132
E. D. 9. CORNER AND ANTHONY 0. DAVIES
Concerning zooplankton, Jargensen (1966) has expressed the view
that the well-developed feeding mechanisms of these animals are consistent with the importance of particulate material as a source of food.
Jarrgcnsen (1966) also makes the important point that the use of either
detritus or dissolved material as a principal food by zooplankton is
inconsistent with the view that grazing is an important factor in
controlling the size of the phytoplankton population in the sea.
D. Laboratory studies on assimilation
Harvey et al. (1935) observed that large numbers of faecal pellets
were produced by zooplankton when feeding on phytoplankton in the
English Channel during spring. Undigested plant cells were detected
in the contents of the faecal pellets, and this led to the view that the
plant food present in a diatom bloom might be poorly digested. Twenty
years were to pass before this problem was examined in the laboratory,
t,he first definitive study being made by Marshall and Orr (1955a)
using the copepod C. jinmarchicus.
In Calanus, digestion takes place in the wide anterior part of the gut.
As the food passes into the narrow posterior end it is gradually compacted into a faecal pellet which is subsequently ejected, enclosed in a
pellicle that is probably chitinous. Marshall and Orr (1955s) cultured
a number of diatoms and phytoflagellates in media containing radioactive phosphorus and fed the [32P]-labelIed plants to the animal.
Feeding took place in the dark and the experimental vessels were
slowly rotated in a vertical plane so that the plant food would not settle
out. After about 21 h, the animals, eggs and faecal pellets were removed
and measured for radioactivity. The sum of the quantities of 3aP in
these three fractions was assumed to give the total 32P ingested ; and
the amount in the faecal pellets, when subtracted from this total, gave
the quantity of 32P digested. This latter quantity, expressed as a percentage of the former, gave the assimilation efficiency-that is, the
percentage of ingested food digested and absorbed.
It should be noted that the experimental technique was such that
any small amount of 32P retained in the gut of the animal would have
been estimated as part of the assimilated fraction and not as potential
faecal material. This may have led to a slight over-estimate of assimilation efficiency. On the other hand, apart from faecal pellets the
animal also excretes soluble products of metabolism. To measure this
soluble excretion Marshall and Orr (1955a) fed Calanus on a rich
culture of [32P)-lrcbelled algae for about one week, then transferred the
animals to fresh sea water and estimated the fall in level of 32P in the
bodies over several weeks. They found that, compared with living
E. D. 9. CORNER AND ANTHONY 0. DAVIES
Concerning zooplankton, Jargensen (1966) has expressed the view
that the well-developed feeding mechanisms of these animals are consistent with the importance of particulate material as a source of food.
Jarrgcnsen (1966) also makes the important point that the use of either
detritus or dissolved material as a principal food by zooplankton is
inconsistent with the view that grazing is an important factor in
controlling the size of the phytoplankton population in the sea.
D. Laboratory studies on assimilation
Harvey et al. (1935) observed that large numbers of faecal pellets
were produced by zooplankton when feeding on phytoplankton in the
English Channel during spring. Undigested plant cells were detected
in the contents of the faecal pellets, and this led to the view that the
plant food present in a diatom bloom might be poorly digested. Twenty
years were to pass before this problem was examined in the laboratory,
t,he first definitive study being made by Marshall and Orr (1955a)
using the copepod C. jinmarchicus.
In Calanus, digestion takes place in the wide anterior part of the gut.
As the food passes into the narrow posterior end it is gradually compacted into a faecal pellet which is subsequently ejected, enclosed in a
pellicle that is probably chitinous. Marshall and Orr (1955s) cultured
a number of diatoms and phytoflagellates in media containing radioactive phosphorus and fed the [32P]-labelIed plants to the animal.
Feeding took place in the dark and the experimental vessels were
slowly rotated in a vertical plane so that the plant food would not settle
out. After about 21 h, the animals, eggs and faecal pellets were removed
and measured for radioactivity. The sum of the quantities of 3aP in
these three fractions was assumed to give the total 32P ingested ; and
the amount in the faecal pellets, when subtracted from this total, gave
the quantity of 32P digested. This latter quantity, expressed as a percentage of the former, gave the assimilation efficiency-that is, the
percentage of ingested food digested and absorbed.
It should be noted that the experimental technique was such that
any small amount of 32P retained in the gut of the animal would have
been estimated as part of the assimilated fraction and not as potential
faecal material. This may have led to a slight over-estimate of assimilation efficiency. On the other hand, apart from faecal pellets the
animal also excretes soluble products of metabolism. To measure this
soluble excretion Marshall and Orr (1955a) fed Calanus on a rich
culture of [32P)-lrcbelled algae for about one week, then transferred the
animals to fresh sea water and estimated the fall in level of 32P in the
bodies over several weeks. They found that, compared with living