WEBB and KEYES: SWIMMING KINEMATICS OF SHARKS 



blacktip shark, <t> was of the order of 0.57T over 

 the range of swimming speeds studied. 



In Equation (1), a\ and a2 are constants, and 

 therefore, / or X can be varied at any speed to 

 keep (p>0.5n. However, such changes also affect 

 thrust. For example, if A varies with speed, com- 

 pensatory changes in tail-beat frequency and/or 

 tail-beat amplitude must occur to balance thrust 

 and drag at a given speed. The blacktip shark 

 modulated both. Therefore, the modulation of 

 wavelength, amplitude, and frequency with 

 speed can be explained in terms of mechanical 

 advantages from an interaction between widely 

 spaced median fins. It should also be noted that 

 the early rate of increase of amplitude along the 

 body in sharks, occurring near the first dorsal 

 fin, might increase the strength of the vortex 

 sheet. This could enhance thrust, perhaps more 

 than would occur with patterns of increasing 

 amplitude seen in fusiform teleosts. 



Adaptive flow interactions between median 

 fins as suggested by Lighthill (1970) apply to the 

 established flow patterns of a steadily swimming 

 fish. Therefore, the common body form and kine- 

 matic patterns of sharks appear to be adapta- 

 tions for cruising. Some sharks, analogous in 

 form to tuna (group 1 of Thomson and Simanek 

 1977), are obviously highly specialized for cruis- 

 ing (Lighthill 1975; Lindsey 1978), but the pres- 

 ent observations suggest that other sharks are 

 also specialized through the utilization of other 

 principles, exploiting more anguilliform propul- 

 sion and a more elongate, flexible body. The dis- 

 tribution of the median fins along the body is 

 very similar among sharks (Thomson and Sima- 

 nek 1977). This suggests that such cruising adap- 

 tations are relatively common. Furthermore, 

 sharks are frequently negatively buoyant, when 

 continuous forward motion is important in swim- 

 ming free from the bottom. This argues for the 

 importance of cruising in the routine behavior of 

 sharks, and hence the importance of any mecha- 

 nisms to enhance thrust and efficiency in steady 

 swimming. 



Comparative observations on teleosts also sug- 

 gest that in general, sharks are specialized in 

 cruising. Experimental studies have shown that 

 optimal design for transient swimming patterns 

 (angular and linear acceleration) differs from, 

 and is largely exclusive of, that for steady swim- 

 ming such as cruising (see review by Webb in 

 press). In teleosts, optimal morphological fea- 

 tures for steady swimming include a small area 

 anterior to a deep high aspect ratio tail propel- 



ling a fairly rigid body. For maximum accelera- 

 tion, depth (and hence area) should be large over 

 the whole length of a flexible body. Bony fish can 

 achieve some compromise because of their flex- 

 ible fins, but in general design for unsteady 

 acceleration activity appears most important 

 (Webb 1982). Compromises are excluded for 

 sharks because they do not have collapsible fins. 

 In addition, the separation of the median fins re- 

 duces the area of the body available to generate 

 large forces for acceleration. Some sharks, e.g., 

 the horn shark, Heterodontus francisci, have 

 somewhat enlarged median fins that suggest a 

 greater importance of acceleration. In general, a 

 more posterior location of the first dorsal fin is 

 associated with larger area fins, as in Ginglymo- 

 stoma cirratum that could similarly improve 

 acceleration. In this case the more posterior loca- 

 tion of the first dorsal fin may be at the cost of 

 reducing </> below 0.57T. However, the body and 

 fin form typical of sharks (Thomson and Sima- 

 nek 1977) probably provides for poor accelera- 

 tion performance. 



In conclusion, sharks appear to have exploited 

 their different structural capacities to specialize 

 for cruising when swimming. 



ACKNOWLEDGMENTS 



This work was completed while P. W. Webb 

 was an NRC/NOAA Research Associate on leave 

 from the University of Michigan. I am indebted 

 to R. Lasker and J. R. Hunter for their hospital- 

 ity. The authors thank Sea World for providing 

 specimens and facilities. 



LITERATURE CITED 



Alexander, R. McN. 



1968. Animal mechanics. Univ. Washington Press, 

 Seattle, 346 p. 

 Aleyev, Y. G. 



1977. Nekton. Junk, The Hague, 435 p. 

 Bainbridge, R. 



1958. The speed of swimming of fish as related to size and 

 to the frequency and amplitude of the tail beat. J. Exp. 

 Biol. 35:109-133. 

 1963. Caudal fin and body movement in the propulsion 

 of some fish. J. Exp. Biol. 40:23-56. 

 Breder, C. M., Jr. 



1926. The locomotion of fishes. Zoologica (N.Y.) 4:159- 

 297. 

 Brett, J. R., and J. M. Blackburn. 



1978. Metabolic rate and energy expenditure of the spiny 

 dogfish, Squalus acanthias. J. Fish. Res. Board Can. 

 35:816-821. 



811 



