BRILL: STANDARD METABOLIC RATES OF TROPICAL TUNAS 



that follow), pulmonary diffusing capacity, mito- 

 chondrial volume and capillary density in muscles 

 were shown to be limiting factors in achieving high 

 MMR's. From these studies Weibel et al. (1981) pro- 

 posed that, at maximum rates of aerobic metabo- 

 lism, there is no excess capacity at any level in the 

 respiratory chain. In other words, to achieve high 

 MMR's, a complete series of anatomical/physiologi- 

 calAjiochemical adaptations must be present. And, 

 as shown in Table 4, these adaptations are present 

 in tunas. 



Table 4.— Adaptations of tunas for high nnaximum metabolic rates. 



Large gill surface areas 

 Thin secondary lamella in the 



gills 

 High hematocrit, high hemoglobin 



levels (i.e., high blood O2 



carrying capacity) 

 High maximum cardiac output 

 Elevated muscle temperatures 



High muscle myoglobin levels 



High muscle mitochondrial 



density 

 High muscle capillary density 

 High muscle aerobic enzyme 



activity levels 



Muir and Hughes 1969 



Muir and Brown 1971 



Klawe et al. 1963; Jones 



et al. 1986 

 Poupa and Lindstrom 1983 

 Stevens and Neill 1978 

 Stevens 1982 

 George and Stevens 1978 

 Stevens and Carey 1981 

 George and Stevens 1978 

 Hulbert et al. 1979 

 Hulbert et al. 1979 



Guppy et al. 1979 



One of tunas' adaptations for high MMR's are gills 

 with large respiratory surface areas. However, high 

 rates of oxygen uptake are inexorably linked with 

 high osmoregulatory costs, since gills that permit 

 high rates of oxygen uptake must also permit high 

 rates of water and ion movements. This is especially 

 true in marine fishes like tunas where seawater and 

 blood osmolality are approximately 1,000 and 400 

 mosm, respectively (Bourke 1983). Rao (1968), 

 Farmer and Beamish (1969), Nordhe and Leffler 

 (1975), and Furspan et al. (1984) estimated that the 

 cost of osmoregulation can account for 27 to 50% 

 of the SMR. The gills are a main osmoregulatory 

 effector organ (Evans 1979), and Daxboeck et al. 

 (1982) found that gill tissue respiration alone can 

 account for 27% of the SMR in trout. The SMR, 

 therefore, is obviously strongly influenced by osmo- 

 regulatory cost, which in turn is strongly influenced 

 by gill surface area. Ultsch (1973, 1976) came to a 

 similar conclusion after finding that the SMR's of 

 aquatic (i.e., gill breathing) salamanders were con- 

 trolled by respiratory (i.e., gill) surface area. 



Muir and Hughes (1969) measured the total sec- 

 ondary lamellar gill surface (i.e., respiratory) area 

 in skipjack tuna, yellowfin tuna, and bluefin tuna, 

 Thunnus thunnus. They found total secondary 

 lamellar areas for 1 kg tunas to be an order of 



magnitude or more larger than 1 kg bass or roach. 

 Also, they found gill areas were proportional to body 

 weight and the exponent to be 0.85 for the combined 

 data from the three tuna species. This exponent is 

 significantly different from the exponents I found 

 for the effect of body weight on SMR's. It appears 

 that in tunas, the SMR is not strictly determined 

 by secondary lamellar surface area, although high 

 osmoregulatory costs are most likely the main cause 

 of tunas' high SMR's. Also, the difference between 

 the effct of body size on SMR and gill respiratory 

 area implies that larger tunas have greater scope 

 of activity than smaller fishes, as has been shown 

 to occur in other teleosts (Hughes 1984). 



ACKNOWLEDGMENTS 



I wish to thank Christofer H. Boggs, Peter G. 

 Bushnell, Terry Foreman, Carol Hopper, Robert 

 Olson, and E. Don Stevens for reviewing this paper 

 and providing useful criticisms and suggestions; 

 Robert E. Bourke for obtaining aholehole and rain- 

 bow trout; and David Jones for lending the respir- 

 ometer for aholehole. 



LITERATURE CITED 



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