Silky sharks are found throughout the Pacific, Atlantic, and Indian Oceans between 40°N latitude and 40°S latitude. They favor sub-tropical waters and are among the world’s most abundant shark species. They are highly migratory sharks, but have been known to concentrate in the Gulf of Aden, the Gulf of Mexico, and along the coast of southern Baja California. Those located in the Atlantic Ocean tend to move with the Gulf Stream and the migrations of tuna, their primary food source. Populations of silky sharks in both the Pacific and Indian Oceans also take part in migratory patterns by moving toward slightly higher latitudes during the summer months. (Camhi, et al., 2008; Gilbert and Schlernitzauer, 1966; MarineBio.org, 2012)
Silky sharks are mostly found in the coastal and oceanic waters of tropical oceans, mainly at temperatures above 23°C. They primarily inhabit continental and insular shelves, but have also been found over deep water reefs and in open ocean, slope, and shallow, coastal water habitats. This species has been found at depths of up to 500 m, and records show that they have been seen in waters as shallow as 18 meters. It has been noted that this species has a wider latitudinal distribution along continental shelves compared to the open ocean or along insular shelves. (Camhi, et al., 2008; Clarke, et al., 2011; MarineBio.org, 2012; Michael, 2005)
During various stages of the life cycle, silky sharks transition between different habitats. For the first few years of life, juveniles live in nursery grounds and lead a demersal or semi-pelagic lifestyle. As they grow older and reach an average young adult length of about 130 cm, they migrate offshore to deeper waters. At this stage, they often join and travel with large schools of pelagic fish such as tuna, ensuring a constant food supply. Adult silky sharks return seasonally to continental and insular shelf areas in order to feed and reproduce. However, they tend to spend most of their time in deeper waters. (Camhi, et al., 2008; Clarke, et al., 2011)
Unlike most members of the genus Carcharhinus, the pectoral fins of this species are sickle-shaped. The first dorsal fin is relatively small, with a rounded apex, which originates behind the pectoral fins. The second dorsal fin is also very small, with a long trailing tip that almost reaches the precaudal pit, which is a notch on the dorsal side of the shark that is located where the caudal fin, or tail fin, begins. Silky sharks are the largest members of their genus, reaching up to 346 kg in mass and 3.5 m in length. Females grow to be much larger than males. Upon reaching maturity, female silky sharks range from 2.1 to 2.3 m (7 to 7.5 ft) in length whereas male silky sharks range from 1.8 to 2.1 m (6 to 7 ft). (Camhi, et al., 2008; Garrick, et al., 1964; Knickle, 2012; MarineBio.org, 2012; Michael, 2005; Thomson and Simanek, 1977)
Silky sharks get their name from the silky feel of their hide. Their skin, as in other shark species, is covered with dermal denticles. However, the unusually dense packing of these structures in this species makes their skin feel much softer to the touch than the rougher skin that is commonly associated with sharks. Another distinctive feature of silky sharks is the shape of their teeth. They have between 14 to 17 teeth on each side their upper jaws, and these teeth are notched or serrated rather than concave, which is the condition in most other species of sharks. (Garrick, et al., 1964; Knickle, 2012; MarineBio.org, 2012; Michael, 2005)
The dorsal coloration of this species can vary greatly, from a dark brown to a blue-grey color. The ventral surface is generally white, but in some individuals the ventral surface of the pelvic and pectoral fins can have darkly colored tips. (Garrick, et al., 1964; Knickle, 2012; MarineBio.org, 2012; Michael, 2005)
Silky sharks give birth to live young, providing placentally derived nutrition throughout the developmental process. In females, the oviducts are modified to form uteri, with only the right ovary being functional. The embryos develop in longitudinally oriented individual chambers, with their heads pointing anteriorly in the uterus. When silky sharks are born, they range in length from 70 to 75 cm. Juveniles rapidly grow an additional 25 to 35 cm by their first winter, which is thought to enhance their survival. (Gilbert and Schlernitzauer, 1966; Joung, et al., 2008; MarineBio.org, 2012; Martin, 2007)
Male silky sharks release pheromones; however, it is uncertain as to whether or not the pheromones are used to attract mates, ward off competition, mark territory, or some combination of the three. Additionally, studies have shown that no sexual segregation exists within silky shark populations. Pheromones do not play a role in determining social structure, meaning that silky sharks do not travel together solely for mating purposes. Rather, size appears to be the determing factor in social structure, with co-travelling generally being of the same size class. (Gilbert and Schlernitzauer, 1966; Knickle, 2012; MarineBio.org, 2012; Martin, 2007; Musick and McMillan, 2002)
Mating rituals of silky sharks, if they exist, are unknown. During the mating process, the male inserts his claspers into the female's cloaca, releasing sperm. Males mate with multiple females during a breeding season. In tropical waters, silky sharks do not have a set breeding season and mate year-round. Silky sharks located in the warm temperate waters of the Gulf of Mexico have a set breeding period during the summer months of June, July, and August. (Carrier, et al., 2004; Knickle, 2012; MarineBio.org, 2012)
