Chlorophyll a conversion and gut passage time for the pelagic tunicate Oikopleura vanhoeffeni (Appendicularia)

We determined the chlorophyll (Chl) a conversion efficiency, defined as the conversion of Chl a into pheopigments and non-fluorescent products, for the cold-water appendicularian Oikopleura vanhoeffeni using Ge-68 as conservative tracer. In both laboratory experiments and in freshly collected animals, there was enough undegraded Chl a present in the gut to provide an index of feeding activity for individual animals. In laboratory experiments with diatoms, Chl a conversion efficiency was predictable with an average of 79%. The mean gut passage time (0.8 h) determined using corn starch and diatoms as markers was not influenced by food concentration or animal size over a trunk length range of 1.8-5 mm. When experimentally determined Chl a conversion efficiency and gut passage times were applied to O. vanhoeffeni from Logy Bay (Newfoundland), the estimated clearance rates were in close agreement with previously published values using several different techniques. We therefore suggest that the modified gut pigment technique is a useful tool to assess in situ ingestion rates of O. vanhoeffeni.