130
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
ecology and trophology (Ivlev 1961). The use of 14C solves these questions by
simple experiments. As labeled food in these experiments, the optimal kind is
selected from previous assessment of the food spectrum of the animal under
investigation.
As basic parameters of nutrition (e.g., consumption + assimilation) in this
kind of experiments, either the index of assimilation CalC, % or the absolute
rate of food assimilation A, ).1g C Sp.-l h- 1 , can be accepted. The procedure estimate CalC was presented above (see Sect. 3.5.2). The procedure to determine
the assimilation rate A is also very simple, if assimilability I of the food which
is used in the experiment has been previously determined according to the
balance study (see below). In this case, the value of R e, radioactivity of the
ingested food, is measured during the experiment. The values of A can then
be calculated as follows: A = Re Cr I1100).1g C Sp.-l h- 1 , where Re is the radioactivity of ingested food, cpmsp.-l calculated per 1 h of incubation of consumer with the labeled food; Cr is the inverse specific radioactivity of food,
).1g C cpm- 1 ; and I the assimilability of a given kind of food, %.
The experiments are carried out as follows. A series of vessels are prepared containing volumes of water and numbers of animals so that during the
experiment the concentrations of food in the vessels do not decrease to less
than 20% of the initial amount. An appropriate amount of labeled food suspension is added to the vessels, its range of concentration varying from
0.01-0.05 mg/l to 20-25 mg/l wet biomass. During the experiment, the water in
the aquaria must be periodically and carefully mixed by slow bubbling of air
to prevent precipitation of food. The level of food concentration must also be
checked at the beginning and end of the experiment by counting the radioactivity on the filters after filtration of subsamples taken from the experimental
vessels. During the experiment, either the CalC values or the values of A are
determined. The difference will be in the duration of the exposure to the
labeled food. When CalC values are to be estimated, this exposure could be
up to 2 to 4 h, with subsequent transfer of consumers to the vessels with the
unlabeled food (see Sect. 3.5.2.1), while to estimation of A, exposure to the
labeled food must be no longer than the time necessary for its digestion
without subsequent incubation with the unlabeled food. After the experiment
ends, the animals are gently washed of the labeled food and placed directly
into scintillation vials.
The values of CalC (or A) thus estimated are plotted on the ordinate of
the graph against the food concentration values expressed in mg 1-1 of carbon
or wet biomass. The curve thus obtained shows the dependence of the intensity of nutrition of the consumer on food concentration. This optimum concentration will correspond to that point at which the curve levels out (Figs
3.11-3.13). The threshold food concentration can be determined by the curve
at which the values of A are plotted. The respiration in consumers (M) is measured by the oxygen bottle method (or with the radiocarbon one, see below,
Sect. 3.6.4) and is expressed in carbon units per sp. per h (M = ).1gCsp.-lh- 1 ),
as A. The point on the curve where the assimilation rate is equal to M will
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
ecology and trophology (Ivlev 1961). The use of 14C solves these questions by
simple experiments. As labeled food in these experiments, the optimal kind is
selected from previous assessment of the food spectrum of the animal under
investigation.
As basic parameters of nutrition (e.g., consumption + assimilation) in this
kind of experiments, either the index of assimilation CalC, % or the absolute
rate of food assimilation A, ).1g C Sp.-l h- 1 , can be accepted. The procedure estimate CalC was presented above (see Sect. 3.5.2). The procedure to determine
the assimilation rate A is also very simple, if assimilability I of the food which
is used in the experiment has been previously determined according to the
balance study (see below). In this case, the value of R e, radioactivity of the
ingested food, is measured during the experiment. The values of A can then
be calculated as follows: A = Re Cr I1100).1g C Sp.-l h- 1 , where Re is the radioactivity of ingested food, cpmsp.-l calculated per 1 h of incubation of consumer with the labeled food; Cr is the inverse specific radioactivity of food,
).1g C cpm- 1 ; and I the assimilability of a given kind of food, %.
The experiments are carried out as follows. A series of vessels are prepared containing volumes of water and numbers of animals so that during the
experiment the concentrations of food in the vessels do not decrease to less
than 20% of the initial amount. An appropriate amount of labeled food suspension is added to the vessels, its range of concentration varying from
0.01-0.05 mg/l to 20-25 mg/l wet biomass. During the experiment, the water in
the aquaria must be periodically and carefully mixed by slow bubbling of air
to prevent precipitation of food. The level of food concentration must also be
checked at the beginning and end of the experiment by counting the radioactivity on the filters after filtration of subsamples taken from the experimental
vessels. During the experiment, either the CalC values or the values of A are
determined. The difference will be in the duration of the exposure to the
labeled food. When CalC values are to be estimated, this exposure could be
up to 2 to 4 h, with subsequent transfer of consumers to the vessels with the
unlabeled food (see Sect. 3.5.2.1), while to estimation of A, exposure to the
labeled food must be no longer than the time necessary for its digestion
without subsequent incubation with the unlabeled food. After the experiment
ends, the animals are gently washed of the labeled food and placed directly
into scintillation vials.
The values of CalC (or A) thus estimated are plotted on the ordinate of
the graph against the food concentration values expressed in mg 1-1 of carbon
or wet biomass. The curve thus obtained shows the dependence of the intensity of nutrition of the consumer on food concentration. This optimum concentration will correspond to that point at which the curve levels out (Figs
3.11-3.13). The threshold food concentration can be determined by the curve
at which the values of A are plotted. The respiration in consumers (M) is measured by the oxygen bottle method (or with the radiocarbon one, see below,
Sect. 3.6.4) and is expressed in carbon units per sp. per h (M = ).1gCsp.-lh- 1 ),
as A. The point on the curve where the assimilation rate is equal to M will