MAGNUSON: ADAPTATIONS OF SCOMBROIDS AND XIPHOIDS 



characters were surveyed from the literature 

 and are presented in Table 8. To determine 

 whether the hydrostatic requirements of swim- 

 ming have had some influence on or interact 

 with the maximum size, the occurrence of 

 gas bladders and the occurrence of long pectoral 

 fins were compared for small, medium, and 

 large scombroid and xiphoid fishes (Table 9). 

 The distinction between short and long fins 

 was arbitrarily set at a pectoral length of 

 20% or more of body length at any point in 

 the life history. 



Larger species apparently require a gas 

 bladder but smaller species do not. All of the 

 14 species ^ 70 kg had gas bladders but only 

 26% of 19 species ^ 6 kg had gas bladders 

 (Table 9). Medium size fish, 7-69 kg, were 

 intermediate. The largest species without a 

 gas bladder was Scomberomorus commerson 

 at 59 kg (Table 8). Apparently the presence 

 of a gas bladder is obligatory for those species 

 greater than 60 kg. The gas bladder overcomes 

 some weight in water, reduces minimum re- 

 quired speeds, and results in a design that 

 could reduce energy expenditures for the larger 

 species. 



The larger fishes also tend to have relatively 

 longer pectoral fins. Of the 15 species ^ 70 kg, 

 67% have long pectorals at some stage in their 

 life history but none of the 25 species < 6 kg 

 ever have long pectorals (Table 9). Medium 

 size fish, 7-69 kg, were intermediate. 



The two adaptations that make slower speeds 

 possible, large pectorals and a gas bladder. 



Table 9. — Occurrence of a gas bladder and the occur- 

 rence of long pectoral fins among scombroid and xiphoid 

 fishes with different maximum masses. (A summary of 

 Table 8.) 



have some tendency to occur together. Of 24 

 species without a gas bladder, 92% (22 species) 

 have short pectoral fins and of 25 species with 

 a gas bladder only 48% (12 species) have short 

 pectoral fins (Table 8). Apparently small species 

 require neither large pectorals nor a gas bladder 

 to have a required swimming speed slow enough 

 to maintain. For these species, in most cases, 

 the advantages of not having a gas bladder 

 or the drag associated with large pectoral fins 

 have overweighed the advantage of a slow 

 required speed in the course of evolutionary 

 history. Apparently large species have achieved 

 speeds slow enough for a consistent overall 

 design by the joint effects of pectorals and 

 the presence of a gas bladder. For these species 

 if the gas bladder were large enough, large 

 pectorals were not necessary; but if the gas 

 bladder were not large enough, selection oc- 

 curred for large pectorals. 



Thus large species have a gas bladder and 

 may or may not have long pectoral fins, while 

 small species may or may not have a gas 

 bladder and in neither case have long pectorals. 

 In addition, large xiphoids with the large gas 

 bladder may be able to use the pectoral for 

 functions other than lift production. For 

 example, the reduction of pelvic fins among 

 xiphoids means that the pectorals must also 

 produce any downward forces often ascribed 

 to the pelvics. 



The joint influences of maximum mass, body 

 density, and lifting areas on minimum possible 

 speeds give some perspective to the problem 

 of large size (Figure 10). Surfaces of minimum 

 swimming speeds were calculated for hypo- 

 thetical 1-, 10-, and 100-kg scombroids with a 

 lift coefficient of 1.0 for the pectoral fins. The 

 keel was assumed to produce no lift. As 

 expected, the smaller the gas bladder, that 

 is, the greater {Df - Dg), the faster the fish 

 must swim to maintain hydrostatic equilibrium. 

 Also, the smaller the pectoral fins [Af^), the 

 faster the fish must swim. 



For small scombroids of 1 kg, changes in 

 the density have little influence on required 

 speed. And once the pectoral fins reach at least 

 50 cm-, further increases in area have little 

 influence on required speed. A 1-kg scombroid 

 would be about 40 cm long. Fish this long 



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