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Characteristics, distribution and ecology
Taxonomische classification
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Author: (LeSueur, 1818)

Field Marks:
A large grey shark with a fairly short, broadly rounded snout, low anterior nasal flaps, fairly large eyes, broad, triangular, rather low, erect and semioblique-cusped serrated anterolateral teeth without cusplets in upper jaw, lower teeth erect and narrowcusped, usually 14/14 rows of anteroposterior teeth, a low interdorsal ridge, large falcate pectoral fins, a moderatesized first dorsal with a short rear tip and origin about opposite free rear tips of pectoral fins, a small, low second dorsal, and no conspicuous markings on fins.

Diagnostic Features:
A fairly slender species (up to about 3.7 m). Snout short to moderately long and broadly rounded; internarial width 1 to 1.4 times in preoral length; eyes circular and moderately large, their length 1 to 2.1% of total length; anterior nasal flaps low and poorly developed; upper labial furrows shortand inconspicuous; hyomandibular line of pores just behind mouth corners not conspicuously enlarged; gill slits moderately long, third 2.7 to 4% of total length and less than a third of first dorsal base; usually 14/14 rows of anteroposterior teeth in each jaw half but varying from 14 to 15/13 to 15; upper teeth with broad, triangular, strongly serrated, rather low erect to slightly oblique cusps, that smoothly merge into crown feet which have slightly coarser serrations but no cusplets; lower teeth with erect, moderately broad, serrated cusps and transverse or sometimes arched roots. A low interdorsal ridge present. First dorsal fin moderate-sized and semifalcate, with a pointed or narrowly rounded apex and posterior margin curving ventrally from fin apex; origin of first dorsal fin usually over or slightly anterior to the pectoral free rear tips; inner margin of first dorsal moderately short, a third of dorsal base or less; second dorsal fin small and low, its height 1.5 to 2.3% of total length, its inner margin fairly long and 1.6 to 2.1 times its height; origin of second dorsal about over anal origin; pectoral fins large and falcate, with narrowly rounded or pointed apices, length of anterior margins about 17 to 22% of total length; 173 to 194 total vertebral centra, 86 to 97 precaudal centra.Tips of most fins dusky but not black or white. An inconspicuous white band on flank.

Geographical Distribution:
Western Atlantic: Southern Massachusetts and Georges Bank to Florida, Bahamas, Cuba, northern Gulf of Mexico, and Nicaragua; southern Brazil. Eastern North Atlantic: ? Portugal, ? Spain, ? Morocco, ? Madeira, ? western Mediterranean, Canary and Cape Verde Islands, Senegal, Sierra Leone. Western Indian Ocean: South Africa, Mozambique, Madagascar, ? Red Sea. Western Pacific: Japan, China, Viet Nam, Australia (New South Wales, Queensland, Western Australia), New Caledonia. Eastern Pacific: Southern California to Gulf of California, Revillagigedo Islands, possibly Chile.

Habitat and Biology:
A common, coastal-pelagic. inshore and offshore warm temperate and tropical shark of the continental and insular shelves and oceanic waters adjacent to them, that ranges from the surf zone to well out to sea and from the surface to 400 m depth. It does not prefer areas with reduced salinities and tends to avoid estuaries. Adults of the species occupy an overlapping intermediate offshore coastal habitat between other similar species of Carcharhinus such as more strictly inshore coastal species such as C. plumbeus, the offshore deep-benthic C. altimus, oceanic species such as C. falciformis and C. longimanus, and island species such as C. albimarginatus and C. galapagensis. Adult dusky sharks are often seen offshore and commonly follow ships.

