The dusky shark (Carcharhinus obscurus) has a long streamlined body that is brown or gray above and white below. There is a faint lateral stripe that extends to the pelvic fin. Juveniles have a dark colouration on the tip of the caudal fin and the underside of the pectoral fins that is less pronounced in adults. The first dorsal fin is much larger than the second with a dermal ridge found between the two dorsal fins. The upper lobe of the caudal fin is larger than the lower lobe. The pectoral fins are long and curved on the trailing edge. The dusky shark can grow to 4 meters (13 feet) in length. The upper teeth of the dusky shark are broad and triangular with a serrated edge, but while the front teeth are virtually erect, the others are slightly oblique. The lower teeth differ since they have narrow, erect cusps with more finely serrated edges than the upper teeth.
The dusky shark is a cosmopolitan species that occurs along continental coastlines in tropical and temperate waters. It ranges from Nova Scotia to Cuba (including the northern Gulf of Mexico) and from Nicaragua to southern Brazil in the western Atlantic and from southern California to the Gulf of California in the eastern Pacific. It is also found in the Mediterranean, Indian and western Pacific, including Madagascar and Australia.
It occurs along continental shorelines where it ranges from shallow inshore waters to the outer reaches of the continental shelf and adjacent oceanic waters. Although generally a bottom feeder, it can be found from the surface to a depth of 400 m (1240 ft). Adults of this species tend to avoid areas of low salinity and rarely enter estuaries. The young congregate in very shallow coastal water (nurseries) in estuaries and bays from New Jersey to Cape Hattaras.
This species is known to be highly migratory in the western Atlantic and eastern Pacific, moving north during the summer months and south in the winter. Males and females undertake these seasonal migrations separately.
Along the coast of South Africa, immature dusky sharks are known to undergo sex-segregated migrations with females moving north and males moving south. However, this phenomenon is complicated by the occurrence of temperature-regulated seasonal migration with sharks traveling southward during summer and northward in winter. This, in turn, is further complicated by the tendency for some sharks to move into deeper water during summer months.
In comparison, dusky sharks in western Australia undergo distinct seasonal migrations with both adolescents and adults moving inshore during summer and autumn.
The diet of this shark includes small schooling fish such as sardines and anchovies, as well as larger fish like tuna, mackerel, flatfish and eels. The dusky shark also eats dogfish, catsharks, skates and rays.
Females become sexually mature at about 3.0 meters (10 feet), while males become sexually mature at 2.9 meters (9.5 feet). They are viviparous and females give birth to a litter of 6 to 14 pups. After 16 months gestation the pups are delivered at a size of approximately 95 cm (3 feet).