. Indeed, while no individual copepod could be identified in the digestive chyme past the mid-gut of these larvae, partially digested Artemia could be seen as far as the anus in those fed on Artemia. Although the rate of development and of differentiation of the digestive tract of first-feeding halibut larvae appeared unaffected by the nature of their prey items, our results show a difference in the absorption sites of copepods vs. Artemia. These findings are of considerable significance to the rearing techniques used in the commercial production of halibut juveniles.