188
THE BIOLOGY OF EUPHAUSIIDS
as peristalsis also occurs in the hind-gut. Retro-peristaltic movements
also take place in the mid-gut from the posterior end forwards; acoording to Vonk (1960) this is necessary because digestion occurs in the
hind-part of the mid-gut while absorption occurs in the fore-part.
Of what does the diet of euphausiids consist? Detailed studies of
the food of Meganyctiphanes norvegica and Thysanoessa raschii have
been made by us and considerable, although less detailed, knowledge
of the food of other euphausiids is available. So far, nobody has
observed euphausiids feeding in their normal habitat and the only
way to determine what they eat is to collect net hauls and to fix the
animals in a preservative, usually formalin, so that the contents of
their stomachs can be examined at leisure in the laboratory. The most
recent studies (Fisher and Goldie, 1959 ; Mauchline, 1960, 1966a) were
made in this way. The percentage of animals in a sample with any one
food in their stomachs provides an indication of the importance of, and
possibly the preference for, that type of food. Particulate matter,
whether inorganic matter derived from the bottom or organic debris,
both suspended and deposited, was the most abundant constituent of the
stomach contents of Meganyctiphanes norvegica and Thysanoessa raschii.
The degree of abundance relative to other foods in the stomach varied
seasonally and the amount of particulate matter present among the
stomach contents of Meganyctiphunes norvegica was noticeably less
during the spring increase of plankton when M . norvegica was feeding
most actively on plankton. Macdonald (1927a) and Meschkat (1 936)
were the first to draw attention to the great importance of detritus in
the diet of euphausiids, especially during the winter months. Einarsson
(1945) has even gone so far as to suggest that detritus may be an essential constituent of the diet of M . norvegica. Macdonald (1927a) stressed
the correlation between the amount of particulate matter in suspension
in the water or in the mud on the bottom and the numbers of
M . norvegica present in the area. Hickling (1925) studied the feeding
of euphausiids in the St. George's Channel area, between Wales and
Ireland, and found that Nyctiphanes couchii, Meganyctiphanes norvegica,
and species of Thysanoessa had all been feeding on detritus and often
had their food baskets, mouthparts, and stomachs full of it. The
detritus consisted of plant fragments, crustacean remains, diatoms
(especially Paralia and Coscinodiscus species), and inorganic grit.
Hjort and Ruud (1929) observed that Meganyctiphanes norvegica fed
on detritus and sinking organisms in Norwegian coastal waters.
Ponomareva (1955, 1959b) and Ponomareva et al. (1962) have shown
the importance of detritus in the diets of Thysanopoda tricuspidata,
Pseudeuphausia latifrons, Euphausia paciJica, Thysanoessa inermis,
THE BIOLOGY OF EUPHAUSIIDS
as peristalsis also occurs in the hind-gut. Retro-peristaltic movements
also take place in the mid-gut from the posterior end forwards; acoording to Vonk (1960) this is necessary because digestion occurs in the
hind-part of the mid-gut while absorption occurs in the fore-part.
Of what does the diet of euphausiids consist? Detailed studies of
the food of Meganyctiphanes norvegica and Thysanoessa raschii have
been made by us and considerable, although less detailed, knowledge
of the food of other euphausiids is available. So far, nobody has
observed euphausiids feeding in their normal habitat and the only
way to determine what they eat is to collect net hauls and to fix the
animals in a preservative, usually formalin, so that the contents of
their stomachs can be examined at leisure in the laboratory. The most
recent studies (Fisher and Goldie, 1959 ; Mauchline, 1960, 1966a) were
made in this way. The percentage of animals in a sample with any one
food in their stomachs provides an indication of the importance of, and
possibly the preference for, that type of food. Particulate matter,
whether inorganic matter derived from the bottom or organic debris,
both suspended and deposited, was the most abundant constituent of the
stomach contents of Meganyctiphanes norvegica and Thysanoessa raschii.
The degree of abundance relative to other foods in the stomach varied
seasonally and the amount of particulate matter present among the
stomach contents of Meganyctiphunes norvegica was noticeably less
during the spring increase of plankton when M . norvegica was feeding
most actively on plankton. Macdonald (1927a) and Meschkat (1 936)
were the first to draw attention to the great importance of detritus in
the diet of euphausiids, especially during the winter months. Einarsson
(1945) has even gone so far as to suggest that detritus may be an essential constituent of the diet of M . norvegica. Macdonald (1927a) stressed
the correlation between the amount of particulate matter in suspension
in the water or in the mud on the bottom and the numbers of
M . norvegica present in the area. Hickling (1925) studied the feeding
of euphausiids in the St. George's Channel area, between Wales and
Ireland, and found that Nyctiphanes couchii, Meganyctiphanes norvegica,
and species of Thysanoessa had all been feeding on detritus and often
had their food baskets, mouthparts, and stomachs full of it. The
detritus consisted of plant fragments, crustacean remains, diatoms
(especially Paralia and Coscinodiscus species), and inorganic grit.
Hjort and Ruud (1929) observed that Meganyctiphanes norvegica fed
on detritus and sinking organisms in Norwegian coastal waters.
Ponomareva (1955, 1959b) and Ponomareva et al. (1962) have shown
the importance of detritus in the diets of Thysanopoda tricuspidata,
Pseudeuphausia latifrons, Euphausia paciJica, Thysanoessa inermis,