Escape response of the rotifer Polyarthra: a high-speed cinematographic analysis

Summary

Cinefilms of unconstrained P. vulgaris at 17°C were taken at a low magnification (∼2x) and 120–200 fps to analyze body movements during swimming and escape responses mediated by movements of the 12 lateral, bladelike appendages or paddles. Cinefilms of partially constrained P. vulgaris and P. dolichoptera at 16°C were taken at a higher magnification (∼10x) and 300 fps, using interference contrast optics, to resolve paddle movements during escape responses. When swimming, P. vulgaris moved at a velocity of 0.348±0,025 (S.E.) mm·s^-1 (2.64 body lenghs·s^-1), having a Reynolds number of 0.05. During escape responses, P. vulgaris traveled 1.947±0.124 (S.E.) mm (15 body lengths) during 0.0564±0.0038 (S.E.) s, continuously moving at a velocity of 35.7±1.2 (S.E.) mm·s^-1 (270 body lengths·s^-1) and having a Reynolds number of 5. During these responses, P. vulgaris tumbled sinuously but mostly-88.9±2.3 (S.E.) %-in a constant direction; the angular change in direction from one frame to the next was 28±2 (S.E.) degrees, but the sign of the change in direction frequently alternated. Escape responses are caused by 1–3 cycles of paddle movements. In each cycle, the rigid paddles move up asynchronously until they are all directly overhead, and then they move downwards to their original resting positions, again asynchronously. Polyarthra's body moves along the flight path during all phases of this cycle. A single cycle may take as little as 26 ms, 13 ms for the paddles to elevate and 13 ms for them to descend. The asynchronous upward and downward movements of each of the 12 paddles explain why Polyarthra's body tumbles continuously through its low Reynolds number, viscous environment. Escape responses generally were initiated by contact with another rotifer. In one P. dolichoptera response, the time lag between such contact and the initiation of paddle elevation was about 7 ms. The very short lag time, great velocity, considerable displacement, and unpredictable directionality of Polyarthra's escape response make it a very effective defense against capture by some invertebrate predators.