ECOLOGY: INDIAN OCEAN SHARKS 551 



while the taxonomy of the squaloid sharks is confused (at least in the present 

 study area) and their distribution can be considered only in general terms. 

 Nevertheless, enough information is at hand to make a first analysis of the 

 distribution of sharks along the east coast of southern Africa. 



Adequate description of the distribution of sharks involves more than 

 mere delineation of geographical ranges, as noted by Stewart Springer (1967). 

 Factors such as size and sex distributions in different depths must be con- 

 sidered as well as the interactions of different species in the same area. Since 

 many sharks enjoy nothing more than a meal of other sharks, the adults of 

 small species are subject not only to direct competition from the young of 

 larger species but also to predation by adults of larger species. Further com- 

 plications may be introduced by species such as Carcharodon carcharias, 

 which may have different dentitions, hence different diets, in juveniles and 

 adults. 



The most notable feature of the distribution of many shark species is 

 segregation by size and sex. Segregation by sex is apparent even among 

 very young sharks as shown by, for example, Carcharhinus obscurus (Bass 

 et al. 1973), Galeus arae (Bullis 1967), Galeorhinus zyopterus (Ripley 1946), 

 and Heterodontus portusjacksoni (McLaughlin and O'Gower 1971). One 

 possible reason for this early segregation by sex is discussed later. Segregation 

 among the larger species is predominantly lateral although some segregation 

 by depth takes place. Among the smaller species segregation is predominantly 

 according to depth, and population differences may occur within relatively 

 short geographic ranges. The larger species usually have wider geographic 

 ranges, with populations at least partly isolated from one another and dif- 

 fering in size, vertebral numbers, litter size and markings plus, probably, 

 many features not easily discerned by human beings. 



It is clear that even a basic analysis of the distribution of sharks in a given 

 area is complex. The danger exists that too much attention to detail may result, 

 to put it proverbially, in not being able to see the wood for the trees. The 

 approach here is to treat a few species in detail by way of introduction, then 

 consider the galeoid sharks as a whole. Special attention is paid to the genus 

 Carcharhinus as an illustration of the way several closely related species can 

 coexist in one geographical area. The distribution of the nongaleoid sharks is 

 mentioned to complete a picture of the distribution of sharks in the sea off 

 the east coast of southern Africa. 



Carcharhinus obscurus 



The dusky shark, Carcharhinus obscurus (Figure 3), is a relatively large shark 

 known from all three major oceans. Born about 80 to 90 cm long, males 

 mature at about 280 cm and grow to about 325 cm, while females mature at 

 about 300 cm and grow to about 360 cm. Armed with serrated cutting teeth 

 in the upper jaws, C. obscurus feeds mostly on bottom-living prey, although 

 it also eats pelagic animals. Elasmobranchs, chiefly demersal forms, are a 

 regular component of its diet. 



