136
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
The estimation of the filtration rate (or the clearing rate), F" has been and
still remains a beloved pursuit in experimental trophology (Rigler 1971a,b;
Nygaard and Hessen 1990). Most of the nutritional research using various
radioisotopic markers CH, 14C, 32p, 33P) was aimed at the estimation of Fr;
however, the uses of this analysis are limited. The value of filtration rate, F"
in fact shows the rate of food consumption (food ration C): C = FrQllg
CSp.-l h-l, if Q is the mean food concentration in the experimental vessel
(biomass), IlgCml-\ vice versa, having estimated (by some means) the food
ration, C, expressed in the same units, it is possible to calculate the filtration
rate: Fr = CIQmlh-1sp.-l. When using the radiocarbon method, Fr = R/rmll- 1
Sp.-l, where Ri is the radioactivity of the ingested food, e.g., the radioactivity
of animals, cpmsp.-l h- 1 ; see Sect. 3.5.2, and r the radioactivity of suspended
labeled food, cpmml- 1 • The application of the latter method for Fr estimation
became even more common with the appearance of radioisotopic methodology (Marshall and Orr 1995; Nauwerck 1959; Burns and Rigler 1967; Kibby
1971; Rigler 1971a,b).
At the same time it is quite obvious that, having the possibility to measure
the food rations (ingestion rate) directly with radioisotopic labels, there is no
reason at all to estimate (or calculate) the filtration rate, F" which itself is a
comparatively meaningless value. It reflects neither the velocity of filtration
flow nor filtration efficiency, because no animal filterer clears the water at once
with 100% efficiency. The determination of filtration rate Fr by measuring the
difference in food concentration makes sense when for some reason direct
estimation of the food rations creates a problems. In this case, calculation via
Fr might present an acceptable alternative. This could be the case especially
when dealing with tiny grazers like small protozoa or mixotrophic planktonic
microalgae, which are difficult to separate from the labeled food (Hollibaugh
et al.1980; Roman and Rublee 1981; Lessard and Swift 1985; Caron et al. 1993).
Another case is the estimation of food rations in very large objects such as
the clams, tunicates, or fish. Exact measurements of radioactivity (Ri) of food
ingested by them during a short-term incubation are difficult. In this case, the
estimation of food ration via measurement of the filtration rate might be a
reasonable solution, especially as, with benthic filterers, this estimation could
be adequate, unlike with planktonic filterers, which deposit their excretions on
the bottom.
3.5.7.2 Technique for Fr Determination
Estimation of filtration rate is based on measuring the decrease in food concentration as indicated by the difference in radioactivity of suspended food in
parallel vessels, one blank without animal grazers (ri) and another experimental, with them (r2). In practice, in Fr estimations, the ro value is often estimated in the experimental vessel just before placing into it the also animals,
which is not right. During the exposure period (usually 1 to 3 h, depending on
Radioisotopic Methods for the Study of Nutrition in Aquatic Animals
The estimation of the filtration rate (or the clearing rate), F" has been and
still remains a beloved pursuit in experimental trophology (Rigler 1971a,b;
Nygaard and Hessen 1990). Most of the nutritional research using various
radioisotopic markers CH, 14C, 32p, 33P) was aimed at the estimation of Fr;
however, the uses of this analysis are limited. The value of filtration rate, F"
in fact shows the rate of food consumption (food ration C): C = FrQllg
CSp.-l h-l, if Q is the mean food concentration in the experimental vessel
(biomass), IlgCml-\ vice versa, having estimated (by some means) the food
ration, C, expressed in the same units, it is possible to calculate the filtration
rate: Fr = CIQmlh-1sp.-l. When using the radiocarbon method, Fr = R/rmll- 1
Sp.-l, where Ri is the radioactivity of the ingested food, e.g., the radioactivity
of animals, cpmsp.-l h- 1 ; see Sect. 3.5.2, and r the radioactivity of suspended
labeled food, cpmml- 1 • The application of the latter method for Fr estimation
became even more common with the appearance of radioisotopic methodology (Marshall and Orr 1995; Nauwerck 1959; Burns and Rigler 1967; Kibby
1971; Rigler 1971a,b).
At the same time it is quite obvious that, having the possibility to measure
the food rations (ingestion rate) directly with radioisotopic labels, there is no
reason at all to estimate (or calculate) the filtration rate, F" which itself is a
comparatively meaningless value. It reflects neither the velocity of filtration
flow nor filtration efficiency, because no animal filterer clears the water at once
with 100% efficiency. The determination of filtration rate Fr by measuring the
difference in food concentration makes sense when for some reason direct
estimation of the food rations creates a problems. In this case, calculation via
Fr might present an acceptable alternative. This could be the case especially
when dealing with tiny grazers like small protozoa or mixotrophic planktonic
microalgae, which are difficult to separate from the labeled food (Hollibaugh
et al.1980; Roman and Rublee 1981; Lessard and Swift 1985; Caron et al. 1993).
Another case is the estimation of food rations in very large objects such as
the clams, tunicates, or fish. Exact measurements of radioactivity (Ri) of food
ingested by them during a short-term incubation are difficult. In this case, the
estimation of food ration via measurement of the filtration rate might be a
reasonable solution, especially as, with benthic filterers, this estimation could
be adequate, unlike with planktonic filterers, which deposit their excretions on
the bottom.
3.5.7.2 Technique for Fr Determination
Estimation of filtration rate is based on measuring the decrease in food concentration as indicated by the difference in radioactivity of suspended food in
parallel vessels, one blank without animal grazers (ri) and another experimental, with them (r2). In practice, in Fr estimations, the ro value is often estimated in the experimental vessel just before placing into it the also animals,
which is not right. During the exposure period (usually 1 to 3 h, depending on