Marine Species Identification Portal : Silky shark

Author: (Bibron, 1839)

Field Marks:
A large, dark, slim, oceanic grey shark with moderately long rounded snout, moderately large eyes, oblique-cusped serrated teeth in the upper jaw, upper teeth with basal cusplets or very strong serrations, usually 15/15 rows of anteroposterior teeth, an interdorsal ridge, long narrow pectoral fins, a moderatesized first dorsal with its origin behind the pectoral rear tips, a low second dorsal with a greatly elongated inner margin and rear tips, and no conspicuous markings.

Diagnositic Features:
A large, fairly slender species (up to about 3.3 m). Snout moderately long and rounded; internarial width 1.2 to 1.6 times in preoral length; eyes circular and moderately large, their length 1.2 to 2.7% of total length; upper labial furrows short and inconspicuous; hyomandibular line of pores just behind mouth corners not conspicuously enlarged; gill slits moderate-sized, the third 2.9 to 3.6% of total length and less than 2/5 of first dorsal base; usually 15/15 rows of anteroposterior teeth in each jaw half but varying from 14 to 16/13 to 17; upper teeth with fairly narrow, strongly serrated, erect to moderately oblique cusps, welldelimited from crown feet, feet with heavy serrations or small cusplets; lower teeth with erect, narrow, smoothedged cusps and transverse roots. A narrow interdorsal ridge present. First dorsal fin moderate-sized and falcate, with narrowly to broadly rounded apex and posterior margin curving ventrally from fin apex; origin of first dorsal fin behind pectoral free rear tips; inner margin of first dorsal long, about half dorsal base or slightly more or less; second dorsal fin very small and low, its height 1.3 to 2.2% of total length,its inner margin long and 1.6 to 3 (usually over 2) times its height; origin of second dorsal over or slightly behind anal origin; pectoral fins large (especially in adults, shorter in young), narrowly falcate, with narrowly rounded or pointed apices, length of anterior margins about 14 to 22% of total length; 199 to 215 total vertebral centra, 98 to 106 precaudal centra. Colour dark grey or grey brown above, sometimes nearly blackish, white below; tips of fins other than first dorsal dusky but not black-tipped; an inconspicuous white band on flank.

Geographical Distribution:
Oceanic and coastal, circumtropical. Western Atlantic: Massachusetts to southern Brazil, including Gulf of Mexico and Caribbean Sea: Central Atlantic from St. Paul's Rocks. Eastern Atlantic: Madeira, Atlantic Spain, Senegal to northern Angola. Indian Ocean: Madagascar, Mozambique, Tanzania, Comores and Aldabra Island, between Somalia and Maldive Islands, Oman, Red Sea, Sri Lanka. Western Pacific: Thailand, the Philippines, New Caledonia, New Zealand and China (including Taiwan Island). Central and eastern Pacific: Caroline, Hawaiian, Phoenix and Line Islands, westward to Cocos, Revillagigedo, Clipperton and Malpelos Islands, southern Baja California to Peru.

Habitat ano Biology:
An abundant offshore, oceanic and epipelagic and littoral, tropical shark, found near the edge of continental and insular shelves but also far from land in the open sea. It occasionally occurs inshore where the water is as shallow as 18 m; in the open ocean it occurs from the surface down to at least 500 m depth.

The silky shark is often found over deepwater reefs and near insular slopes. Water temperatures of 23° to 24°C have been recorded where it occurs. It is anactive, quick-moving, aggressive shark in the water, but defers to the more sluggish but stubbornly persistant oceanic whitetip shark. When approached by divers individuals have been seen to perform a "hunch" display, with back arched, head raised and caudal fin lowered, possibly as a defensive threat display.

Population dynamics and structure are poorly known. Longline sampling in the eastern and central Pacific shows this shark to be much more abundant offshore near land than in the open ocean, unlike the blue shark (Prionace glauca) and the oceanic whitetipe shark (Carcharhinus longimanus), which occur with it. One is tempted to speculate that this shark is perhaps less well-adapted to oceanic life than the whitetip and blue sharks, and that its greater activity is best supported in offshore areas close to land masses that have higher productivity of prey species than the open ocean. The sluggishness, opportunistic feeding habits, and long pectoral fins of the blue and whitetip sharks may be energy-saving adaptations for life in the open sea; the blue shark additionally has gillraker papillae that apparently adapt it to preying on small pelagic animals. Sketchy data shows no strong tendency for sexual segregation in the silky shark, but this may very well occur. There is size segregation, with young occurring on offshore nursery areas and adults seaward from them. This is one of the three most common oceanic sharks, along with the blue and oceanic whitetip sharks, and one of the more abundant large marine organisms.

Viviparous, with a yolk-sac placenta; number of young 2 to 14 per litter. There seems to be no pronounced seasonality in birth of young. The gestation period is not known. In the western North Atlantic nursery areas for the young of this shark occur along the outer edge of the continental shelf and on oceanic banks in the Caribbean.

Primarily a fish-eater, eating pelagic and inshore teleosts including sea catfish, mullets, mackerel, yellowfin tuna, albacore, and porcupine fish, but also squid, paper nautiluses, and pelagic crabs. Associated with schools of tuna, and earning the ire of tuna purse seiners for the damage it does to nets and catches; it is called the 'neteater shark' in the tropical eastern Pacific.

The silky shark is generally regarded as dangerous or potentially dangerous to people, particularly because of its size and abundance offshore, although no attacks have been attributed to it. Because of its lesser aggressiveness and apparently more restricted diet, it may very well be less dangerous than the oceanic whitetip shark.

Size:
Maximum about 330 cm, males maturing at about 187 to 217 cm and reaching 270 to 300 cm; females maturing at 213 to 230 cm and reaching at least 305 cm; size at birth about 70 to 87 cm.

A length-weight curve for Cuban sharks is:
WT =0.8782 x 10 5 total length3 091 (Guitart Manday, 1975).

Interest to Fisheries:
This species is very commonly taken by pelagic longline fisheries but is also taken in fixed bottom nets. Important fisheries exist in the Gulf of Mexico and Caribbean Sea, but probably also elsewhere. Its meat is utilized fresh and dried salted for human consumption; its hide for leather; its fins for shark-fin soup; and its liver is extracted for oil, which has high vitamin A content.

Type material:
Holotype: Museum National d'Histoire Naturelle, Paris, MNHN 1134, 528 mm female fetus. Type Locality: Cuba.

Silky shark (Carcharhinus falciformis)