198
THE BIOLOGY OF EUPHAUSIIDS
in the plankton (Fig. 70). Calanus cristatus Brady was the dominant
copepod in the winter plankton but did not occur in the food. Calanus
tonsus Brady occurred commonly in the plankton and the food but the
dominant copepod in the winter food, Metridia paci$ca Brodsky, was
not a dominant planktonic organism. The species in the summer food
of the euphausiids were Calanus tonsus, a dominant form in the
plankton, and Metridia paci$ca, again a minor species of the plankton.
Ponomareva concludes that the euphausiids catch copepods selectively
and that luminous copepods are more subject to predation than nonluminous.
Several analyses, some based on very few samples of stomach
contents, of the food of many species of euphausiids are available in the
literature (Table VIII). Several of the descriptions of the " food ') of
euphausiids refer to material present in the food basket and since there
is a high probability that this material entered it while the euphausiids
were in the net it cannot, therefore, be considered in any analysis of
their natural food.
Bentheuphausia amblyops, species of Thysanopoda, Meganyctiphanes
norvegica, species of Nyctiphanes, Euphausia, and Thysanoessa are
omnivorous, that is they can feed on bottom material, particulate
matter (by filtering), and living organisms. Their mouthparts are well
adapted for this broad diet. A further examination of the food of
Pseudeuphausia latij'rons would, bearing in mind the similar structure
of its mouthparts, probably show it to have an omnivorous diet.
Species in the remaining three genera, Nematoscelis, Nematobrachion,
and Stylocheiron have mouthparts which are generally less heavily
setose than those of species in other genera. Also, as in some species of
Thysanoessa, they have one pair of the anterior thoracic limbs greatly
elongated and associated with this modification of the limbs is the
division of the eyes into two lobes. These modifications are thought to
be for carnivorous feeding but nobody has yet discovered the actual
function of these long limbs in the living animals because most of these
species are oceanic and several live a t subsurface levels. The distal
segments of the elongated limbs are curiously modified, in some species
to form a " claw ", but on the other hand, the limbs are so long and
thin that they probably lack strength and only observations on living
animals will confirm their function in predatory feeding. Stomach
analyses of these species, however, indicate that they can obtain food
by filter feeding because diatoms, dinoflagellates, radiolarians, and
tintinnids have all been recorded as present among the contents.
Consequently, these species also appear to be omnivorous. There is no
information available on the food of Tessarabrachion oculaturn and this
THE BIOLOGY OF EUPHAUSIIDS
in the plankton (Fig. 70). Calanus cristatus Brady was the dominant
copepod in the winter plankton but did not occur in the food. Calanus
tonsus Brady occurred commonly in the plankton and the food but the
dominant copepod in the winter food, Metridia paci$ca Brodsky, was
not a dominant planktonic organism. The species in the summer food
of the euphausiids were Calanus tonsus, a dominant form in the
plankton, and Metridia paci$ca, again a minor species of the plankton.
Ponomareva concludes that the euphausiids catch copepods selectively
and that luminous copepods are more subject to predation than nonluminous.
Several analyses, some based on very few samples of stomach
contents, of the food of many species of euphausiids are available in the
literature (Table VIII). Several of the descriptions of the " food ') of
euphausiids refer to material present in the food basket and since there
is a high probability that this material entered it while the euphausiids
were in the net it cannot, therefore, be considered in any analysis of
their natural food.
Bentheuphausia amblyops, species of Thysanopoda, Meganyctiphanes
norvegica, species of Nyctiphanes, Euphausia, and Thysanoessa are
omnivorous, that is they can feed on bottom material, particulate
matter (by filtering), and living organisms. Their mouthparts are well
adapted for this broad diet. A further examination of the food of
Pseudeuphausia latij'rons would, bearing in mind the similar structure
of its mouthparts, probably show it to have an omnivorous diet.
Species in the remaining three genera, Nematoscelis, Nematobrachion,
and Stylocheiron have mouthparts which are generally less heavily
setose than those of species in other genera. Also, as in some species of
Thysanoessa, they have one pair of the anterior thoracic limbs greatly
elongated and associated with this modification of the limbs is the
division of the eyes into two lobes. These modifications are thought to
be for carnivorous feeding but nobody has yet discovered the actual
function of these long limbs in the living animals because most of these
species are oceanic and several live a t subsurface levels. The distal
segments of the elongated limbs are curiously modified, in some species
to form a " claw ", but on the other hand, the limbs are so long and
thin that they probably lack strength and only observations on living
animals will confirm their function in predatory feeding. Stomach
analyses of these species, however, indicate that they can obtain food
by filter feeding because diatoms, dinoflagellates, radiolarians, and
tintinnids have all been recorded as present among the contents.
Consequently, these species also appear to be omnivorous. There is no
information available on the food of Tessarabrachion oculaturn and this