the lines (Fig. 7.9a) are proportional to the filtering rates. Food levels were allowed to
drop, falling exponentially in a clearance rate relation (straight line on the semi-log
plot), then replenished. At low food levels, filtering is faster, over 500 ml h−1 for one
tested animal (Fig. 7.9b), than at medium or high levels, at which it cleared at 69 ml h
−1 (Fig. 7.9b). Evidently, appendicularians have a saturable functional response,
saturating for this animal at diatom concentration greater than 10^7 cells ml−1. The time
series for one animal feeding at low food concentrations showed progressive slowing
as the filters of its house clogged and deteriorated (Fig. 7.9a). Upon replacing its
house, a matter of a few minutes, it returned to a high rate. Apparently a house can
deteriorate substantially before replacing it is worth the cost. Actual rates in these
experiments varied greatly between individuals – more than was explained by their
size.
Fig. 7.9 Time course of cell abundance in containers with feeding O. vanhoeffeni.
Clearance rates in ml h−1 are given beneath each downslope. Upward arrows represent
additions of concentrated food particles. (a) Specimen feeding at around 250 ml h−1.
Its feeding rate declined as its house aged, then recovered when it made a new house.
(b) Specimen with high clearance rates at low food concentrations (<1100 cells ml−1),
lower rates at higher concentrations, indicating that ingestion becomes saturated.
(^) (From Bochdansky & Deibel 1999.)