2 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 115 



Proportional differences. — In the following account the fea- 

 tures of the embryo whale shark are compared for the most part 

 with those of the adult (total length 17' 3") from Acapulco, Mexico, 

 figm-ed in Bigelow and Schroeder (1948). As dimensions of both 

 these specimens are already available in their respective accounts, 

 there is no need to reproduce such measiu-ements here. Instead, 

 I shall give, in general terms, the major differences between the 

 embryo and adult, followed in each case by a figm-e in parentheses 

 which is the proportional difference expressed as a percentage of total 

 length. 



The differences are: the adult is slightly broader headed (0.5), longer 

 headed (1.4), and noticeably shorter tailed (6.3); the adidt eye is strik- 

 ingly smaller (0.9), but the gUl-openings are longer (0.3 to 2.2); the first 

 dorsal (2.6), second dorsal (8.4), anal (7.8), and pelvic fins (3.2) 

 are fm-ther rearward in the adult; the first dorsal fin is proportionately 

 higher (1.2), but its base is shorter (0.9) in the adult (however, the 

 reverse is the case for both height and base length — about 2.0 and 0.3 

 respectivel}^ — in the second dorsal and anal fins); the distance between 

 the fu'st and second dorsal fins and between the anal fin and subcaudal 

 origin are longer in the adult (4.8 and 3.4 respectively); the pectoral 

 fin of the adult has a much longer distal margin (5.5) than that of the 

 embryo, though the anterior margins are comparable; the lengths of 

 the upper and lower lobes of the caudal fin are considerably shorter 

 (7.3 and 4.2 respectively) in the adult than in the embryo. 



The above differences are indicative of the growth change which 

 the whale shark undergoes. Understanding such growth change is 

 important in studying sharks because of the frequent need to rely on 

 proportional dimensions to distinguish species. The pattern of growth 

 change is by no means identical in aU sharks, but there do appear to 

 be conmion featm-es (Beebe and Tee- Van, 1941, p. 107; Maschlanka, 

 1955, p. 12; S. Springer, 1960, p. 9; Garrick, 1960, p. 546), such as 

 accelerated growth in the trunk region, as compared to the head and 

 tail, which thus proportionately decrease with increasing total length; 

 a tendency for the pectoral fin to increase its relative length or at 

 least remain stable (though Carcharhinus longimanus (Poey) shows 

 a relative shortening of pectoral fin from juvenile to adult, as noted 

 by S. Springer, 1960, p. 9); a broadening of the head region; and a 

 noticeable decrease in eye diameter. Dorsal fin heights tend to 

 increase relatively in the galeoid sharks (but C. longimanus is again 

 an exception), while in the squaloid sharks the reverse generally 

 holds true (Garrick, 1960, p. 548). 



The indicated growth change of the embryo whale shark fits the 

 above pattern reasonably well except that the head length in the 

 embryo is relatively shorter than that of the adult. This may be 



