6. FOOD AND FEED'INCI
189
T. longipes, T . raschii, Stylocheiron abbreviatum, and S. maximum.
Many of the early workers on euphausiids overlooked the occurrence of
detritus in the diet. Thus Holt and Tattersall (1905a)) in their study of
the euphausiids of the Irish continental slope, mentioned copepods,
euphausiids and decapod larvae, fragments of the pteropod mollusc
Spiralis, and other matter, including a fish larva, in the food of Meganyctiphanes norvegica. Bigelow (1926) did not specifically name detritus,
including inorganic particles, among the items he listed in the diet of
M . norvegica from the Gulf of Maine ; these items were algal filaments
and diatom debris, vegetable particles, and small crustaceans, especially
copepods and the cladoceran Evadne. In this area, too, Meganyctiphanes
norvegica was also observed to feed on bits of herring refuse from the
sardine factories on the neighbouring shores.
Other components of the diet of M . norvegica in Loch Fyne were, in
decreasing order of importance, Crustacea, dinoflagellates, diatoms,
algae, fern sporangia, and dipteran egg membranes. The Crustacea were
fragmented before reaching the stomach but it was possible to make a
broad division of them into copepod and eucaridan fragments.
Although M . norvegica undoubtedly consumes the small species of
copepods available in Loch Fyne, these were not identified in the
stomachs but fragments of the larger species, Calanus finmarchicus and
Pareuchaeta norvegica (Boeck) (Euchaeta norvegica) were recognized.
When eucaridan Crustacea are eaten, fragments of them may be identified but a much surer indication of their presence among the stomach
contents is provided by the occurrence of whole, or parts of, compound
eyes. Most of these were eyes of euphausiids, characterized by hexagonal
corneas and contrasting with the square ones of the decapod Crustacea.
The compound eyes occurred most commonly in the stomachs during
the period June to October and during the late winter. Taking into
account the composition of the plankton a t these times it seems likely
that cannibalism occurs when the proportion of euphausiids to other
planktonic organisms is high either because of an abundance of
euphausiids, as in the late summer when the season's larvae are growing
fast, or because of a scarcity of other planktonic organisms, as in the
winter. On the other hand, during the early summer euphausiids are
far out-numbered and exceeded in biomass by the copepods which are
then the most abundant source of animal protein in the plankton around
the euphausiids; consequently, copepods are much more likely to be
caught and eaten a t this time.
A group of Protophyta occurring a t times in enormous numbers in
the sea are the dinoflagellates. A few species are toxic and may multiply
rapidly into such dense populations as to cause the deaths of fish and
189
T. longipes, T . raschii, Stylocheiron abbreviatum, and S. maximum.
Many of the early workers on euphausiids overlooked the occurrence of
detritus in the diet. Thus Holt and Tattersall (1905a)) in their study of
the euphausiids of the Irish continental slope, mentioned copepods,
euphausiids and decapod larvae, fragments of the pteropod mollusc
Spiralis, and other matter, including a fish larva, in the food of Meganyctiphanes norvegica. Bigelow (1926) did not specifically name detritus,
including inorganic particles, among the items he listed in the diet of
M . norvegica from the Gulf of Maine ; these items were algal filaments
and diatom debris, vegetable particles, and small crustaceans, especially
copepods and the cladoceran Evadne. In this area, too, Meganyctiphanes
norvegica was also observed to feed on bits of herring refuse from the
sardine factories on the neighbouring shores.
Other components of the diet of M . norvegica in Loch Fyne were, in
decreasing order of importance, Crustacea, dinoflagellates, diatoms,
algae, fern sporangia, and dipteran egg membranes. The Crustacea were
fragmented before reaching the stomach but it was possible to make a
broad division of them into copepod and eucaridan fragments.
Although M . norvegica undoubtedly consumes the small species of
copepods available in Loch Fyne, these were not identified in the
stomachs but fragments of the larger species, Calanus finmarchicus and
Pareuchaeta norvegica (Boeck) (Euchaeta norvegica) were recognized.
When eucaridan Crustacea are eaten, fragments of them may be identified but a much surer indication of their presence among the stomach
contents is provided by the occurrence of whole, or parts of, compound
eyes. Most of these were eyes of euphausiids, characterized by hexagonal
corneas and contrasting with the square ones of the decapod Crustacea.
The compound eyes occurred most commonly in the stomachs during
the period June to October and during the late winter. Taking into
account the composition of the plankton a t these times it seems likely
that cannibalism occurs when the proportion of euphausiids to other
planktonic organisms is high either because of an abundance of
euphausiids, as in the late summer when the season's larvae are growing
fast, or because of a scarcity of other planktonic organisms, as in the
winter. On the other hand, during the early summer euphausiids are
far out-numbered and exceeded in biomass by the copepods which are
then the most abundant source of animal protein in the plankton around
the euphausiids; consequently, copepods are much more likely to be
caught and eaten a t this time.
A group of Protophyta occurring a t times in enormous numbers in
the sea are the dinoflagellates. A few species are toxic and may multiply
rapidly into such dense populations as to cause the deaths of fish and