578 ECOLOGY AND BEHAVIOR 



would seem to be an ideal habitat for O. taurus and probably is, except for 

 the lack of suitable nursery grounds within migratory distance. As noted by 

 Bass et al. (1975c), "the Mozambique channel may possibly be crossed by an 

 occasional vagrant O. tarus, but this wide expanse of deep water presents a 

 real obstacle to regular migrations by any species which normally lives on 

 the continental shelf." 



A further point arising from this train of thought is whether sharks stay in 

 a particular habitat because they have to (for some physiological reason) or 

 because they have no particular reason to move out of it. A consideration of 

 the sharks Triaenodon obesus, Negaprion acutidens, and Odontaspis taurus 

 can clarify this rather obscure statement. Both T. obesus and N. acutidens 

 occur in shallow water off Mozambique and Tongaland but are rarely, if 

 ever, found in the turbid and slightly cooler waters to the south of St. Lucia. 

 Adult O. taurus have the same basic distribution, except that pregnant fe- 

 males swim southward to the nursery areas in the southern and eastern Cape. 

 Both T. obesus and N. acutidens complete their life cycles in tropical waters 

 and therefore have no need to move into subtropical regions. If O. taurus 

 had nursery areas in tropical rather than subtropical waters it would also 

 appear to be confined to tropical seas. That certain sharks do not normally 

 inhabit cold, warm, clear, turbid, oxygen-deficient, or brack waters may re- 

 flect a preference rather than a physiological inability to tolerate such condi- 

 tions. 



Carcharhinus leucas is well known for its ability to enter freshwater and 

 uses rivers and lake systems along the east African coast as nursery areas. The 

 young sharks also occur in the sea and are quite capable of living in normal 

 marine salinities. If this species did not use rivers as nursery areas (remember- 

 ing that the adult sharks do not normally enter freshwater in the region) 

 would anyone have suspected that it was capable of osmoregulatng in fresh- 

 water? Carcharhinus amboinensis, a species very similar to C. leucas in 

 morphology, is common on the Natal coast (more so than C. leucas) but has 

 never been recorded in any of the river systems along the east African coast. 

 The logical explanation of these distributions would be that C. leucas and 

 C. amboinensis avoid competition by having different nursery areas, C. leucas 

 in freshwater and C. amboinensis in the sea. 



At Madagascar, however, C. leucas appears to be absent and C. amboinensis 

 is found close inshore and possibly also in freshwater. This shark has been 

 reported from freshwater bays along the New Guinea coast where strong 

 river outflows displace the seawater, but it is not known whether it actually 

 ascends the river systems (L. F. Filewood, personal communication). C. 

 leucas and C. amboinensis may avoid competition by having nursery grounds 

 in different regions, both of them in freshwater. In the southwest Indian 

 Ocean, C. leucas uses river systems along the African coast while C. 

 amboinensis may possibly use those of Madagascar and other islands. Re- 

 search into the habits of these two species in strictly tropical regions should 

 yield some interesting results. One can also speculate as to how many sharks 

 not normally found in freshwater are physiologically capable of adjusting to 

 such an environment. 



