was largely upheld by Leonard Compagno in his 1988 phenetic study, and by Gavin Naylor in his 1992 allozyme sequence study. Naylor was able to further resolve the interrelationships of the "ridge-backed" branch of Carcharhinus, finding that the dusky shark, Galapagos shark, oceanic whitetip shark, and blue shark (Prionace glauca) comprise its most derived clade. Distribution and habitat The range of the dusky shark extends worldwide, albeit discontinuously, in tropical and warm-temperate waters. In the western Atlantic Ocean, it is found from Massachusetts and the Georges Bank to southern Brazil, including the Bahamas and Cuba. In the eastern Atlantic Ocean, it has been reported from the western and central Mediterranean Sea, the Canary Islands, Cape Verde, Senegal, Sierra Leone, and possibly elsewhere including Portugal, Spain, Morocco, and Madeira. In the Indian Ocean, it is found off South Africa, Mozambique, and Madagascar, with sporadic records in the Arabian Sea, the Bay of Bengal, and perhaps the Red Sea. In the Pacific Ocean, it occurs off Japan, mainland China and Taiwan, Vietnam, Australia, and New Caledonia in the west, and from southern California to the Gulf of California, around Revillagigedo, and possibly off northern Chile in the east. Records of dusky sharks from the northeastern and eastern central Atlantic, and around tropical islands, may in fact be of Galapagos sharks. Mitochondrial DNA and microsatellite evidence suggest that Indonesian and Australian sharks represent distinct populations. Residing off continental coastlines from the surf zone to the outer continental shelf and adjacent oceanic waters, the dusky shark occupies an intermediate habitat that overlaps with its more specialized relatives, such as the inshore sandbar shark, the pelagic silky shark (C. falciformis) and oceanic whitetip shark, the deepwater bignose shark, and the islandic Galapagos shark and silvertip shark (C. albimarginatus). One tracking study in the northern Gulf of Mexico found that it spends most of its time at depths of , while making occasional forays below ; this species has been known to dive as deep as . It prefers water temperatures of , and avoids areas of low salinity such as estuaries. The dusky shark is nomadic and strongly migratory, undertaking recorded movements of up to ; adults generally move longer distances than juveniles. Sharks along both coasts of North America shift northward with warmer summer temperatures, and retreat back towards the equator in winter. Off South Africa, young males and females over long disperse southward