many species of
fish live in water
at depths where little solar ultraviolet light would penetrate.
Therefore, cutaneous synthesis of cholecalciferol in
fish by the
action of ultraviolet light on 7-dehydrocholesterol would seem to
be unlikely. To explain the high cholecalciferol content of
fish it has
been assumed that it is either produced endogenously by some
undiscovered enzymatic synthesis or else it is accumulated from
the diet in a food chain that originated in organisms containing
7-dehydrocholesterol, which are exposed to ultraviolet light at the
water surface. Attempts to demonstrate enzymatic synthesis
of cholecalciferol in
fish liver from radioactively labelled
7-dehydrocholesterol [5] or from labelled acetate [6] have given
no indication of metabolic synthesis. On the other hand, vitamin D
has been found in zooplankton at the ocean surface [7] which
could be the source of the vitamin D that is eventually found in the
liver lipid of deep sea
fish such as cod.