Summary

The kinematics of turning manoeuvres and the distance-time performance in escape responses of startled angelfish (Pterophyllum eimekei) are investigated employing high-speed cinematography (400 Hz). All escape responses observed are C-type fast-starts, in which the fish assumes a C shape at the end of the initial body contraction (stage 1). Kinematic analysis of the subsequent stage (stage 2) allows the response to be classified into two types: single bend (SB), in which the tail does not recoil completely after the formation of the C, and double bend (DB), in which it does.

The two types of response have different total escape angles (measured from the subsequent positions of the centre of mass, SB 120.0°; DB 73.3°, P<0.005), different stage 2 turning angles (in the same direction as stage 1 for SB, 11.0°; in the direction opposite to stage 1 for DB, −21.9°: P<0.0005) and different maximum angular velocities in the direction opposite to the initial one (SB −8.08 rad s⁻¹; DB −56.62 rad s⁻¹: P<0.001). There are no significant differences in stage 1 kinematics for the two types of escape. Stage 1 turning angle is linearly correlated to stage 2 turning angle for DB only (P<0.01; r²=0.60) and to total escape angle for both types of response (P<0.0001; r²=0.80). Stage 1 duration is linearly correlated to stage 1 turning angle (P<0.0001; r²=0.83) and to total escape angle (P<0.0001; r²=0.72) for both types of escape.

Distance-time performance is also different in the two response types, mainly because of differences in stage 2 (maximum velocity for SB 0.99 ms⁻¹; maximum velocity for DB 1.53 ms⁻¹: maximum acceleration for SB 34.1 ms⁻²; maximum acceleration for DB 74.7 ms⁻²: P<0.0001 in both cases). As a result, there are significant differences in the performance throughout the whole response (maximum velocity 1.02 ms⁻¹ and 1.53 ms⁻¹ for SB and DB fast-starts, respectively; maximum acceleration 63.2 ms⁻² and 91.9 ms⁻² for SB and DB fast-starts, respectively) as well as within a fixed time (0.03 s). Overall, higher distance-time performances associated with smaller angles of turn are found in DB than in SB responses.

Comparison with previous studies reveals that angelfish have a good fast-start performance despite specializations for low-speed swimming. In addition, the angelfish turning radius (0.065±0.0063 L, where L is body length; mean±2 S.E.) is lower than that previously reported for any fish.