Rapid Jumps and Bioluminescence Elicited by Controlled Hydrodynamic Stimuli in a Mesopelagic Copepod, Pleuromamma xiphias.

Abstract

Actively vertically migrating mesopelagic copepods are preyed upon by a wide variety of fishes and invertebrates. Their responses to predatory attacks include vigorous escape jumps and discharge of bioluminescent material. Escape jumps and bioluminescent discharges in the calanoid copepod Pleuromamma xiphias were elicited by quantified hydrodynamic disturbances. Brief weak stimuli (peak water velocity 64 +/- 21 {mu}m s-1) elicited weak (peak force 6.5 dynes) propulsive responses ("jumps") and no bioluminescence. Moderate stimuli (1580 +/- 780 {mu}m s-1) produced strong propulsive responses consisting of long trains of coordinated power strokes by the four pairs of swimming legs ("kicks"). Peak forces averaged 42 dynes. Strong stimuli (5520 +/- 3420 {mu}m s-1) were required to elicit both a jump and a bioluminescent discharge. In several cases, multiple stimuli were needed to evoke bioluminescence, given the limits on stimulus magnitude imposed by the apparatus. Repeated bioluminescent discharges could be evoked, but this responsiveness waned rapidly. Latencies for the jump response (14 +/- 4 ms) were shorter than for the accompanying bioluminescent discharge (49 +/- 26 ms). The higher threshold for eliciting bioluminescent discharge compared to escape jumps suggests that the copepods save this defense mechanism for what is perceived to be a stronger threat.