Retention efficiency of sub-micrometer particles by the pharyngeal filter of the pelagic tunicate Oikopleura vanhoeffeni

Abstract

Retention efficiencles of 0.6 to 7 l m diameter particles were determined for the pelagic tunicate Oikopleura vanhoeffenj by offering individuals a graded series of latex microspherrs 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-liv~ng bacteria (0.6 pm) were retained with a mean efficiency of 44 t 8 % ( ? 95 % CI), whereas 1, 3 and 7 Gm beads were retained with mean efficiencies of 58 5 8 "h, 88 + 6 " ; ) and 91 + 3 ' ? , S , 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 pm bedds) 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 d o salps. We conclude that 0 . vanhoeffeni is capable of mediating a shunt of microbial biomass directly from bacteria, other picoplankton and small nanoflagellates to large metazoans, ~ncluding known fish predators of appendicularians.