THE EFFECT OF SIZE ON THE KINEMATICS AND PERFORMANCE OF ANGELFISH (PTEROPHYLLUM-EIMEKEI) ESCAPE RESPONSES

Angelfish (Pterophyllum eimekei) employ two types of fast-starts, single bend (SB) and double bend (DB). Higher performance (distance travelled, velocity, acceleration) and smaller angles of turn are found in DB responses than in SB responses. The percentage of DB responses is inversely related to body size. Small fish employ DB responses for all escape angles, whereas larger fish commonly show SB responses. This is due to a decrease in maximum DB escape angles for larger fish. Escape angles for pooled SB and DB responses are size-independent. Distance covered within a given time, maximum velocity, and maximum acceleration for pooled SB and DB responses are size-independent. With the exception of the smallest fish, angelfish show submaximal performance when making large turns. We suggest that angelfish employ a particular type of response depending on their predators' strike angle. Extremely small and large fish use principally DB and SB responses, respectively. The employment of SB and DB responses in small and large fish, respectively, may be restricted to a narrow range of predator strike angles.