Relationship between functional response and gut transit time in the calanoid copepod Acartia clausi: role of food quantity and quality

The relationship between ingestion rate and gut transit time of the calanoid copepod Acartia clausi was examined in laboratory experiments with five different diets: (i) living cells of the diatom Thalassiosira weissflogii, (ii) detrital cells of the same diatom, (iii) 50:50 mix of the two previous diets on a protein basis, (iv) dinoflagellate cells of Prorocentrum micans and (v) Prorocentrum minimum. Ingestion followed a Holling type 2 response for diets 1 and 4 and a linear one for diets 2, 3 and 5. Gut transit time varied with food abundance only when the copepods were fed with the living diatom. The gut evacuation rate increased with the concentration of T. weissflogii with values of 0.010, 0.020, 0.032, 0.042 min–1, corresponding to gut transit time of 97, 50, 31 and 24 min, measured at 50, 110, 130 and 275 mg protein L–1, respectively. Copepods fed with dinoflagellates, mixed and pure detrital diets exhibited longer and similar gut transit times ranging from 85 to 166 min, depending on diet. The coupling between ingestion rate and gut transit time measurements is discussed in the context of copepod feeding strategies.