13. EUPHAUSIIDS IN THE MARINE ECONOMY
383
many animals in the sea, especially the pelagic cephalopods, many
species of fish, the baleen whales and, of course, the birds of the
Antarctic. The following fish which have high concentrations of
vitamin A in their livers feed directly on euphausiids and consequently
probably obtain most of Lheir vitamin from them: the cod, Gadus
morhua ; the saithe, Pollachius virens ; the herring, Clupea harengus ;
the capelin, Mallotus villosus ; the torsk, Brosmius brosme Ascanius ;
the hake, Nerluccius merluccius. Several other species of fish feed on
these fish, for instance the halibut Hippoglossus hippoglossus (L.) on
torsk, and so obtain their supplies of vitamin A indirectly from the
euphausiids. The euphausiids provide most of the vitamin A as the
1 l-cis isomer and the cephalopods, fish and whales must presumably be
able to convert this to the all-trans form of vitamin A, the form which
they mainly contain.
A schematic representation of the passage of vitamin A through
marine food chains can therefore be drawn (Fig. 137). Fisher and Kon
(1959) attempted to assess whether the concentrations of vitamin A in
euphausiids could account for the reserves of this vitamin present in the
livers of the baleen whales. The liver of a baleen whale can weigh
1 ton and have a total weight of 1 kg of vitamin A present. Since
euphausiids, for example Meganyctiphanes norvegica, can have concentrations of vitamin A of the order of 5 pglg present, 1 ton of these
animals will contain approximately 5 g of the vitamin-1 ton of
euphausiids being the weight of one good meal for a whale. Not all of
the vitamin in the euphausiids is absorbed by the digestive organs of
the whale but the concentrations which do pass out in the faeces are
much lower than in the original meal of euphausiids. Further, it is the
ll-cis isomer of vitamin A that is provided by the euphausiids and
experiments with vitamin A-deficient rats suggest that there is a 25%
yield of the all-trans isomer when the rats are supplied with the ll-cis
isomer. I n the whale, however, the efficiency of conversion may be
much higher and so it may be that the whales can obtain their vitamin
A reserves from a few hundred good meals; to test this hypothesis,
rather large-scale feeding experiments are required.
Since predatory organisms such as whales and fish cannot absorb
vitamin A from their food with 100% efficiency, the faeces form a
secondary source of vitamin A utilized by organisms such as filterfeeding copepods and benthic invertebrates, the latter in turn being the
food of many demersal fish. How much vitamin A reaches vertebrates
via this route as compared to that reaching them via such vitamin A
producing organisms as the molluscs, is uncertain as yet.
Another contribution which euphausiids may make to the marine
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383
many animals in the sea, especially the pelagic cephalopods, many
species of fish, the baleen whales and, of course, the birds of the
Antarctic. The following fish which have high concentrations of
vitamin A in their livers feed directly on euphausiids and consequently
probably obtain most of Lheir vitamin from them: the cod, Gadus
morhua ; the saithe, Pollachius virens ; the herring, Clupea harengus ;
the capelin, Mallotus villosus ; the torsk, Brosmius brosme Ascanius ;
the hake, Nerluccius merluccius. Several other species of fish feed on
these fish, for instance the halibut Hippoglossus hippoglossus (L.) on
torsk, and so obtain their supplies of vitamin A indirectly from the
euphausiids. The euphausiids provide most of the vitamin A as the
1 l-cis isomer and the cephalopods, fish and whales must presumably be
able to convert this to the all-trans form of vitamin A, the form which
they mainly contain.
A schematic representation of the passage of vitamin A through
marine food chains can therefore be drawn (Fig. 137). Fisher and Kon
(1959) attempted to assess whether the concentrations of vitamin A in
euphausiids could account for the reserves of this vitamin present in the
livers of the baleen whales. The liver of a baleen whale can weigh
1 ton and have a total weight of 1 kg of vitamin A present. Since
euphausiids, for example Meganyctiphanes norvegica, can have concentrations of vitamin A of the order of 5 pglg present, 1 ton of these
animals will contain approximately 5 g of the vitamin-1 ton of
euphausiids being the weight of one good meal for a whale. Not all of
the vitamin in the euphausiids is absorbed by the digestive organs of
the whale but the concentrations which do pass out in the faeces are
much lower than in the original meal of euphausiids. Further, it is the
ll-cis isomer of vitamin A that is provided by the euphausiids and
experiments with vitamin A-deficient rats suggest that there is a 25%
yield of the all-trans isomer when the rats are supplied with the ll-cis
isomer. I n the whale, however, the efficiency of conversion may be
much higher and so it may be that the whales can obtain their vitamin
A reserves from a few hundred good meals; to test this hypothesis,
rather large-scale feeding experiments are required.
Since predatory organisms such as whales and fish cannot absorb
vitamin A from their food with 100% efficiency, the faeces form a
secondary source of vitamin A utilized by organisms such as filterfeeding copepods and benthic invertebrates, the latter in turn being the
food of many demersal fish. How much vitamin A reaches vertebrates
via this route as compared to that reaching them via such vitamin A
producing organisms as the molluscs, is uncertain as yet.
Another contribution which euphausiids may make to the marine
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