Biological Oceanography

(ff) #1

response.


(^) Some diatoms encased in hard opal are far too large and stiff to swallow. However,
Jansen (2008) has observed by simple microscopy that Temora can grasp a hatbox-
shaped cell of Coscinodiscus with diameter a third of its length and chew a hole in
one side. This causes the cytoplasm to withdraw to the opposite side, likely due to
release of turgor pressure. So, the copepod rotates the cell, bites another hole, sucks
out the cytoplasm, and drops the frustule.
(^) Reports can be expected soon from video studies of copepods generating feeding
currents (as opposed to ambush feeding) without being tethered. Being held in place
changes the flow patterns. In the meantime, flow around an untethered copepod while
it feeds has been examined by PIV. A thin (1 mm) plane of laser light is projected
through an aquarium containing small particles that reflect the light. Video cameras
record the fields of particles in the plane at frequencies on the order of 60 Hz, and the
field of fluid velocities is determined (by analyses at several levels of sophistication)
from the shifts in position of the same particles in successive frames. Catton et al.
(2007) provide images (Plate 7.1) of the flow field around copepodites of Euchaeta
antarctica cruising in the illuminated plane at 1–2 cm s−1, views looking down on the
dorsal side and at the side. Locomotion at this low speed is driven by a sculling
motion of the antenna. Flow is accelerated only a very short distance in front of the
animal, about half the body length, minimizing predator-alerting disturbance in the
direction of travel. However, some flow carries water in from well to the side and
over the antennules, likely for olfactory evaluation. Much of the water necessarily
pushed from in front of the animal passes over it dorsally. Water is drawn up from
below the body into the area of the feeding limbs, presumably checked for food scent,
then accelerated under the center rear of the body, and dispersed aft and down.
Comparative pictures for tethered specimens are in the paper by Catton et al., indeed
showing that flow in their vicinity is strongly modified, particularly with zones of
acceleration extending much farther in all directions. Plankters sensing predators or
prey in fluid adjacent to their bodies respond to shear (strain rates) on the order of 0.5
s−1 (that is, Δcm s−1 along stream per cm across stream). The gradients in velocity
shown in Plate 7.1 when converted to shear by Catton et al. show 0.5 s−1 extending
over about two body lengths and three body diameters. To move at all, some enhanced
risk of predation has to be accepted; evidently, nothing ventured, nothing gained. The
lesser volume of water accelerated near the free-swimming copepod also implies that
less work is required for cruise feeding than would be calculated (e.g. van Duren et al.
2003) for a tethered animal. The work involved in cruise feeding appears to be a small
component of the animal’s overall energy budget.


Feeding Rates and Factors Affecting Them

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