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 newl y designed flow-through device and (3) observation of the behavior of indiv idual appendicularians using video. Experimentally determined clearance rat e decreased approximately 2-fold with a 100-fold increase in food concentra tion in both the static and flow-through experiments. 'Feeding effort' (i.e . the product of time spent feeding and tail beat frequency), decreased gra dually with increasing food concentration and elapsed time, however, the ma gnitude of the behavioral response was smaller than the reduction expected from the experimentally determined clearance rates. Saturation was not reac hed, even in the highest particle concentrations tested approximating the p eak of a spring diatom bloom (500-1300 mu g C 1(-1)). The time interval bet ween defecation of fecal pellets (average: 16 min) was influenced by the pr esence of the phayngeal filter, however, did not change with food concentra tion. Constant gut passage time irrespective of food concentration shows th at the volume of individual fecal pellets can be used as a proxy for ingest ion rate. Our findings are consistent with a hypothesis incorporating the c logging 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.