Abstract
We constructed a general relationship between prey concentration and absorption efficiency (AE; previously termed assimilation efficiency) in the cosmopolitan calanoid copepod Acartia tonsa . The relationships between observed AEs and prey concentrations developed differently on different prey species. While AEs decreased asymptotically with prey concentration on the cryptophyte Rhodomonas baltica , the chlorophyte Dunaliella tertiolecta generated a v‐shaped response, whereas AEs remained constant on the diatom Thalassiosira weissflogii . We used these observations to develop a mechanistic gut model in which gut carbon concentrations depend on ingestion rate and either egestion rate or absorption rate. To investigate which rate was more important for AE, two different model modes were run: absorption was set to vary according to absorptive enzyme kinetics and egestion set to relate linearly to gut carbon concentration, or the contrary, egestion was set to vary according to enzyme kinetics and absorption set to relate linearly to gut concentration. This approach revealed that copepod carbon AE is controlled primarily by egestion rate kinetics and is independent of absorptive enzyme kinetics. Accordingly, we suggest a simplified regression model for the functional response of AE in pelagic copepods based solely on rates of ingestion and egestion.