194
THE BIOLOGY O F EUPHAUSIIDS
is available the euphausiids feed by day and night but an individual
animal probably does not feed 24 hr a day but has resting periods, as
indicated by the percentage of animals in any one haul with empty or
nearly empty stomachs.
The qualitative aspects of the food of Euphausia superba are fairly
well known and are described by Marr (1962). Barkley (1940) showed
that the diatom Fragilariopsis antarctica (Castracane) Hustedt was the
most frequent and abundant food present in the stomachs of Euphausia
superba, comprising about 80% of all food organisms. Hart (quoted
by Marr, 1962) augmented Barkley’s list of food species and the
importance of the small, spineless diatoms in the diet of this animal
became evident. Hustedt (1958) again emphasizes the importance of
li’ragilariopsis antarctica in the diet but increases the length of the list
of other diatoms that are also eaten. Most of the diatoms that are
reported as common in the stomachs of Euphausia superba have
strongly silicified frustules and their frequent occurrence could be
related to this ; less strongly silicified frustules may be digested quickly
and hence give rise to the paucity of many other diatom species in the
stomachs. Indeed, Hart (1934) says that the stomach contents of
E . superba resemble “green porridge ” in areas where the poorly
silicified oceanic species of Chaetoceros predominate. A “ green mush ”
(Mauchline, 1960) and a “ flocculent detritus ” (Macdonald, 1927a)
have been described in the stomachs of Meganyctiphanes norvegica and
the present authors think these terms are synonymous but whether this
material, which is amorphous, originated from digested phytoplankton
or nanoplankton is as yet unknown. The silicoflagellate, Distephanus
regularis (= D. speculum var. regularis Lemmermann) and the chrysophycean, Phaeocystis sp., are recorded, along with twelve or so diatom
species, as the dominant food organisms of Euphausia superba. The
presence of adult copepods in the stomachs has not been recorded but
copepodite stages of copepods have been noted as a food. With the
seasonal changes in diet found in Meganyctiphanes norvegica and
Thysanoessa ra,schii coupled to the fact that the majority of samples of
Euphausia superba analysed for stomach contents were taken in the
Antarctic summer, it is probable that an examination of the autumn
and winter diets of this species might produce a rather different picture
of the food. Diatoms would be scarce and other foods would have to
suffice. It seems unlikely, and this is supported by the report of remains
of copepodites in the stomachs, that this one euphausiid is a herbivore
and strict filter feeder when, as will be shown, almost all other euphausiids investigated appear to be omnivorous. Copepods such as Calanus
Jinmarchicus are herbivorous when phytoplankton is available but can
THE BIOLOGY O F EUPHAUSIIDS
is available the euphausiids feed by day and night but an individual
animal probably does not feed 24 hr a day but has resting periods, as
indicated by the percentage of animals in any one haul with empty or
nearly empty stomachs.
The qualitative aspects of the food of Euphausia superba are fairly
well known and are described by Marr (1962). Barkley (1940) showed
that the diatom Fragilariopsis antarctica (Castracane) Hustedt was the
most frequent and abundant food present in the stomachs of Euphausia
superba, comprising about 80% of all food organisms. Hart (quoted
by Marr, 1962) augmented Barkley’s list of food species and the
importance of the small, spineless diatoms in the diet of this animal
became evident. Hustedt (1958) again emphasizes the importance of
li’ragilariopsis antarctica in the diet but increases the length of the list
of other diatoms that are also eaten. Most of the diatoms that are
reported as common in the stomachs of Euphausia superba have
strongly silicified frustules and their frequent occurrence could be
related to this ; less strongly silicified frustules may be digested quickly
and hence give rise to the paucity of many other diatom species in the
stomachs. Indeed, Hart (1934) says that the stomach contents of
E . superba resemble “green porridge ” in areas where the poorly
silicified oceanic species of Chaetoceros predominate. A “ green mush ”
(Mauchline, 1960) and a “ flocculent detritus ” (Macdonald, 1927a)
have been described in the stomachs of Meganyctiphanes norvegica and
the present authors think these terms are synonymous but whether this
material, which is amorphous, originated from digested phytoplankton
or nanoplankton is as yet unknown. The silicoflagellate, Distephanus
regularis (= D. speculum var. regularis Lemmermann) and the chrysophycean, Phaeocystis sp., are recorded, along with twelve or so diatom
species, as the dominant food organisms of Euphausia superba. The
presence of adult copepods in the stomachs has not been recorded but
copepodite stages of copepods have been noted as a food. With the
seasonal changes in diet found in Meganyctiphanes norvegica and
Thysanoessa ra,schii coupled to the fact that the majority of samples of
Euphausia superba analysed for stomach contents were taken in the
Antarctic summer, it is probable that an examination of the autumn
and winter diets of this species might produce a rather different picture
of the food. Diatoms would be scarce and other foods would have to
suffice. It seems unlikely, and this is supported by the report of remains
of copepodites in the stomachs, that this one euphausiid is a herbivore
and strict filter feeder when, as will be shown, almost all other euphausiids investigated appear to be omnivorous. Copepods such as Calanus
Jinmarchicus are herbivorous when phytoplankton is available but can