Latin name

Carcharhinus falciformis

Other names

Blackspot shark, gray whaler shark, olive shark, ridgeback shark, sickle shark, sickle-shaped shark, sickle silk shark

Identification

The name "Silky" was given to the shark because of its soft, placoid scales. The silky shark has a slender, streamlined body, a rather long, rounded snout with a barely developed skin fold at the front. The medium-sized round eyes are equipped with nictitating membranes. There are short, shallow furrows at the corners of the mouth. There are 14-16 and 13-17 rows of teeth on each side of the upper and lower jaws. The upper teeth are triangular in shape, with strongly serrated edges and a single point; they stand straight in the centre of the jaw, but become increasingly inclined towards the corners. The lower teeth are narrow, straight and have smooth edges. They have five pairs of gill slits of medium length.

The dorsal and pectoral fins are characteristically shaped and can be used to distinguish silky sharks from similar species. The first dorsal fin is relatively small, less than 1/10th of the body length, with its base in line with the free ends of the pectoral fins. It has a rounded posterior tip, the length of the posterior free tip is half the height of the fin. The second dorsal fin is very small, smaller than the anal fin, the length of the free posterior tip is almost twice the length of the fin. There is a ridge between the first and second dorsal fins. Pectoral fins are narrow and crescent-shaped, particularly long in adults. The anal fin has a deep notch on the posterior edge above the free tip. The caudal fin is rather high with a well-developed lower lobe. There is a ventral notch near the tip of the upper lobe. The tip of the upper lobe is just below the tip of the first dorsal fin. The skin is densely covered with overlapping placoid scales. Each scale is diamond-shaped and has a ridge ending in a tooth. The dorsal colouration varies from golden brown to dark grey with a metallic tinge, the belly is snow white with white stripes running down the sides. The fins (except the first dorsal fin) are darker at the tips; this is more noticeable in young sharks. After death, the colouration quickly fades to grey. 

Distribution

The silky shark is ubiquitous in marine waters with temperatures above +23°C. In the Atlantic Ocean it is found from Massachusetts to Spain in the north, from southern Brazil to northern Angola in the south, including the Mediterranean, the Gulf of Mexico and the Caribbean. It lives in the Indian Ocean from the coastal waters of Mozambique to Western Australia, including the Red Sea and the Persian Gulf. In the Pacific Ocean, it is found from southern China and Japan to southern California and the Gulf of California; from Sydney south to northern New Zealand and Chile. Based on differences in their life cycle, four distinct populations of silky sharks can be distinguished in ocean basins around the world. They live in the Northwest Atlantic, the Western and Central Pacific, the Eastern Pacific and the Indian Ocean.

The silky shark is primarily an open-ocean dweller: it is found both near the surface and at depths of up to 200m, but can dive to depths of 500m or more. Observations of sharks in the eastern Pacific and northern Gulf of Mexico have shown that they spend 99% of their time at depths of 50m, with behavioural patterns remaining constant regardless of the time of day. This species prefers to stay close to the continental or island shelf and over coral reefs at depth. In some cases, sharks venture into coastal waters to depths of at least 18 metres. These sharks are highly mobile and travel long distances, although the details of their migrations are poorly understood. There is evidence that individual sharks have swum up to 60 km per day and covered distances of up to 1339 km. Large sharks tend to travel longer distances than smaller sharks. In the Pacific, they spend their summers at high latitudes, especially in Southern Oscillation years. In the North Atlantic, most sharks follow the Gulf Stream north along the east coast of the United States. In the Gulf of Aden they are more common in late spring and summer.

Habitat

Silky sharks are one of the three most common pelagic sharks and, with a population of at least ten million individuals, are considered to be one of the most abundant species of large oceanic animals. Unlike other sharks that live mainly in coastal waters where food is easier to find, these sharks prefer the open ocean. The silky shark is an active, curious and aggressive predator. When it sees an interesting object approaching, it will not pay close attention, but will slowly circle around it, sometimes turning its head from side to side. However, it can react with astonishing speed to any change in its immediate surroundings. These sharks are often found around floating objects such as lags or tethered offshore buoys.

Size

Silky sharks typically grow to 2.5m in length, with the maximum recorded length and weight being 3.5m and 346kg respectively. Females are larger than males. 

Life history and Behavior

Young silky sharks are known to form loosely organised groups, possibly for mutual protection. During migrations, a school may contain more than a thousand sharks. Groups are usually formed according to the size of the individuals and, in the Pacific, probably according to sex. In groups there are clashes: sharks turn sideways towards each other, open their jaws and expose their gills. Sometimes they charge upwards, and when they reach the surface they change direction and glide back down. The meaning of this behaviour is unknown. When confronted, the shark will show threat by arching its back, lowering its tail and pectoral fins and raising its head. It then begins to make small circles, moving stiffly and jerkily and turning sideways towards the perceived threat.

Food and feeding habits

A large predator that feeds primarily on bony fish at all levels of the water column. Its diet includes: tuna, mackerel, sardines, mullet, sea bass, lutjanidae, carangidae, scombridae, squaliformes, eel, lanternfish, triggerfish, and diodontidae. It also feeds on squid, argonauts and crabs. 

Reproduction

Like other members of the genus, silky sharks are viviparous. Once empty, the yolk sac becomes a kind of placenta through which the mother provides nourishment to the embryo. Compared to other viviparous sharks, the placenta of these sharks is less similar to the placenta of mammals because there is no closure between the tissues of the fetus and the mother. In addition, fetal red blood cells are much smaller than maternal red blood cells. Adult females have one functional ovary (right) and two functional uteri, which are divided longitudinally into separate compartments for each embryo. The silky shark is thought to breed throughout the year in most parts of the world, but in the Gulf of Mexico mating and birth occurs in late spring or early summer (May to August). Pregnancy lasts 12 months. Females give birth annually or every two years. A litter contains from 1 to 16 cubs, usually 6 to 12. The pups are born on the reef edge of the continental shelf where there is plenty of food and no large pelagic sharks. During the first year of life, the sharks grow to 25-30cm. After a few months, the young sharks migrate from their birthplace to the open ocean.

The life cycle of silky sharks is habitat dependent. Sharks in the northwest Atlantic tend to be larger than their west-central Pacific relatives at all ages, while sharks in the eastern Pacific tend to be smaller than sharks in other regions. In general, the silky shark has a moderate growth rate compared to other shark species, although this varies considerably between individuals. The maximum lifespan is at least 22 years.

Human interaction

Silky sharks are considered potentially dangerous to humans. However, they rarely come into contact with humans as they prefer to live in the open ocean. Their natural curiosity and boldness allows them to swim close to divers, and the presence of food can excite them and provoke aggressive behaviour. These quolls are more aggressive on reefs than in open water. There are six known cases of silky shark attacks, three of which were unprovoked and none fatal. Silky sharks are targeted in both commercial and recreational fisheries.