6. FOOD AND FEEDING
191
the gut. He found that the faecal pellets of M . norvegica were elongate
and cylindrical, with the ends sometimes rounded, but usually broken
off and frayed; they were about 0.1-0.15 mm broad and up to 4 mrn
long, pellets collected from the sea bottom always being shorter than
those shed in captivity. Crustacean remains occurred frequently in the
pellets and large crustacean spines often projected from either end.
The seasonal and diurnal variations in the diet of M . norvegica and
Thysanoessa raschii must now be considered before going any further
with the discussion of the qualitative aspects of the food of species in
different genera. The seasonal variations in the diets of Meganyctiphanes
norvegica and Thysanoessa raschii have been studied in considerable
detail by the authors and several conclusions from these studies are
applicable to other species of euphausiids in other sea areas.
Seasonal changes in the intensity of feeding of these two species were
examined over periods of two years. The state of fullness of the stomachs
varied in both species, some having very little food, others being full,
and the majority having intermediate amounts. Consequently, we have
only discriminated between empty stomachs, stomachs with some food,
and stomachs that were more than half full. The percentages of animals
with stomachs containing food are shown for the various times of the
year in Fig. 69. Some seasonal variation is evident but interpretation
difficult. Both species seem to feed a t greater intensity in the summer
than in the winter, and feeding is a t its greatest intensity in both
species during the months April to July. There are suggestions of
increased rates of feeding in August when there are often dense populations of young euphausiids and dinoflagellates and again about October
when the autumn plankton increase takes place but the indications are
inconclusive. What appears significant is that the larger Meganyctiphanes norvegica feed more intensively, on average, throughout the
year than do the smaller Thysanoessa raschii.
An examination of the percentage of stomachs containing remains of
crustaceans (Fig. 69) gives an indication of the amount of carnivorous
feeding carried out by these animals. The percentage of Meganyctiphanes norvegica with remains of compound eyes present in their
stomachs is also shown and Fisher and Goldie (1959) made a further
analysis of the occurrence of these compound eyes and concluded that
they were euphausiid eyes and that this cannibalism takes place when
the proportion of euphausiids (late larvae and early adolescents) to other
organisms in the plankton is high, as it is in the late summer and
autumn. No compound eyes were found in the stomachs of Thysanoessa
raschii in the Clyde. General crustacean remains occurred in the
stomachs of both species but, in their occurrence, showed seasonal
7*
191
the gut. He found that the faecal pellets of M . norvegica were elongate
and cylindrical, with the ends sometimes rounded, but usually broken
off and frayed; they were about 0.1-0.15 mm broad and up to 4 mrn
long, pellets collected from the sea bottom always being shorter than
those shed in captivity. Crustacean remains occurred frequently in the
pellets and large crustacean spines often projected from either end.
The seasonal and diurnal variations in the diet of M . norvegica and
Thysanoessa raschii must now be considered before going any further
with the discussion of the qualitative aspects of the food of species in
different genera. The seasonal variations in the diets of Meganyctiphanes
norvegica and Thysanoessa raschii have been studied in considerable
detail by the authors and several conclusions from these studies are
applicable to other species of euphausiids in other sea areas.
Seasonal changes in the intensity of feeding of these two species were
examined over periods of two years. The state of fullness of the stomachs
varied in both species, some having very little food, others being full,
and the majority having intermediate amounts. Consequently, we have
only discriminated between empty stomachs, stomachs with some food,
and stomachs that were more than half full. The percentages of animals
with stomachs containing food are shown for the various times of the
year in Fig. 69. Some seasonal variation is evident but interpretation
difficult. Both species seem to feed a t greater intensity in the summer
than in the winter, and feeding is a t its greatest intensity in both
species during the months April to July. There are suggestions of
increased rates of feeding in August when there are often dense populations of young euphausiids and dinoflagellates and again about October
when the autumn plankton increase takes place but the indications are
inconclusive. What appears significant is that the larger Meganyctiphanes norvegica feed more intensively, on average, throughout the
year than do the smaller Thysanoessa raschii.
An examination of the percentage of stomachs containing remains of
crustaceans (Fig. 69) gives an indication of the amount of carnivorous
feeding carried out by these animals. The percentage of Meganyctiphanes norvegica with remains of compound eyes present in their
stomachs is also shown and Fisher and Goldie (1959) made a further
analysis of the occurrence of these compound eyes and concluded that
they were euphausiid eyes and that this cannibalism takes place when
the proportion of euphausiids (late larvae and early adolescents) to other
organisms in the plankton is high, as it is in the late summer and
autumn. No compound eyes were found in the stomachs of Thysanoessa
raschii in the Clyde. General crustacean remains occurred in the
stomachs of both species but, in their occurrence, showed seasonal
7*