Silky Shark - Biology and Ecology

Biology and Ecology

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 (Orcinus orca). 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 (Sphyrna lewini), 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 (Tursiops sp.), along with 25 grey reef sharks (C. amblyrhynchos) and a lone silvertip shark (C. albimarginatus). Silky sharks are themselves accompanied by juvenile pilot fish (Naucrates ductor), who "ride" the pressure wave ahead of the shark, as well as by jacks, who snatch scraps of food and rub against the shark's skin to scrape off parasites.