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The fast-start swimming of angelfish, Pterophyllum eimekei

University of British Columbia

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).Subsequent kinematics (stage 2) allows for classification of the response 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 result in different total escape angles (measured considering subsequent positions of the center 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 velocity in the direction opposite to the initial one (SB -8.08 rad/s; DB -56.62 rad/s: p<0.001). There are no significant differences in stage 1 kinematics. Stage 1 turning angle is linearly correlated to stage 2 turning angle for DB only (p<0.01; 12=0.60), and to total escape angle for both types of response (p<0.0001; 12=0.80). Stage 1duration 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 between the two response types, mainly due to differences in stage 2 (maximum velocity, SB 0.99m/s; maximum velocity DB, 1.53 m/s: maximum acceleration SB, 34.1 m/s2; maximum acceleration DB, 74.7 m/s2: p<0.0001 in both cases). As a result, significant differences in the performance throughout the whole response (maximum velocity 1.02 m/s and 1.53 m/s for SB and DB respectively; maximum acceleration 63.2 m/s2 and 91.9 m/s2 for SB and B respectively) as well as within a fixed time (0.03 s) are found. Overall, higher distance-time performance associated with smaller angles of turn are found in DB when compared to SB. Comparison with previous studies reveals that angelfish have good fast-start performance despite specializations for low speed swimming in the labriform (pectoral fins) mode. In addition, angelfish turning radii (0.065 ± 0.0063L: Mean ± 2 s.e.) are lower than those previously reported for any fish. The swimming trajectories of angelfish escape responses to mechanical stimulus are determined. Although fish escape trajectories are linearly related to stimulus direction, they vary considerably after the initial turn away from the stimulus. Past studies of escape trajectories in fish and other animals have been analysed by employing linear plots of stimulus angle versus body turning angle. Here, I define escape trajectories as a circular variable, with 0° as stimulus direction. Angelfish escape in non-random trajectories when the stimulus is presented laterally, within an angular zone of approximately 30°-120°(discriminating zone). The circular plot of escape trajectories shows a bimodal pattern that cannot be revealed by linear analysis. Angelfish escape preferentially at 180° and 130° away from the stimulus, maximizing the distance covered from the stimulus and escaping at the limit of their discriminating zone, respectively. P. eimekei correct their responses when turning towards the stimulus, suggesting that escape trajectories are modulated by sensory feedback. Reanalysis of published work on other animals by employing circular histograms of escape trajectories, reveals multimodal patterns which are also not apparent from the linear plots. I suggest that the presence of multiple preferred trajectories may be adaptive in preventing predators from learning any single fixed pattern of response and compensating for it. The effect of size on angelfish escape responses is also investigated. Size has an effect on the type of response performed. Small angelfish 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 sub-maximal performance when making large turns. I 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 either DB or SB responses in small and large fish, respectively, may be restricted to a narrow range of predator strike angles.

Thesis/Dissertation

application/pdf

2008-09-11

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http://hdl.handle.net/2429/1879

Zoology

University of British Columbia

UBCV

DSpace