Vol. 81: 25-30,1992
MARINE ECOLOGY PROGRESS SERIES
Published March 26
ABSTRACT: Retention efficiencies of 0.6 to 7 µm diameter particles were determined for the pelagic tunicate Oikopleura vanhoeffeni by offering individuals a graded series of latex microspheres and measuring the change in volume proportion directly by microscope counts of the number of beads of each size in suspension vs in the gut. Beads the size of free-living bacteria (0.6 µm) were retained with a mean efficiency of 44 ± 8 % (± 95 % CI), whereas 1, 3 and 7 µm beads were retained with mean efficiencies of 58 ± 8 %, 88 ± 6 % and 91 ± 3 %, respectively. These results support our earlier ultrastructural predictions of a coarse pharyngeal filter mesh, and show that the food concentrating filter does not alone determine the size composition of the ingested ration. Retention of sub-micrometer particles (0.6 and 1 µm beads) was dependent on body size, with mean values of <20 % for specimens longer than 3 mm. These retention spectra are similar to those of salps and ascidians, but show that appendicularians retain sub-micrometer particles more efficiently than do salps. We conclude that O. vanhoeffeni is capable of mediating a shunt of microbial biomass directly from bacteria, other picoplankton and small nanoflagellates to large metazoans, including known fish predators of appendicularians.