110
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
is incubated for another 12 h. It will then be ready for the feeding experiment.
The bacterioplankton is this case is not separated from the medium, but, using
appropriate controls, the data thus obtained can be safely corrected for the
presence of dissolved labeled substances remaining in the water together with
the labeled bacterioplankton. The advantages and disadvantages of the use of
tritiated thymidine as a label will be discussed beluw.
In feeding experiments with predatory animals, most often small representatives of zooplankton are used as food objects, such as ciliates, rotifers,
veligers, nauplii of aptemia, and small cladocerans (bosmina, chydorus;
Sorokin 1968, 1993). The labeled ciliates are obtained by cultivation on a suspension of labeled bacteria (Paramecium) or on labeled natural bacterioplankton (Strombidium). After several days, the ciliates graze out practically
the whole load of the labeled bacteria, so it is not even necessary to separate
them from the medium even if it were possible. In any case, the density of the
remaining bacteria not grazed by ciliates should be controlled by direct microscopic count (see Sect. 6.1.2.1). The rotifers are labeled by being fed with
labeled algae such as Chlorella or Chlamydomonas, or with labeled bacteria.
Brachionus are usually used for labeling rotifers, but other species can often
be collected in ponds or small lakes as nearly pure culture. For this, a plankton sample collected by passing some 30-50 I of water through a 40-flm mesh
net is exposed in an aquarium illuminated from one side. The rotifers, being
strongly phototaxic, migrate to the illuminated place and are removed by
pipette for transfer into the filtered water, to which a suspension of labeled
algae or bacteria is added. To separate them from the labeled medium, the
same method with phototaxis could be used if they are small and fragile. Brachionus can be separated with the aid of a 20-flm mesh plankton net using a
special device for washing zooplankton without exposing it to the air (Fig.
3.2F). Veligers can also be labeled in the same way. To obtain them in sufficient
amounts for feeding experiments with 14C, parent mollusks can be injected
with serotonin to induce spawning, after which veligers are raised from the
eggs (Sorokin 1992).
To label cladocerans or calanoid copepods, they are fed with labeled algae
or bacteria, and predatory cyclopids with labeled small animals, ciliates, or
rotifers. The nauplii of artemia can be easily labeled by being fed with labeled
algae such as Tetraselmis. Cladocerans and copepods are usually collected
from plankton samples with a pipette (Fig. 3.3B). To collect such plankton
samples without damaging the plankters, the plankton net should be supplied
with a special end beaker having windows on both sides covered with plankton net (Fig. 3.3A).
The time of labeling depends on the purpose of the experiment. If relative values are to be measured, it can be 2-3 days. If it is balance experiments
to estimate absolute values of C and A, the animals should be fed with the
labeled food as long as possible, for at least 1-2 weeks. Then the animals are
separated from the labeled food by washing them with the aid of the device
shown in Fig. 3.2F.
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
is incubated for another 12 h. It will then be ready for the feeding experiment.
The bacterioplankton is this case is not separated from the medium, but, using
appropriate controls, the data thus obtained can be safely corrected for the
presence of dissolved labeled substances remaining in the water together with
the labeled bacterioplankton. The advantages and disadvantages of the use of
tritiated thymidine as a label will be discussed beluw.
In feeding experiments with predatory animals, most often small representatives of zooplankton are used as food objects, such as ciliates, rotifers,
veligers, nauplii of aptemia, and small cladocerans (bosmina, chydorus;
Sorokin 1968, 1993). The labeled ciliates are obtained by cultivation on a suspension of labeled bacteria (Paramecium) or on labeled natural bacterioplankton (Strombidium). After several days, the ciliates graze out practically
the whole load of the labeled bacteria, so it is not even necessary to separate
them from the medium even if it were possible. In any case, the density of the
remaining bacteria not grazed by ciliates should be controlled by direct microscopic count (see Sect. 6.1.2.1). The rotifers are labeled by being fed with
labeled algae such as Chlorella or Chlamydomonas, or with labeled bacteria.
Brachionus are usually used for labeling rotifers, but other species can often
be collected in ponds or small lakes as nearly pure culture. For this, a plankton sample collected by passing some 30-50 I of water through a 40-flm mesh
net is exposed in an aquarium illuminated from one side. The rotifers, being
strongly phototaxic, migrate to the illuminated place and are removed by
pipette for transfer into the filtered water, to which a suspension of labeled
algae or bacteria is added. To separate them from the labeled medium, the
same method with phototaxis could be used if they are small and fragile. Brachionus can be separated with the aid of a 20-flm mesh plankton net using a
special device for washing zooplankton without exposing it to the air (Fig.
3.2F). Veligers can also be labeled in the same way. To obtain them in sufficient
amounts for feeding experiments with 14C, parent mollusks can be injected
with serotonin to induce spawning, after which veligers are raised from the
eggs (Sorokin 1992).
To label cladocerans or calanoid copepods, they are fed with labeled algae
or bacteria, and predatory cyclopids with labeled small animals, ciliates, or
rotifers. The nauplii of artemia can be easily labeled by being fed with labeled
algae such as Tetraselmis. Cladocerans and copepods are usually collected
from plankton samples with a pipette (Fig. 3.3B). To collect such plankton
samples without damaging the plankters, the plankton net should be supplied
with a special end beaker having windows on both sides covered with plankton net (Fig. 3.3A).
The time of labeling depends on the purpose of the experiment. If relative values are to be measured, it can be 2-3 days. If it is balance experiments
to estimate absolute values of C and A, the animals should be fed with the
labeled food as long as possible, for at least 1-2 weeks. Then the animals are
separated from the labeled food by washing them with the aid of the device
shown in Fig. 3.2F.