Ultrastructure of the pharyngeal filter of the appendicularian Oikopleura vanhoeffeni: Implications for particle size selection and fluid mechanics

Abstract

It has now been established that, contrary to several recent reports, it is the pharyngeal filter of oikopleurids that removes food particles from suspension rather than the food-concentrating filter. Although the ultrastructure of the food-concentrating fllter of several oikopleurid species has been described, nothing is known of the ultrastructure of the pharyngeal filter. We studled the pharyngeal filter of Oikopleura vanhoeffeni using the transmission electron microscope. Mean pore size (3.26 X 6.35 pm) and fiber diameter (203nm) of the pharyngeal filter were surprisingly large, but mean width-to-length ratio of the pores (0.55)and porosity of the filter (91O/O) were similar to values for pharyngeal filters of ascidian and thaliacean tunicates. There was no evidence of fine microfibers between the thick fibers Using the cumulative frequency distribution of pore width of the pharyngeal filter, we predict that particles of 2.4 to 5.8 um nominal size will be retained with 50 % efficiency, and that particles of 4.4 to 6.0 pm nominal size will be retained with 100 % efficiency (values from 4 individuals). Particles the size of suspended bacteria (0.5pm) will be retained with less than 10 % efficiency. Pressure drop across the pharyngeal filter was about 0.14 mm HzO, which is similar to values for mucous-net suspension feeders with ciliary pumping mechanisms.