(^) (Drawings from Alvariño 1965.)
Chaetognaths are frequent and abundant predators in the marine zooplankton,
including at present 98 accepted, holoplanktonic species. They are elongate, usually
transparent cylinders divided by transverse septa into head, trunk, and tail sections.
Adult lengths are 2 to 12 cm. Tail and trunk sections bear thin fins. The head is armed
with long, curved, chitinous, opal-tipped fangs that jam captured prey (mostly
copepods) into the jawless, ventral mouth. The chaetognath hunting strategy is to
hang motionless, waiting for actively swimming animals to pass within striking
distance, a few to 20 cm. Alerted by vibrations transmitted through the water and
sensed by hair-like receptors on the body surface, the worm flicks forward, driven by
longitudinal trunk muscles, and impales the prey with its fangs. Predators with this
strategy make swimming very dangerous for prey; the less swimming, the better. The
copepod genus Metridia has not evolved in accord with this wisdom, individuals
keeping constantly on the move. They are, therefore, among the most common prey of
all pelagic “ambush” predators, particularly arrow worms (Sullivan 1980). Other
copepods are also important. Thuesen et al. (1988) showed that at least some
chaetognaths “still” their prey at capture with tetrodotoxin, a powerful neural poison.
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