ECOLOGY: INDIAN OCEAN SHARKS 581 



category. Although visible evidence of this habit is often seen in the form of 

 mating scars on females and in sexual dimorphism in tooth shape, the only 

 description of this type of behavior is that given by Clark (1975) for a species 

 of Carcharhinus (either C. spallanzani or C. amblyrhynchos) in the Red 

 Sea: "... earlier we had seen 15 sharks of this genus, Carcharhinus, in a 

 courtship frenzy. They were milling around in a loose group. Then a female 

 broke away from the throng and swam upward with a male following and 

 biting her repeatedly, tearing the edges of her fins, slashing the sides of her 

 body, leaving a crescent of tooth marks on her flanks." 



Distinctive Markings— Distinctive markings on the body and fins 

 might be used in conjunction with postural displays to show them off to 

 better advantage or might serve as an efficient means of communication 

 between sharks swimming in a relaxed "normal" manner. The markings 

 found on many sluggish, bottom-dwelling sharks serve to camouflage them 

 rather than to make them more conspicuous, and it is among the pelagic 

 species that distinctive markings are chiefly found. In the deeper parts of the 

 oceans many shark species have complex patterns of photophores that may 

 be of use in maintaining the integrity of schools of the same species (Springer 

 1967). 



Variations among distinctive markings can be seen in the genus Carcharhinus 

 as recorded off the east coast of southern Africa. Thirteen species with 

 tropical distributions are found in inshore waters of this region, and these 

 can be divided into two groups on the basis of the presence or absence of 

 distinctive markings on the adult sharks. 



Species without distinctive markings—Six species occur: C. leucas, 

 C. amboinensis, C. obscurus, C. falciformis, C. altimus, and C. plumbeus. The 

 first five tend to have dusky tips to the fins in juveniles which fade and may 

 be imperceptible in the adults. C. plumbeus is completely plain from birth to 

 adulthood. C. obscurus and C. falciformis are inhabitants of the outer parts 

 of the continental shelf and possibly do not come into regular social contact 

 with other species of their genus. The same can be supposed for C. altimus, 

 which is normally found near the bottom on the continental shelf. 



Species with distinctive markings— Seven species occur here, C. 

 brevipinna, C. limbatus, C spallanzani, C sorrah, C. melanopterus, C. 

 albimarginatus, and C. sealei. The distinctive markings of these species may 

 be present from birth as in C. albimarginatus and C. sorrah or may appear 

 some time after birth as in C brevipinna, the newborn young of which have 

 no markings at all. On the whole, these are inshore sharks commonly found 

 in clear water and visual distinctions might help considerably in specific 

 recognition. C. plumbeus is also commonly found with these species but 

 might be distinctive in its lack of markings. In the Mauritius-Seychelles area 

 the pale color of this shark is responsible for its vernacular name of "requin 

 blanc." C. leucas and C. amboinensis may occur in the same waters as these 

 sharks. Although distinctive markings are probably not the only method by 

 which these various species recognize one another, they are probably helpful 



