Vitamin A plays an essential role in vision as a component of the visual pigments and is required for the maintenance of the mucus-secreting epithelial tissues of the body. Its role in protecting mucous membranes and developing bone tissue is thought to be due to its involvement in the metabolism of mucopolysaccharides. Vitamin A serves an important role during embryonic development, where it regulates cellular differentiation and proliferation. Vitamin A is also required for normal immune function (West et al., 1991; Blazer, 1992; Bowers, 1997; Olivier, 1997).
Vitamin A1(retinol) is found in mammals, whereas both vitamin A1 and vitamin A2 (3-dehydroretinol, or 3,4-didehydroretinol) are found in fishes (Braekkan et al., 1969; Barua, et al., 1977; Massonet et al., 1981; Goswami, 1984a; Lee, 1987; Matsuno, 1991; Storebakken et al., 1993). Vitamin A2 is typically the dominant form in freshwater fishes, although significant amounts have been found in seawater resident Atlantic salmon (Salmo salar) (Storebakken et al., 1993) and yellowtail (Seriola quinqueradiata) (Matsuno, 1991). In the fish, the oxidative conversion of retinol to 3-dehydroretinol occurs (Lambertsen and Olaf, 1969; Hata et al., 1973; Schiedt et al., 1985; Goswami, 1984b; Davies and Davies, 1986; Katsuyama and Matsuno, 1988; Matsuno, 1991; Storebakken et al., 1993). It has been suggested that in tilapia and yellowtail (Seriola quinqueradiata), the dihydroxycarotenoids such as astaxanthin, zeaxanthin, lutein and tunaxanthin are bioconverted into vitamin A2(Katsuyama and Matsuno, 1988; Matsuno, 1991).
Trout can use beta-carotene as a vitamin A precursor, but only when the water temperature is greater than 9†C (Poston, 1969c; Poston et al., 1977). It was also shown that they can use astaxanthin, zeaxanthin and canthaxanthin as provitamin A sources (Schiedt et al., 1985). Dupree (1966, 1970) found that channel catfish could use beta-carotene as a vitamin A source only if the dietary concentration exceeded 2,000 IU per kg. Channel catfish were found to convert beta-carotene to vitamin A1 and A2 in about a 1:1 ratio (Lee, 1987). It has been suggested that beta-carotene and canthaxanthin can be biotransformed in the liver of tilapia into vitamin A1 (Katsuyama and Matsuno, 1988). The efficiency of conversion of various provitamin A compounds has not been studied.
Carotenoids have been shown to have roles other than as a source of provitamin A. The reported actions of carotenoids have been on reproduction, photoresponse and behavior, respiration, health and immunology, and as antioxidants (Goodwin, 1986; Torrissen, 1990; Bendich, 1991; Fujii, 1993; Christiansen et al., 1995b; Torrissen and Christiansen, 1995; Devresse et al., 1997).
Astaxanthin was found to be essential for alevins of Atlantic salmon (Salmo salar) during the first-feeding and juvenile periods, regardless of vitamin A supplementation (Christiansen et al., 1994, 1995a; Christiansen and Torrissen, 1996). Astaxanthin has also been shown to increase the survival of crustaceans (Chien and Jeng, 1992; Nègre-Sadargues et al., 1993; Menasveta et al., 1995). It has been suggested that Penaeus semisulcatus requires astaxanthin, not retinol per se (Dall, 1995).