Silky shark

Appearance

Slim and streamlined, the silky shark has a fairly long, rounded snout with barely developed flaps of skin in front of the nostrils. The circular, medium-sized eyes are equipped with nictitating membranes. Short, shallow furrows are present at the corners of the mouth. Fourteen to 16 and 13–17 tooth rows are found on either side of the upper and lower jaws, respectively. The upper teeth are triangular and strongly serrated, with a notch in the posterior edge; they are erect at the center and become more oblique towards the sides. The lower teeth are narrow, erect, and smooth-edged. The five pairs of gill slits are moderate in length.

The dorsal and pectoral fins are distinctive and help to distinguish the silky shark from similar species. The first dorsal fin is relatively small, measuring less than a tenth as high as the shark is long, and originates behind the free rear tips of the pectoral fins. It has a rounded apex, an "S"-shaped rear margin, and a free rear tip about half as long as the fin is tall. The second dorsal fin is tiny, smaller than the anal fin, with a drawn-out free rear tip up to three times as long as the fin is tall. A narrow dorsal ridge runs between the dorsal fins. The pectoral fins are narrow and sickle-shaped, and particularly long in adults. The anal fin originates slightly ahead of the second dorsal fin and has a deep notch in the posterior margin. The caudal fin is fairly high with a well-developed lower lobe.

The skin is densely covered by minute, overlapping dermal denticles. Each dermal denticle is diamond-shaped and bears horizontal ridges leading to posterior marginal teeth, which increase in number as the shark grows. The back is metallic golden-brown to dark gray and the belly is snowy white, which extends onto the flank as a faint lighter stripe. The fins darken at the tips; this is more obvious in young sharks. The coloration quickly fades to a dull gray after death. One of the larger members of its genus, the silky shark commonly reaches a length of 2.5 m, with a maximum recorded length and weight of 3.5 m and 346 kg, respectively. Females grow larger than males.

Distribution

The silky shark has a cosmopolitan distribution in marine waters warmer than 23 °C. In the Atlantic Ocean, it is found from the U.S. state of Massachusetts to Spain in the north, and from southern Brazil to northern Angola in the south, including the Mediterranean Sea, Gulf of Mexico, and Caribbean Sea. It occurs throughout the Indian Ocean, as far south as Mozambique in the west and Western Australia in the east, including the Red Sea and the Persian Gulf. In the Pacific Ocean, the northern extent of its range runs from southern China and Japan to southern Baja California and the Gulf of California, while the southern extent runs from Sydney, Australia, to northern New Zealand to northern Chile. Based on life history differences, four distinct populations of silky sharks have been identified in ocean basins worldwide: in the northwestern Atlantic, in the western and central Pacific, in the eastern Pacific, and in the Indian Ocean.

Behavior

The silky shark is one of the three most common pelagic sharks along with the blue and oceanic whitetip sharks, and counts among the most numerous large oceanic animals in the world with a population of at least tens of millions. Compared to the other two species, it is less strictly pelagic with the greatest numbers found in offshore waters associated with land, where food is more readily obtained than farther out in the truly open ocean. The silky shark is an active, inquisitive, and aggressive predator, though it will defer to the slower but more powerful oceanic whitetip shark in competitive situations. When approaching something of interest, it may seem inattentive, sedately circling and sometimes swinging its head from side to side. However, it can respond with startling swiftness to any shift in its immediate surroundings. This shark is often found around floating objects such as logs or tethered naval buoys.

Younger silky sharks are known to form large, loosely organized aggregations, possibly for mutual defense. During migrations, over a thousand individuals may gather. These groups are generally segregated by size, and in the Pacific perhaps also by sex. Silky sharks within a group have been observed to "tilt", presenting their full lateral profile towards each other, as well as gape their jaws or puff out their gills. On occasion, sharks have also been seen suddenly charging straight up, veering away just before reaching the surface and gliding back down to deeper water. The significance of these behaviors is unknown. When confronted, the silky shark may perform a threat display, in which it arches its back, drops its tail and pectoral fins, and elevates its head. The shark then proceeds to swim in tight loops with a stiff, jerky motion, often turning broadside towards the perceived threat.

