COMPAGNO and SPRINGER; NEW GENUS OF CARCHARIIINID SHARKS 



but all are far smaller than those present in 

 oviparous scyliorhinids. The condition of nida- 

 mental glands and eggshells indicate that lago 

 omanensis is livebearing, with oviductal egg 

 counts suggesting a litter of 2 to 10 young. The 

 relatively small size of egg yolks implies that a 

 maternal source of nourishment is provided the 

 embryos unless the young are extremely small at 

 birth. 



SIZE 



lago is one of the smaller carcharhinids. In 

 the Carcharhinidae, ScoUodon, the Protozygaeyia 

 group in Rhidoprionodon, and Mustelus have spe- 

 cies nearly or quite as small as /. omanensis, 

 though Eridacnis species are even smaller. One 

 of the latter, E. radcliffei, is apparently the smal- 

 lest carcharhinid and one of the smallest sharks, 

 with males mature at 186 mm and females at 

 216 mm. 



Size disparity between the sexes is a common 

 phenomenon among elasmobranchs, in all known 

 cases with females larger than males. In lago 

 omanensis this disparity is very marked ; our 

 largest male (365 mm) was only 63% as long 

 as the corresponding female (582 mm) and 

 weighed but one-sixth as much. 



FOOD 



Stomachs of two specimens contained remains 

 of unidentified fish, in one a fish head 32 mm 

 long and in the other a 50-mm section of the 

 posterior trunk of a fish estimated to have been 

 more than 200 mm long. 



DISTRIBUTION 



Table 1 shows the distribution of 16 of 17 

 known specimens of lago omanensis, all except 

 the holotype from IIOE Cruise 4B. Only three 

 other shark specimens, all Mustelus sp., were 

 collected during Cruise 4B from 81 trawling 

 stations in the northern Arabian Sea. This total 

 of only 19 shark specimens of two species is much 

 lower than the expected catch for comparable 

 gear in many other areas of continental shelf 

 and slope. 



A possible explanation for the low incidence of 

 sharks in the catches lies in frequent presence 



of ijoorly oxygenated water near the bottom 

 along the coast between the Gulf of Kutch and 

 the Gulf of Oman (See Banse, 1968, for a gen- 

 eral account of the hydrography of part of this 

 area). Sharks of species commonly held in ma- 

 rine aquaria are thought to require a high dis- 

 solved oxygen level for survival although studies 

 to verify this for particular species have not 

 been made. 



Low oxygen concentration in water at the bot- 

 tom, 0.22 to 0.77 ml/liter, is associated with five 

 of the six IIOE stations at which lago omanensis 

 was taken. It appears that this species may be 

 exceptionally tolerant to low oxygen levels, even 

 at the moderately warm (16.24° to 22.39° C, or 

 about 61.3° to 72.4° F) water it apparently in- 

 habits. In the Red Sea, Marshall and Bourne 

 (1964) reported that their unidentified carcharh- 

 inoid (which may be lago omanensis or a close 

 relative) occurred at depths down to 2195 m. 

 As this area and these depths may have oxygen 

 concentrations lower than 1 ml/liter at the end 

 of summer (Richards, 1957), the Marshall and 

 Bourne shark may be able to survive oxygen 

 levels as low as known lago omanensis appar- 

 ently does in the Arabian Sea. 



Gibbs and Hurwitz (1967) regarded the great- 

 er development of gill lamellae in the stomiatoid 

 fish, Chauliodus pammelas compared with that 

 in C. sloani as an adaptation to the low oxygen 

 habitat of C. pammelas. We looked at struc- 

 tures having respiratory functions in lago oman- 

 ensis but found nothing to suggest such an 

 adaptation. /. omanensis, however, has no 

 closely allied species as a basis for comparison. 



ACKNOWLEDGMENTS 



We thank Dr. N. B. Marshall, British Museum 

 (Natural History) for assistance in comparisons 

 of the holotype with specimens of the IIOE ser- 

 ies, Maarten Korringa, Stanford University, for 

 suggesting the generic name, and Mrs. Martha 

 J. Mitchill of Kent Cambridge Scientific Inc., 

 and Stanford University, for permitting one of 

 the writers (Compagno) to use a Cambridge 

 Stereoscan scanning electron microscope in her 

 care. 



625 



