FISHERY BULLETIN: VOL. 70, NO. 3 



In the specimens with largest white areas, the 

 region corresponding to that of confluence in 

 some younger specimens was dark gray thickly 

 peppered with light gray spots (Figure 6). In 

 some adults, the gular area is also flecked with 

 light spots (Figure 4). In all adults the degree 

 of contrast between the flipper band and the 

 gular region is much reduced over that in youn- 

 ger animals. The cape remains obscure (see pho- 

 tograph of specimens in water in Perrin, 1968). 

 The oblique mark adjacent to the genital region 

 is obliterated in most adults, but in some can 

 still be faintly discerned (Figure 6). The thin 

 light gray line delineating the upper margin of 

 the flipper band may in adults be prominent or 

 so faint as to appear absent. 



In adult males, the dorsal fin is canted forward 

 to a greater or lesser degree (Figures 7, 8, and 

 9). Development of this feature is correlated 

 with development of a protuberant keel, com- 

 posed of connective tissue, just posterior to the 

 anus. The function of these features is unknown. 

 However, comparison of the habits of this spin- 

 ner with those of the Hawaiian spinner, in which 

 the features are very weakly developed (see be- 

 low) , provides a basis for a deductive hypothesis. 

 In the eastern Pacific, spinners commonly occur 

 together with spotted porpoise {S. graffmcmi) in 

 mixed schools comprised of several hundred, or 

 even thousands, of individuals of each species. 

 In Hawaiian waters, spinners and spotted por- 

 poise [there referred to 5. attenuata (Gray) ] do 

 not occur in mixed schools (personal communi- 

 cation from K. S. Norris) . In Hawaiian waters, 

 then, most interspecific contacts could be as- 

 sumed to be head-on and made at some distance, 

 while in the mixed schools of the eastern Pa- 

 cific interspecific contacts must be relatively 

 more frequent, made at shorter distances, and 

 for the most part lateral. It can be hypothesized 

 that the features function in species recognition 

 in a sexual context, perhaps, for example, to in- 

 sure that oestrous females make advances to 

 males of the proper species. 



WHITEBELLY SPINNER 



In this animal the ventrum is white. In all 

 young specimens examined, the axillary and gen- 



FiGURE 7.— Adult eastern spinner. 

 Male, 175 cm, from 3°N, 87°W, July 19 

 or 20, 1970. Perrin field no. WFP47; 

 U.S. National Museum no. 396026 

 (complete skeleton). Photographed 

 after frozen for several months and 

 thawed in water. 



Figure 8a and b. — Adult eastern spin- 

 ner. Male from off Acapulco, Mexico. 

 Photographed soon after death. Spe- 

 cimen not saved. 



Figure 9. — Adult eastern spinner. 

 Male, 186 cm, from 12°20'N, 92°53'W, 

 March 10, 1968; San Diego Natural 

 History Museum no. 21427 (complete 

 skeleton and cast). Photographed in 

 frozen state after several months in 

 frozen storage. 



ital white regions were confluent (Figure 11) as 

 in only some young eastern spinners. A gain, not 

 much change occurs in the pattern until late sub- 

 adulthood (Figures 12, 13, and 15) when the 

 margin of the ventrum becomes speckled. The 

 main point of diflference between these animals 

 and the eastern specimens, aside from the larger 

 ventral light areas, is that there is much greater 

 contrast between the dorsal cape and the lateral 

 field. The cape can even be discerned in some 

 of the photographs of frozen specimens (Figure 

 13). In some subadults (Figure 13) an anteri- 

 orly narrowing dark gray mark can be seen in 

 the area between the flipper insertions. Other 

 diff"erences between this and the eastern form 

 are discussed below. As in the eastern spinner, 

 individual variation among adults (Figures 16, 

 17, 18, 19, 22, and 23) is most pronounced in 

 extent of the light ventrum, but the lower end 

 of the range is about equal to the upper end for 

 the eastern spinner. Also again, there is con- 

 siderable variation in intensity of speckling 

 (compare Figures 16 and 22), and the supra- 

 flipper-band stripe may be eff"ectively absent 

 (Figure 19). The flippers are usually dark on 

 both sides, but may be speckled above (Figure 



988 



