On the possibility of the application of gut fluorescence method for studies of herbivorous zooplankton feeding

The gut fluorescence method has been widely used to study feeding in herbivorous zooplankton. This method is based on two assumptions: 1) there is no considerable pigment destruction during gut passage; 2) gut evacuation rate of non-feeding animals is the same as that of actively feeding animals. At present, the evidence on both assumptions is controversal, and the question on the possibility of using the gut fluorescence method is still open. The results presented showed that on the average 80% of ingested pigments are destroyed to nonfluorescent products in the gut of marine copepod Calanus marshallae. Gut evacuation rate for these copepods transfered to filtered water was considerably lower than that of feeding copepods. A change of food type (peridinea or diatoms) resulted in the changes of gut evacuation rate, The obtained data impose serious restrictions on the applicability of the gut fluorescence method for quantitative estimation df zooplankton grazing rates.