For scombrids, which swim continuously and 

 rely upon forward motion to ventilate their gills, 

 the existence of a relatively high speed for the 

 division of labor between red and white muscles, 

 has been assumed primarily on the basis of work 

 done by Rayner and Keenan (1967). These inves- 

 tigators concluded that in the skipjack tuna, red 

 muscle alone powered cruise swimming and white 

 muscle only became active at burst velocities. The 

 initial objective of Rayner and Keenan's study was 

 to demonstrate contractile properties of red mus- 

 cle, and to this end they blocked white muscle 

 activity (pentobarbital) and worked exclusively 

 with tranquilized (propriopromazine) or sedated 

 fish. Moreover, their specimens were restrained in 

 a fixed position and artificially ventilated by per- 

 fusion tubes in the mouth. Thus the movements by 

 these skipjack tunas that were identified as "low 

 frequency swimming," were in fact only casually 

 related to the swimming requirements for gill 

 ventilation and hydrostatic equilibrium; both are 

 controlling factors in normal swimming (Magnu- 

 son 1973; Roberts 1975). 



Our results with S. japonicus contrast in that 

 they show both red and white muscles function in 

 low-speed swimming. Also, Dizon and Brill (A. E. 

 Dizon, Southwest Fisheries Center Honolulu 

 Laboratory, National Marine Fisheries Service, 

 NOAA, Honolulu, HI 96812. Pers. commun., Sep- 

 tember 1977) recorded red and white EMG's from 

 yellowfin tuna, Thunnus albacares, and found 

 that white muscle activity begins at swimming 

 velocities of <3 BL/s — a speed only slightly above 

 the minimum for hydrostatic equilibrium and well 

 below maximal burst capabilities (Magnuson 

 1973). These observations indicate that in fast- 

 swimming scombrids, patterned staging of red and 

 white muscle activity may differ in that activity 

 begins in white fibers at very low speeds, and that 

 both red and white muscle remain active through- 

 out a wide range of sustainable speeds as well as at 

 burst velocities. Implicit in this idea is the pre- 

 sence of a high scope for aerobic activity in scom- 

 brid white muscle which has been recently dem- 

 onstrated for the skipjack tuna (Guppy et al. in 

 press). Also required by the hypothesis are 

 specializations in red muscle for high-speed con- 

 traction which is supported by the findings of 

 Johnston and Tota (1974) that high levels of 

 myofibrillar ATPase occur in the red muscle of 

 bluefin tuna, T. thynnus. 



What physiological advantage might be gained 

 by a 1°C thermal excess during fast swimming? 



Assuming a Qjo of 2 then a 10% increase in 

 metabolism would afford about a 2-3% rise in 

 swimming speed, but an insignificant change in 

 overall swimming efficiency (Webb 1971). An in- 

 teresting speculation is that the extensive heat- 

 exchanging vascular network used for en- 

 dothermy in the scombrids may have initially 

 evolved to meet the high oxygen requirements of 

 red and white myotomal muscle. More metabolic 

 heat is produced during aerobic respiration and 

 natural selection may have proceeded toward a 

 vascular design that maximized oxygen delivery, 

 yet augmented muscle function by conserving 

 heat and insulating the swimming musculature 

 from ambient conditions. 



Acknowledgments 



This work was conducted at the Southwest 

 Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service (NMFS), NOAA where 

 John L. Roberts was supported by a NOAA Senior 

 Research Associateship. We thank J. R. Hunter, 

 R. Lasker, and G. D. Sharp for advice and many 

 stimulating discussions. H. T. Hammel of the 

 Scripps Institution of Oceanography advised us on 

 the preparation and use of thermocouples. Q. Bone 

 and J. R. Hunter critically read drafts of this paper 

 and made many suggestions. Useful technical ad- 

 vice and assistance were provided by J. Brown and 

 R. Leong of NMFS. 



Literature Cited 



Baldwin, F. M. 



1923. Comparative rates of oxygen consumption in marine 

 forms. Proc. Iowa Acad. Sci. 30:173-180. 

 BlLINSKl, E. 



1974. Biochemical aspects offish swimming. /nD.C.Ma- 

 lins and J. R. Sargent ( editors), Biochemical and biophysi- 

 cal perspectives in marine biology. Vol. 1, p. 239-288. 

 Academic Press, N.Y. 



BONE, Q. 



1966. On the function of the two types of myotomal muscle 

 fibre in elasmobranch fish. J. Mar. Biol. Assoc. U.K. 

 46:321-349. 



1975. Muscular and energetic aspects of fish swim- 

 ming. In T. Y.-T. Wu, C. J. Brokaw, and C. Brennen 

 (editors), Swimming and flying in nature. Vol. 2, p. 493- 

 528. Plenum Press, N.Y. 



BRAEKKAN, O. R. 



1959. A comparative study of vitamins in the trunk mus- 

 cles of fishes. Fiskeridir. Skr. Ser. Teknol. Unders. 

 3(8): 1-42. 



Carey, F. G., J. M. Teal, J. W. Kanwisher, K. D. Lawson, 

 AND J, S, Beckett, 



1971. Warm-bodied fish. Am. Zool. 11:137-145. 



866 