This shark is strongly migratory in temperate and subtropical areas in the eastern North Pacific and western North Atlantic, moving north during the warmer months of summer and retreating south when the water cools. Off the southern coast of Natal, South Africa a nursery area occurs, where newborn sharks of 80 to 90 cm are resident; larger immature sharks over 90 cm move out of this area, with females tending to move north and males south, but there is some overlap in this partial sexual segregation. This pattern is complicated by seasonal, temperature-related migrations as elsewhere in the range of these sharks, going southward in spring and summer and northward in winter, and also a tendency for the sharks to move into deeper water during cooler months. Additionally, there may be other factors affecting the distribution of these young sharks, as may be true off Durban, South Africa, where they move into the surf zone in spring and summer and move offshore in autumn and winter, although inshore water temperatures are about the same. Still larger immature sharks up to 220 cm long may move south to southern Natal, butwhen they become adolescent at up to 280 cm, they tend to move north of Natal along with adults into waters of southern Mozambique. The young form large feeding schools or aggregations.

Viviparous, with a yolk-sac placenta; number of young per litter 3 to 14, with South African sharks averaging more young (about 10) than those from Florida; sex ration approximately 1:1 in the fetuses of South African and Floridian sharks, and the same for adults in Florida. There may be no correlation between maternal size and litter size in this shark, unlike some other species of Carcharhinus. Birth may occur over a long time span of several months in a given region, and has been reported as occurring from late winter to summer. In South African waters birth may occur year-round with an increase in autumn. In pregnant female sharks caught off Florida in the winter there are two size-classes of young, those 43 to 70 cm and full or near full-term fetuses of 85 to 100 cm. These classes may indicate either biannual staggered birth seasons with a gestation period of 8 or 9 months or a long gestation period of about 16 months. Whatever the case, females apparently mate in alternate years; mating in the western Atlantic occurs in the spring. Females move inshore to drop their young, then depart the nursery area. Adults may mature at an age of about 6 years and live to at least 18 years. The young are readily kept in aquaria.

Dusky sharks eat a wide variety of reef, bottom, and pelagic bony fishes, including sardines, menhaden and herring, anchovies, eels, lizardfish, cuskeels, needlefish, mullet, barracuda, goatfish, groupers, porgies, grunts, croakers, bluefish, spadefish, jacks, hairtails, mackeral, tunas and spanish mackeral, soles, flounders and cither flatfishes, flatheads, and gurnards, as well as angelsharks, sawsharks, spiny dogfish (Centrophorus and Squalus), catsharks (Halaelurus), smooth-hounds (Mustelus), other grey sharks (C. limbatus and C. brevipinna), skates, butterfly rays, crabs, lobsters, shrimp, octopi, cuttlefish, squid, starfish, barnacles, bryozoans, whale meat, and occasional garbage. Unlike the bull (C. leucas) and tiger (Galeocerdo cuvier) sharks mammalian carrion, oddities and garbage are apparently uncommon items in the diet of this species.

The dusky shark is apparently dangerous to people, although very few attacks by it have been recorded, and very little is known of its behaviour in relation to divers, swimmers or surfers. Because of its large size when adult, it should be considered a potential hazard where it occurs. Some attacks on people off islands such as Bermuda that were attributed to this species were probably caused by the closely similar Galapagos shark, (Carcharhinus galapagensis).

Young dusky sharks are readily preyed on by other big sharks, including sandtiger (Eugomphodus),great white (Carcharodon), bull (Carcharhinus leucas), and tiger (Galeocerdo) sharks, which help to regulate the population size of this species. Reduction of these species off Natal, South- Africa through an efficient shark gillnetting programme to protect bathing beaches has apparently resulted in an increase in juvenile dusky sharks there.

Maximum size possibly over 400 cm, males maturing at about 280 cm and reaching at least 340 cm; females maturing between 257 and 300 cm and reaching at least 365 cm; size at birth 69 to 100 cm.

Interest to Fisheries:
A common offshore shark regularly caught with longlines, also hook-and-line and set bottom nets. It is utilized fresh, dried salted, frozen and smoked for human consumption; hides used for leather; fins for shark-fin soup base; and liver oil extracted for vitamins.

Some records of this species from Madeira and the Mediterranean Sea may be based on C. galapagensis, according to Garrick (1982).

Type material:
Holotype: None. Type Locality: North America.

Dusky shark (Carcharhinus obscurus)