190
THE BIOLOGY OF EUPHAUSIIDS
other organisms and are then popularly known as red tides, these dense
populations occasionally occurring, for example, off the coast of
Florida. Most species of dinoflagellates, however, are harmless and
some form an important part of the diet of euphausiids. Meganyctiphanes norvegica and ThysanoEssa raschii in Loch Fyne feed on them
especially in the late summer and autumn when their stomachs may be
packed with them. The species most frequently found in their stomachs
belong to the genera Ceratium, Dinophysis, Phalacroma, Prorocentrum,
and Peridinium. Dense populations of tintinnids also occur a t certain
times of the year in certain sea areas and euphausiids feed on these as
they do on the dinoflagellates.
The first link in the marine food chain, or the base of the food
pyramid, consists of unicellular plants. Diatoms are the most abundant and are eaten by Meganyctiphanes norvegica and Thysanoessa
raschii but they are never very numerous in individual stomachs and
are not a t any time predominant in the contents. The most commonly
occurring are species of Paralia and Thalassiosira and more rarely
Coscinodiscus, Rhizosolenia, Navicula, and Biddulphia. Seasonal
variations in the numbers of stomachs containing diatoms usually
correspond with the fluctuations in the concentrations of these
organisms in the plankton. Other marine plants which occurred in a
small percentage of stomachs throughout the whole year are
filamentous algae. The Antarctic Euphausia superba is the only
euphausiid so far reported which appears to feed predominantly on
phytoplankton, at least during the summer months ; all other species,
where details of their diets are available, are like Meganyctiphanes
norvegica and Thysanoessa raschii and do not feed directly on diatoms
to any great degree. Nemoto (1968b) estimated the quantities of
chlorophyll-a and phaeophytin-type pigments in the stomach, intestine
and faecal pellets of nine species of euphausiids. The greatest quantities
of these pigments were present in Euphuusia similis. The chlorophyll-a,
present in the stomachs of surface-living euphausiids, is degraded in the
intestine to phaeo-pigments and these degraded pigments are the ones
that are predominant in the stomachs of deep-sea species such as
Bentheuphausia amblyops. Nemoto suggests that detrital phytoplankton
containing phaeopigments contributes significantly to the diets of the
deep-living species. An earlier but less complete study of the food of
Meganyctiphanes norvegica in Loch Fyne was made by Macdonald
(1927a) who tended to overestimate the importance of diatoms in its diet.
Moore (1931) used a different technique in attempting to investigate
the food of various marine invertebrates. He examined their faecal
pellets, thus ensuring that whatever was identified has passed through
THE BIOLOGY OF EUPHAUSIIDS
other organisms and are then popularly known as red tides, these dense
populations occasionally occurring, for example, off the coast of
Florida. Most species of dinoflagellates, however, are harmless and
some form an important part of the diet of euphausiids. Meganyctiphanes norvegica and ThysanoEssa raschii in Loch Fyne feed on them
especially in the late summer and autumn when their stomachs may be
packed with them. The species most frequently found in their stomachs
belong to the genera Ceratium, Dinophysis, Phalacroma, Prorocentrum,
and Peridinium. Dense populations of tintinnids also occur a t certain
times of the year in certain sea areas and euphausiids feed on these as
they do on the dinoflagellates.
The first link in the marine food chain, or the base of the food
pyramid, consists of unicellular plants. Diatoms are the most abundant and are eaten by Meganyctiphanes norvegica and Thysanoessa
raschii but they are never very numerous in individual stomachs and
are not a t any time predominant in the contents. The most commonly
occurring are species of Paralia and Thalassiosira and more rarely
Coscinodiscus, Rhizosolenia, Navicula, and Biddulphia. Seasonal
variations in the numbers of stomachs containing diatoms usually
correspond with the fluctuations in the concentrations of these
organisms in the plankton. Other marine plants which occurred in a
small percentage of stomachs throughout the whole year are
filamentous algae. The Antarctic Euphausia superba is the only
euphausiid so far reported which appears to feed predominantly on
phytoplankton, at least during the summer months ; all other species,
where details of their diets are available, are like Meganyctiphanes
norvegica and Thysanoessa raschii and do not feed directly on diatoms
to any great degree. Nemoto (1968b) estimated the quantities of
chlorophyll-a and phaeophytin-type pigments in the stomach, intestine
and faecal pellets of nine species of euphausiids. The greatest quantities
of these pigments were present in Euphuusia similis. The chlorophyll-a,
present in the stomachs of surface-living euphausiids, is degraded in the
intestine to phaeo-pigments and these degraded pigments are the ones
that are predominant in the stomachs of deep-sea species such as
Bentheuphausia amblyops. Nemoto suggests that detrital phytoplankton
containing phaeopigments contributes significantly to the diets of the
deep-living species. An earlier but less complete study of the food of
Meganyctiphanes norvegica in Loch Fyne was made by Macdonald
(1927a) who tended to overestimate the importance of diatoms in its diet.
Moore (1931) used a different technique in attempting to investigate
the food of various marine invertebrates. He examined their faecal
pellets, thus ensuring that whatever was identified has passed through