showed an intriguing aspect of this detection mechanism. They took repeated films of
E. pileatus after various durations of experience with a given phytoplankton food.
Inexperienced feeders responded to cells at a mean distance of 273 μm from the
maxilla, while those that had had the food available for 24 or more hours responded to
them at 345 μm. The experienced copepods also responded to 31% of cells appearing
in the vicinity of the maxilla, as opposed to only 12% for the inexperienced. Price and
Paffenhöfer took this to be evidence that olfaction is indeed the sense by which
phytoplankton are detected. At least it shows that experience tells the animal that
weaker signals of the effective kind are associated with food. Bundy et al. (1998)
have shown that copepods also will move to and capture polystyrene beads that are
not likely to have much odor. They speculate that the presence of a bead distorts the
streamlines of the feeding current as the flow approaches the animal’s boundary layer,
and flow-sensing organs (innervated setae) on the limb surfaces, particularly the
antennule, detect the distortions.
(^) While the mechanism described by Koehl and Strickler is definitely filter feeding, it
is not that of a strainer cleaning plants out of a thin soup to get together a thick
porridge for swallowing. Rather, each food particle is found, then moved closer,
separated from surrounding water, evaluated, and eaten or rejected individually. We
know from large differences in feeding mouthparts that not all filter-feeding copepods
collect particles by this exact mechanism. One variant mechanism, that of Diaptomus
sicilis, a freshwater copepod, is described by Vanderploeg and Paffenhöfer (1985),
again based on films of tethered individuals. In D. sicilis, the limbs generate a flow
from anterior to posterior. Particles coming quite close to the maxillar setae will
invoke a fling and clap response comparable to that of Eucalanus. However, part of
the flow is deflected along the inner surface of the maxillar setules which seem to
funnel it. Some particles appear to be ingested from this directed flow without any
fling and clap, a passive capture mode. However, Vanderploeg and Paffenhöfer insist
that the setal web does not act as a filter.
(^) Video studies at 1600–2200 Hz (Kiørboe et al. 2009) of some small copepods, the
largely neritic, calanoid genus Acartia and the widespread and abundant cyclopoid
genus Oithona, describe feeding without generating a feeding current (ambush
feeding) on motile cells approaching the antennules. When a cell approaches within
∼200 μm, the thoracic legs flick back in a posterior to anterior sequence, similar to
escape swimming (Chapter 8) but less sustained, driving the body forward over the
cell. The lunge of the copepod does not significantly push the food particle away, and
the tail fan may be used to rotate the mouth area next to the particle. This is followed
by opening of the mouth limbs, similarly to the Eucalanus feeding-limb sequence,
drawing the cell close for capture by the maxillae. Time from initiation of the lunge to
final capture varies, several flings of the mouth limbs can be required, but it is short,
typically 3–30 ms in Oithona. This leaves very little time for any prey escape