Potential predators of the silky shark include larger sharks and killer whales. Known parasites of this shark include the isopod "Gnathia trimaculata", the copepod "Kroeyerina cortezensis", and the tapeworms "Dasyrhynchus variouncinatus" and "Phyllobothrium" sp. Silky sharks frequently intermingle with schools of scalloped hammerheads, and have been known to follow marine mammals. One account from the Red Sea describes 25 silky sharks following a large pod of bottlenose dolphins, along with 25 grey reef sharks and a lone silvertip shark. Silky sharks are themselves accompanied by juvenile pilot fish, which "ride" the pressure wave ahead of the shark, as well as by jacks, which snatch scraps of food and rub against the shark's skin to scrape off parasites.

Habitat

Primarily an inhabitant of the open ocean, the silky shark is most common from the surface to a depth of 200 m, but may dive to 500 m or more. Tracking studies in the tropical eastern Pacific and northern Gulf of Mexico have found that cruising silky sharks spend 99% of their time within 50 m of the surface, and 80–85% of their time in water with a temperature of 26–30 °C; the pattern was constant regardless of day or night. This species favors the edges of continental and insular shelves, often over deepwater reefs and around islands. Its range extends farther north and south along continental margins than in oceanic waters. On occasion, it may venture into coastal waters as shallow as 18 m. Silky sharks are highly mobile and migratory, though the details of their movements are little-known. Tagging data have recorded individual sharks moving up to 60 km per day, and covering distances up to 1,339 km. Larger sharks generally move longer distances than smaller ones. In the Pacific Ocean and possibly elsewhere, it spends the summer at slightly higher latitudes, particularly during warmer El Niño years. In the northern Atlantic, most sharks follow the Gulf Stream northward along the U.S. East Coast. In the Gulf of Aden, it is most common in late spring and summer.

Food

The silky shark is an opportunistic predator, feeding mainly on bony fishes from all levels of the water column, including tuna, mackerel, sardines, mullets, groupers, snappers, mackerel scads, sea chubs, sea catfish, eels, lanternfishes, es, triggerfishes, and porcupinefishes. It may also take squid, paper nautilus, and swimming crabs, and fossil evidence indicates it scavenged on whale carcasses. Good feeding opportunities can draw silky sharks in large numbers; one such feeding aggregation in the Pacific has been documented "herding" a school of small fishes into a compact mass and trapping it against the surface, whereupon the sharks consumed the entire school. When attacking tightly packed fish, silky sharks charge through the ball and slash open-mouthed, catching the prey fish at the corners of their jaws. Although multiple individuals may feed at once, each launches its attack independently.

Studies conducted off the Florida coast and the Bahamas have shown that silky sharks are highly sensitive to sound, in particular low-frequency, irregular pulses. Experiments in which these sounds were played underwater attracted sharks from hundreds of meters away. Silky sharks likely orient to these sounds because they are similar to the noise generated by feeding animals such as birds or dolphins, thus indicating promising sources of food. These studies have also demonstrated that a silky shark attracted by one sound will quickly withdraw if that sound abruptly changes in amplitude or character; this change need not be a sound produced by a predator to evoke the reaction. Over repeated exposures, silky sharks habituate to the sound change and stop withdrawing, though it takes them much longer to do so compared to the bolder oceanic whitetip shark.

The bite force of a 2-m-long silky shark has been measured at 890 newtons. A well-established association exists between this species and tuna: off Ghana, almost every tuna school has silky sharks trailing behind, and in the eastern Pacific, these sharks inflict such damage to tuna fishing gear and catches that fishery workers have given them the moniker "net-eating sharks". Silky sharks and bottlenose dolphins compete when both species target the same school of fish; the amount eaten by the dolphins decreases relative to the number of sharks present. If a large number of sharks is present, they tend to remain inside the prey school, while the dolphins consign themselves to the periphery, possibly to avoid incidental injury from the sharks' slashing attacks. Conversely, if a large enough group of dolphins gathers, they become able to chase the sharks away from the prey school. Regardless of which one dominates, the two predators do not engage in any overtly aggressive behavior against each other.