EFFECTS OF FATTY ACIDS, VITAMIN
A AND IODINE ON METAMORPHOSIS IN ATLANTIC HALIBUT (Hippoglossus
hippoglossus L.).
K. Hamre, M.
Moren, J. Solbakken, K. Pittmann,
M. Berntssen, I. Opstad
Atlantic halibut larvae were fed
either DHA-selco enriched Artemia or copepods from first feeding.
In fish that had been fed Artemia, only 7% had normal pigmentation
and 10% normal eye migration. The numbers for fish fed copepods were 68
and 88%, respectively. Malpigmented fish fed Artemia
were depigmented, while those fed copepods had ambicoloration.
The differences in development were probably nutrient dependent, since all other
conditions were similar for the two groups.
There are several differences in
nutrient composition between the two feed organisms. We will
concentrate on fatty acids, vitamin A and iodine as a precursor for thyroid
hormone. Fatty acids, vitamin A and thyroid hormone have all been shown
to affect development of flatfish larvae and they are ligands to
nuclear receptors that one of these nutrients or an interaction between
them may be the cause of abnormal development of flatfish larvae.
Larvae fed copepods had
dramatically higher body levels of DHA and EPA and lower levels of
arachidonic acid (ARA) than larvae fed Artemia. The
DHA/EPA ratio was similar in the two groups, but the EPA/ARA ratio
was more than 4 times higher in larvae fed copepods than in
larvae fed Artemia.
Larvae fed copepods had higher
body levels of total retinol than larvae fed Artemia, but
the difference was due to higher levels of retinyl esters, whereas the levels
of free retinol and retinal were similar in the two groups. Enriched Artemia
and
copepods do not contain retinoids, and the larval vitamin A source is mainly
astaxanthin from copepods or canthaxantin from Artemia.
Atlantic halibut juveniles convert these carotenoids to retinol at rates
sufficient to cover the vitamin A requirement at the levels of carotenoids
found in Artemia
and copepods.
The level of iodine was 700
times higher in copepods than in Artemia and
3-4 times higher in larvae fed copepods than in larvae fed Artemia.
There was a significantly higher level of T4
in larvae
fed copepods during the " window of opportunity", 25-35 days after
first- feeding In an experiment where Atlantic halibut larvae were fed Artemia
enriched
in iodine up to the levels found in copepods, there was a significant effect
on the body level of iodine and a nonsignificant tendency of higher
levels of thyroid hormone, but no effect on pigmentation or eye migration.
It is concluded that Artemia
probably
offers sufficient access to vitamin A precursors to cover the
larval requirement and does not contain toxic levels or forms of retinoids.
More research should be done to elucidate possible effects of iodine on
development of Atlantic halibut larvae. Fatty acid composition is still the
most likely candidate for causing abnormal development in Atlantic halibut
larvae.
(National Institute of
Nutrition and Seafood Research (NIFES), Bergen, Norway, e-mail: kristin.hamre@nifes.no)