Force production during pereiopod power strokes in Calanus finmarchicus

Copepods achieve the dramatic speeds of 300–1000 body lengths/s during escape reactions. We investigated the details of this behavior in Calanus finmarchicus. Pereiopod movements during the power strokes were monitored in tethered individuals using high-speed video, while simultaneously measuring force production. At 8–10 °C, the power strokes for each pair of pereiopods registered as separate peaks in the force record, with the largest force being produced by the fourth and third pairs. Peak forces of 500–600 μN were attained within 3.5 ms of initiation of the power stroke sequence. During the power strokes, the pereiopods maximized their surface area by splaying distal segments and setae. During the return stroke, the pereiopods and the setae collapsed, minimizing surface area and thus generating only a weak reverse force. During power stroke sequences in free swimming C. finmarchicus, multiple peaks in acceleration (200 m s−2) corresponded to the power strokes of different pereiopod pairs. During the escape, C. finmarchicus produced maximum swimming speeds of 800 mm s−1 and an estimated muscle-mass specific power output of 300 W kg−1. Comparisons to other organisms indicate that this behavioral performance is particularly powerful and fast. How the copepods achieve this performance remains to be determined.