Developing steelhead trout eggs and sac fry were examined for
wet, dry and lipid weights and for fatty acid composition over the
period from fertilization until the disappearance of the yolk sac, 69
days after fertilization. The yolk sac and embryo or fry were
separated shortly before hatching, and the above determinations
were made on each compartment for succeeding samples.
Losses in the dry and lipid weights of the whole system were
not apparent until the time of hatching. Yolk constituents decreased
uniformly after hatching, while the embryo or fry gain in wet, dry
and lipid weights ceased, 54 days after fertilization.
After 54 days, consumption of lipid by the fry resulted in no
net deposition of lipid in the fry, implying that the amount of lipid
transported out of the yolk after this time was used for production
Fatty acids were analyzed by gas chromatography. The identity
of the major components was confirmed using retention times and
where possible, comparison with standards. Chain length was established
by hydrogenation and the number of double bonds by thin-layer
chromatography on silicic acid-silver nitrate.
Gas chromatograms of the fatty acid ester mixtures showed
a peak due to a polar component which proved to be cholesterol. Being
unstable and with its relatively long retention time,cholesterol
normally would not be expected to interfere in the analysis of fatty
acid esters. This assumption was not valid in this situation.
The embryo and fry were found to have a characteristic fatty
acid composition which, after hatching, differed significantly from
that of the fertilized egg or yolk. A three fold increase in the relative
amount of palmitic acid occurred at the time of hatching.
The embryo preferentially deposited 22:6, 18:0, and 16:0, while
18:1, 16:1, 20:1 and 22:5 were deposited in lesser proportion. This
result implies a requirement in the embryo or fry for the former
group of acids, and in particular for 22:6w3. Since this acid is- not
made de novo in the fish, and belongs to a family of acids found to
be essential to growth, and is retained under starvation and stress,
a requirement for this acid in a phospholipid moiety is suggested.
This requirement is apparent shortly after hatching. The loss of fatty acids from the yolk was found to be much less
selective than the deposition of fatty acids in the fry. Oleic acid
was consistently retained by the yolk. Lipid mixtures transported
out of the yolk were concluded to be of relatively uniform composition,
with no pronounced retention or depletion of any particular class
of lipids in the yolk.
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