Functional feeding response and behavioral ecology of Oikopleura vanhoeffeni (Appendicularia, Tunicata)

We examined the functional feeding response of Oikopleura vanhoeffeni with three different techniques: (1) conventional static experiments, (2) a newly designed flow-through device and (3) observation of the behavior of individual appendicularians using video. Experimentally determined clearance rate decreased approximately 2-fold with a 100-fold increase in food concentration in both the static and flow-through experiments. 'Feeding effort' (i.e. the product of time spent feeding and tail beat frequency), decreased gradually with increasing food concentration and elapsed time, however, the magnitude of the behavioral response was smaller than the reduction expected from the experimentally determined clearance rates. Saturation was not reached, even in the highest particle concentrations tested approximating the peak of a spring diatom bloom (500-1300 mu g C 1(-1)). The time interval between defecation of fecal pellets (average: 16 min) was influenced by the presence of the phayngeal filter, however, did not change with food concentration. Constant gut passage time irrespective of food concentration shows that the volume of individual fecal pellets can be used as a proxy for ingestion rate. Our findings are consistent with a hypothesis incorporating the clogging of the filter surface and changes in viscosity of the food particle suspension inside the food concentrating filter to explain the modulation of clearance rates and behavior. (C) 1999 Elsevier Science B.V. All rights reserved.