MARINE ECOLOGY -
PROGRESS SERIES
Vol. 35: 243-250,1987
Published February 5
2 Department of Biology, Memorial University of Newfoundland, St. John's, Newfoundland A1C 5S7, Canada
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 filter of
several oikopleurid species has been described, nothing is known
of the ultrastructure of the pharyngeal filter. We studied the
pharyngeal filter of Oikopleura vanhoeffeni using the transmission
electron microscope. Mean pore size (3.26 x 6.35 µm)
and fiber diameter (203 nm) of the pharyngeal filter were surprisingly
large, but mean width-to-length ratio of the pores (0.55) and
porosity of the filter (91%) 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 µm
nominal size will be retained with 50% efficiency, and that particles
of 4.4 to 6.0 µm nominal size will be retained
with 100% efficiency (values from 4 individuals). Particles the
size of suspended bacteria (0.5 µm) will be retained with less
than 10% efficiency. Pressure drop across the pharyngeal filter
was about 0.14 mm H20, which is similar to values for mucous-net
suspension feeders with ciliary pumping mechanisms.