However, it does not always work, since predators must evolve matching quickness
and low shear approaches or face extinction in turn. While the jumps are rapidly
initiated and reach high velocity relative to body size, they do not necessarily move
the animal very far from any sizeable predator, only 4 to 6 cm for Acartia spp. of ∼1.3
mm length (Buskey et al. 2002). However, for some larger copepods, sequences of
thoracic leg rowing can last on the order of a second, moving them a half meter or
more from a frightening stimulus. In all cases, rapid movement also accelerates the
surrounding fluid into eddies and wakes that can alert predators possessing motion
sensors to passing meals. Thus, moving may eliminate one danger and create another.
Most of the available results are for adults and late-stage copepodites, but work is in
progress on escape jumps by nauplii. They can move very quickly away from
approaching predators, but not particularly far, <1 cm.
(^) Other zooplankton also have predator avoidance behaviors. Appendicularia burst
out and away from their mucous house-filters when touched. At least solitary salps
accelerate their jet propulsion when touched, blasting out their mucous feeding webs.
Shelled pteropods release their mucous webs when attacked, pull their “wings” into
their shells, and sink rapidly. Chaetognaths, which attack small sources of shear, dart
away from larger ones. Euphausiids stuck to ctenophore tentacles will tail-flip
backward and often escape. Antarctic krill, Euphausia superba, swim evenly spaced
in schools many meters in horizontal dimensions at densities up to more than 25,000
m−3, making en echelon turns and dives. The schools open meters-wide spaces around
the attacking predators and plankton nets. Individuals induced to tail flip
(“lobstering”) can disrupt this organization, something used to advantage by penguins
and other large krill predators (Hamner & Hamner 2000). Some other juvenile and
adult euphausiids (Euphausia, Thysanoessa, Nyctiphanes) have been observed to
school or swarm, sometimes apparently for breeding, but more generally for
avoidance of visual predators. The advantages of schooling are that most of the water
is left empty, requiring more searching by predators, and that the prey gain by
combining their abilities to detect predator approach.
(^) Starvation can be induced in the laboratory. Some species can suspend growth,
reduce metabolism, and wait it out. Others can’t and die quite quickly (Dagg 1977).
Debate regarding how often starvation actually occurs in the field remains unresolved,
but it probably is not very frequent. There are diseases and parasites of zooplankton.
One group of ciliated protozoa, the Apostomatidae (no mouths), have an abundant
“phoront” stage which encysts on the setae of euphausiids and copepods. The smallest
wound anywhere on the host’s body is detected, certainly from the scent of leaking
tissue fluids, and stimulates the phoronts to leave their cysts. They swim to the
wound, enter the body, rapidly consume the tissue by “osmotrophy”, divide repeatedly
and finally exit to seek suitable new hosts. The dead host is just a wisp of exoskeleton.
Gómez-Guttiérez et al. (2006) report that an apostome ciliate parasitoid (animals live