NO. 3539 HAMMERHEAD SHARKS GILBERT 15 



body length. The galeoid sharks, on the other hand, usually reach a 

 much larger size, and thus proportional changes are not likely to be 

 so pronounced within a comparable size range. 



As indicated previously, a straightening of the fins and fin lobes 

 occurs as the shark approaches maturity. In this process the fins 

 (particularly the first dorsal) become increasingly more erect and 

 pointed, whereas the anal fin tends to become somewhat more falcate. 

 In contrast to these true morphometric changes are tlie unfolding of 

 the fins after bu-th, a phenomenon that might be compared with the 

 situation in a newly emerged butterfly. Curling of the fins is most 

 pronounced in embryos of Sphyrna mokarran, although the reason for 

 this is not clear. 



The dermal denticles undergo a noticeable morphological change 

 with an increase in size and age, showing an increased number of 

 "teeth" on the posterior margin. As indicated by Bigelow and 

 Schroeder (1957, p. 24), however, this does not result from changes in 

 the original denticle, but instead occurs as the result of one or more 

 denticle replacements. 



In Sphyrna lewini there is a progressive darkening of the tips of the 

 pectoral fins with increasing size and age, so that in large specimens 

 these extremities are black. This also may be true to a greater or 

 lesser degree for S. zygaena and S. mokarran, although I have not yet 

 substantitated it. 



Variation among specimens of the same species is discussed under 

 the accounts of the individual species. 



Life history and ecology. — No detailed discussion of hammer- 

 head ecology is presented here, since most of the available information 

 has appeared in works by Springer (1939, pp. 9-41; 1940b, pp. 188- 

 194), Bigelow and Schroeder (1948, pp. 407-449), and Baughman and 

 Springer (1950, pp. 96-152). The following is a summary of the in- 

 formation appearing in their papers. 



The larger species of Sphyrna (S. zygaena, S. mokarran, S. lewini, 

 and, presumably, S. couardi) apparently occur both in inshore and 

 offshore waters and have even been known to move into brackish 

 areas. S. tiburo, in contrast, seems to prefer shallow embayments 

 and rarely ventures far from land. Little is known about the habitat 

 of the remaining three species. 



Hammerheads swim near the surface of the water. It is doubtful if 

 any species descends to very great depths, since there have been no 

 verified captures below a hundred fathoms (Stewart Springer, in litt.). 



Sphyrnids feed largely on fish and squid and also may take such 

 items as shrimp, crabs, stingrays, and sharks, including individuals of 

 their own species. The larger hammerheads are regarded as danger- 



