6. FOOD AND FEEDING
193
found that Ceratium sp. occurred commonly in the stomachs in the
autumn of 1957 when it was replaced by Dinophysis sp. and Prorocentrum sp. ; these, again, are reflections of the prevailing composition
of the plankton. Species of Peridiniopsis and Noctiluca also occurred
occasionally in the stomachs of Meganyctiphanes norvegica. The same
species of dinoflagellates were present in the stomachs of Thysanoessa
raschii during the years 1962 to 1964 when the diet of this species was
examined in the Clyde.
The authors examined the diet of the different size classes of
Meganyctiphanes norvegica and Thysanoasa raschii (Fisher and Goldie,
1959; Mauchline, 1960, 1966a) and found that crustaceans and chaetognaths were eaten more frequently by larger animals of the two species
than by smaller; the converse was true for the occurrence of dinoflagellates in the stomachs, these being more common in the stomachs of
smaller euphausiids than larger. Mauchline (1960) found that dinoflagellates were present more frequently in the stomachs of Meganyctiphanes norvegica caught during the night than during the day whereas
crustacean remains were more common in the stomachs during the day.
The dinoflagellates were thus being obtained when the animals had
migrated upwards at night while the crustaceans were being fed on in
deeper water during the day. Fisher and Goldie, however, found that
dinoflagellates and crustaceans were eaten during the day and night.
They point out, however, that during the spring when M . norvegica is
feeding very intensively no relationship existed between the size of
M . norvegica, its vertical distribution in the water column, and the
presence of bottom materials in the stomach. It would seem, therefore,
that the food consists of materials available to them in whatever part
of the water column they are in; if close to the bottom, as they are
during the day, then they feed directly on bottom material and small
crustaceans and if they have migrated towards the surface layers at
night then they extract dinoflagellates or other small organisms by
filter feeding or eat copepods or chaetognaths if available. The authors
found no significant diurnal change in the intensity of feeding of
M . norvegica and Thysanoasa raschii but Ponomareva (1954) found
that T . longipes, T . inermis, and Euphausia pacijka fed at a higher
intensity during the evening and night when they were feeding more
actively in the winter and spring in the Sea of Japan. There may,
indeed, be diurnal changes in the intensity of feeding of euphausiids in
general associated with the probability that in many sea areas the
amount of food available in the surface layers of the sea, into which the
euphausiids migrate at night, is much greater than in the deeper layers
at certain times of the year. The present results suggest that if food
193
found that Ceratium sp. occurred commonly in the stomachs in the
autumn of 1957 when it was replaced by Dinophysis sp. and Prorocentrum sp. ; these, again, are reflections of the prevailing composition
of the plankton. Species of Peridiniopsis and Noctiluca also occurred
occasionally in the stomachs of Meganyctiphanes norvegica. The same
species of dinoflagellates were present in the stomachs of Thysanoessa
raschii during the years 1962 to 1964 when the diet of this species was
examined in the Clyde.
The authors examined the diet of the different size classes of
Meganyctiphanes norvegica and Thysanoasa raschii (Fisher and Goldie,
1959; Mauchline, 1960, 1966a) and found that crustaceans and chaetognaths were eaten more frequently by larger animals of the two species
than by smaller; the converse was true for the occurrence of dinoflagellates in the stomachs, these being more common in the stomachs of
smaller euphausiids than larger. Mauchline (1960) found that dinoflagellates were present more frequently in the stomachs of Meganyctiphanes norvegica caught during the night than during the day whereas
crustacean remains were more common in the stomachs during the day.
The dinoflagellates were thus being obtained when the animals had
migrated upwards at night while the crustaceans were being fed on in
deeper water during the day. Fisher and Goldie, however, found that
dinoflagellates and crustaceans were eaten during the day and night.
They point out, however, that during the spring when M . norvegica is
feeding very intensively no relationship existed between the size of
M . norvegica, its vertical distribution in the water column, and the
presence of bottom materials in the stomach. It would seem, therefore,
that the food consists of materials available to them in whatever part
of the water column they are in; if close to the bottom, as they are
during the day, then they feed directly on bottom material and small
crustaceans and if they have migrated towards the surface layers at
night then they extract dinoflagellates or other small organisms by
filter feeding or eat copepods or chaetognaths if available. The authors
found no significant diurnal change in the intensity of feeding of
M . norvegica and Thysanoasa raschii but Ponomareva (1954) found
that T . longipes, T . inermis, and Euphausia pacijka fed at a higher
intensity during the evening and night when they were feeding more
actively in the winter and spring in the Sea of Japan. There may,
indeed, be diurnal changes in the intensity of feeding of euphausiids in
general associated with the probability that in many sea areas the
amount of food available in the surface layers of the sea, into which the
euphausiids migrate at night, is much greater than in the deeper layers
at certain times of the year. The present results suggest that if food