Gut pigment destruction by the copepod Acartia clausi

The extent of chlorophyll (Chl) a degradation into colorless products by the copepod Acartia clausi was determined by measuring gut fluorescence prior to fecal pellet production. The efficiency of pigment degradation of animals fed at different concentrations of Thalassiosira weissflogii varied between 30.56 and 94.05%, and the quantity of pigments lost during gut passage was directly related to total ingestion. The role of feeding history in pigment destruction was tested. The fraction of ingested Chi a degraded by copepods acclimated to a high food concentration (15 ng Chi a ml(-1)) was greater than the fraction degraded by those acclimated to a low food concentration (1.5 ng Chi a ml(-1)). The percentage of ingested Chi a that A. clausi transformed into fluorometrically undetected compounds was not constant. At present, knowledge of the pigment destruction process indicates that Chi a and Chi a-derived pigments are not considered useful quantitative tracers of the feeding activity of copepods. The results of this study suggest a reassessment of the application of the gut fluorescence method to evaluate grazing activity of the copepod A.clallsi.