6. FOOD AND FEEDING
179
The mouthparts of species in the genera Bentheuphausia, Thysanopoda, Meganyctiphanes, Nyctiphanes, Pseudeuphausia, Euphausia,
Tessarabrachion, and Thysanoessa are eminently suitable for filter
feeding, being heavily setose in most species. The mouthparts, however,
of species in the genera Nematoscelis, Nematobrachion, and Xtylocheiron
are generally less setose and therefore not so well adapted to filter
feeding (Figs 61, 62, and Mauchline, 1967b).
We must now consider how these animals use their appendages for
feeding. Cannon and Manton (1927) describe the feeding mechanisms of
the mysid, Hemimysis lamornae (Couch), and there is a close similarity
between its feeding methods and those of the euphausiids. Most of the
methods described in the next few paragraphs refer particularly to the
two species, Meganyctiphanes norvegica and Thysanoessa raschii, but
because of the great similarity in the structure of the mouthparts of
these two species to that in other species of euphausiids the general
aspects of the descriptions are applicable to other species. Further
support for this assumption will become evident when the food present
in the stomachs of the other species is examined.
The feeding currents produced by the pleopods of euphausiids are
complicated by the simultaneous production of a respiratory current.
The gills are present a t the bases of the thoracic limbs and protrude
from the body free of the carapace. The pleopods beat and produce
currents of water which irrigate the ventral posterior regions of the
thorax (Fig. 63) thus providing a continuous change of water a t the
surfaces of the gills. The gills, themselves, are muscular and can be
observed in almost continuous motion. The animal spends considerable
periods of time beating the pleopods but with the thoracic limbs a t rest.
The feeding currents are produced by the exopodites of the thoracic
limbs which are held outwards from the body. Those on the left-hand
side of the body move in a clockwise direction, those on the right-hand
side in an anti-clockwise direction. A current is produced by each
exopodite and runs upwards to the base of the exopodite and passes
into the food groove between the limbs. The tangential currents from
the exopodites also serve to aerate the gills. The mouthparts have
well-developed muscles and move in such a way as to draw the water
along the food groove and the suspended material and organisms are
strained off mainly by the plumose setae on the bases of the first thoracic
limbs and on the maxillae. This food is then passed to the mandibles by
movements of the mouthparts, and so to the mouth. Experiments
using Meganyctiphanes norvegica showed that if the exopodites of the
thoracic limbs are removed the animal can still feed on starch suspensions; this means that the mouthparts alone can produce currents
179
The mouthparts of species in the genera Bentheuphausia, Thysanopoda, Meganyctiphanes, Nyctiphanes, Pseudeuphausia, Euphausia,
Tessarabrachion, and Thysanoessa are eminently suitable for filter
feeding, being heavily setose in most species. The mouthparts, however,
of species in the genera Nematoscelis, Nematobrachion, and Xtylocheiron
are generally less setose and therefore not so well adapted to filter
feeding (Figs 61, 62, and Mauchline, 1967b).
We must now consider how these animals use their appendages for
feeding. Cannon and Manton (1927) describe the feeding mechanisms of
the mysid, Hemimysis lamornae (Couch), and there is a close similarity
between its feeding methods and those of the euphausiids. Most of the
methods described in the next few paragraphs refer particularly to the
two species, Meganyctiphanes norvegica and Thysanoessa raschii, but
because of the great similarity in the structure of the mouthparts of
these two species to that in other species of euphausiids the general
aspects of the descriptions are applicable to other species. Further
support for this assumption will become evident when the food present
in the stomachs of the other species is examined.
The feeding currents produced by the pleopods of euphausiids are
complicated by the simultaneous production of a respiratory current.
The gills are present a t the bases of the thoracic limbs and protrude
from the body free of the carapace. The pleopods beat and produce
currents of water which irrigate the ventral posterior regions of the
thorax (Fig. 63) thus providing a continuous change of water a t the
surfaces of the gills. The gills, themselves, are muscular and can be
observed in almost continuous motion. The animal spends considerable
periods of time beating the pleopods but with the thoracic limbs a t rest.
The feeding currents are produced by the exopodites of the thoracic
limbs which are held outwards from the body. Those on the left-hand
side of the body move in a clockwise direction, those on the right-hand
side in an anti-clockwise direction. A current is produced by each
exopodite and runs upwards to the base of the exopodite and passes
into the food groove between the limbs. The tangential currents from
the exopodites also serve to aerate the gills. The mouthparts have
well-developed muscles and move in such a way as to draw the water
along the food groove and the suspended material and organisms are
strained off mainly by the plumose setae on the bases of the first thoracic
limbs and on the maxillae. This food is then passed to the mandibles by
movements of the mouthparts, and so to the mouth. Experiments
using Meganyctiphanes norvegica showed that if the exopodites of the
thoracic limbs are removed the animal can still feed on starch suspensions; this means that the mouthparts alone can produce currents