(^) A great deal of discussion has been occasioned by the fact that some, or often most, of the pigment is
actually digested to non-fluorescing breakdown products, with components perhaps assimilated.
Some have supposed that this makes the estimates biased. It would matter if dietary pigment input
were evaluated by fecal pigment output. But that is not the measurement. Both egestion and
destruction are included in the rates based on time-series of whole-body measures. There is no need
to multiply the rate by the ratio of chlorophyll (or pigment) ingested to that defecated, thus
supposedly accounting for the chlorophyll “lost” internally in crustaceans. However, such a
correction can be needed, as in the study of appendicularian feeding by Bochdansky and Deibel, a
study considered in the main text.
(^) There are assumptions to test. Principally, food may be retained in the gut longer when the animal is
in filtered water, than while food is available. Thor and Wendt (2010) have shown that assimilation
efficiency is greater with low rations, likely associated with longer gut retention. Thus, the reliability
of the pigment-loss rate depends upon how fast the animal recognizes that food has been withdrawn.
One test (Ellis & Small 1989) showed that the change to longer retention is slow enough not to
matter. Rates of decline were checked by feeding diatoms labeled with radioactive germanium in the
frustule, then switching to unlabelled food just before the loss-rate determination. This allowed a
comparison based on defecation of germanium-labeled frustules. Pigment-loss rates were greater than
label-loss rates, which is not a problem. Rates of label evacuation were the same for fed animals as
for those switched to filtered seawater, which verifies the assumption that withdrawal of food doesn’t
change behavior instantly. This result is not consistent; Penry and Frost (1991) found that the shift to
filtered water accelerated defecation, and others have seen that also. Some caution is required, but not
correction for digestion of pigment.
(^) A similar approach to ingestion of specific organisms, rather than simply all bearers of chlorophyll, is
under development in several laboratories based on quantitative PCR (qPCR) of species-specific
DNA (Durbin et al. 2007; Nejstgaard et al. 2008). Comparisons are made between the copy number
of a gene, say mitochondrial cytochrome oxidase I or SSU rRNA, in a potential prey and the copy
number of the same sequence in a predator’s gut content (the predator can be ground up whole; it will
not itself have the sequence used as a primer for prey DNA). Digestion of DNA is proving to occur
very rapidly, but qPCR is very sensitive, so the method will at least give qualitative indications that a
particular prey species of interest to an investigator (who has primers for a species-specific stretch of
DNA) has been eaten. The method has yet to be moved to studies in the field.
Some clearance rate estimates and functional response curves are available for other
particle-feeding plankton. Bochdansky et al. (1998) determined clearance rates for a
large appendicularian, O. vanhoeffeni, derived from a modified pigment replacement
method. In this animal, the defecation rate is essentially constant, one pellet every
∼13 minutes regardless of either small temperature changes or food level. It was
observed that the digestive tract consistently contained three fecal pellet volumes of
food, and that the pigment in the gut was only 21% of that required to replace it. Thus,
the turnover of gut content occurs each 39 minutes, and replacing the pigment
requires 4.76 (= 1.0/0.21) times that content for each turnover. The clearance rate in
field-captured O. vanhoeffeni (ml h−1) was determined as the volume of water
containing that scaled-up content each 0.65 h (39 min). Rates depend upon body size,
measured as trunk length: 40 ml h−1 at 3 mm, increasing linearly to 175 ml h−1 at 5
mm.
(^) Rates for large specimens are about 10-fold higher than those of copepods.
Bochdansky and Deibel (1999) show successive estimates of cell abundance in bottles
containing one feeding O. vanhoeffeni. Because of the logarithmic scale, the slopes of
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