Reproductive maturity is reached at 7 to 9 years of age and 2.1 to 2.3 m in length in females, 6 to 7 years and 1.8 to 2.1 m in males. Silky sharks in the Indian Ocean and in the Pacific Ocean achieve maturity at younger ages and at smaller sizes than silky sharks in other areas (approximately two years younger and 0.3 to 0.6 meters shorter). It is thought that the variation in size at maturity might be related to latitude, with sharks in tropical waters (areas of low latitude) tending to grow faster and mature at earlier stages of life. This may be due to warmer waters causing an increase in metabolism, thus speeding up growth rates, but the mechanism responsible is in need of additional research and confirmation. (Joung, et al., 2008; Knickle, 2012; MarineBio.org, 2012; Martin, 2007; Michael, 2005)
In tropical waters, silky sharks breed year round, and in warm-temperate waters, such as the Gulf of Mexico, silky sharks breed only during the summer months (June, July, and August). They breed every two years and typically produce between two and fourteen live offspring per litter. The gestation period averages 12 months. Silky sharks are considered capable predators at birth. (Gilbert and Schlernitzauer, 1966; Joung, et al., 2008; Knickle, 2012; MarineBio.org, 2012; Martin, 2007)
Female silky sharks provide continual nutrition to their developing young through the placenta. The young are also protected due to their development inside their much larger mother's body. Newborn silky sharks receive no additional parental care, as they are highly capable predators at birth. Given the patterns of reproduction known from other elasmobranch species, it is highly unlikely that males provide any investment during the 12 month gestation period. (Gilbert and Schlernitzauer, 1966; Knickle, 2012; MarineBio.org, 2012; Martin, 2007)
The age of silky sharks can be determined by counting the number of growth rings that develop on their vertebrae, with each band representing approximately one year of life. Silky sharks live to be 23 years of age on average, and it is estimated that they can live up 25 years in the wild. There are no records of silky sharks being kept and raised in captivity. (Branstetter, 1987; Joung, et al., 2008; Knickle, 2012; MarineBio.org, 2012; Martin, 2007; Oshitani, et al., 2003)
Silky sharks are generally solitary, but have also been known to travel in loose aggregations or groups. Juveniles in particular primarily travel in groups until they reach maturity, a strategy that is thought to protect them from larger predators. Even at adult stages, silky sharks can be quite social with conspecifics and often intermix with schooling scalloped hammerheads (Sphyrna lewini). Studies have shown that silky sharks may segregate according to size, as travelling groups tend to be composed of individuals of similar size classes. These sharks have been recorded to be active during both the day and at night but they usually reach a peak in daily activity around dawn and around dusk. (Camhi, et al., 2008; Knickle, 2012; MarineBio.org, 2012; Martin, 2007; Michael, 2005)
Silky sharks are a highly migratory species, following the movements of schooling fish such as tuna. They travel alone and in groups depending on the individual shark. Silky sharks are known for their quick and aggressive behavior, and have been seen performing threat displays in which they raise their head, arch their back, and lower their tail. Several shark species display this behavior in situations dealing with territory, mates, and predators. They are very inquisitive and will often make close non-aggresive passes to divers. (Knickle, 2012; Martin, 2007; Nelson, 1977)
While their home range is not well known and appears to be poorly defined, it has been noted that silky sharks favor certain migratory routes and core areas over others. Each silky shark has a unique movement pattern based on prey preference and migration. (Knickle, 2012; MarineBio.org, 2012; Musick and McMillan, 2002; Musyl, et al., 2011; Nelson, 1977)
There has been very little research conducted specifically on how silky sharks communicate and locate prey but, as with other shark species, they have several highly-developed senses. They have a superior sense of smell and can detect a single drop of blood in 100 L of water. They have paired nostrils beneath their snouts, which function as tunnels with two openings separated by a fleshy flap. As the shark swims forward, water flows over the olfactory glands, allowing the shark to “smell” the water. Silky sharks are also very sensitive to electrical frequencies and can accurately hear sounds 80 Hz and below, as well as sounds up to 800 or 1000 Hz. They can hear sounds that are imperceptible to the human ear such as the sounds of struggling prey, or the drumming of bony fish. It is thought that sharks have the ability to determine the direction a sound is coming from using their lateral line, or acousticolateralis system. This system is composed of small bundles of sensory cells called neuromasts which are located in pores along the head and body. (Carrier, et al., 2004; MarineBio.org, 2012; Musick and McMillan, 2002)
Silky sharks have been observed communicating using aggression displays, involving a raised head, arched back, and lowered tail. Males can also communicate by releasing pheromones into the water to attract females and ward off challenging males. (Knickle, 2012; Nelson, 1977)
Silky sharks are generalist carnivores and typically feed on various species of fish, squid, and pelagic crabs, including red crab (Pleuroncodes planipes), jumbo squid (Dosidicus gigas), and chub mackerel (Scomber japonicas). Young silky sharks primarily feed upon jumbo squid, while adult silky sharks consume more red crabs and chub mackerel. Additionally, yellowfin tuna (Thunnus albacares), albacore (Thunnus alalunga), mullet (Mugilidae species), and porcupine fish (Diodon hystrix) have been found in the stomachs of silky sharks. (Cabrera-Chavez-Costa, et al., 2010; Knickle, 2012; MarineBio.org, 2012; Michael, 2005)
Variation in diet of silky sharks depends on the availability and abundance of prey. Other factors that affect their diet include the size and energy content of prey items, and seasonal changes in their availability. They primarily feed on schooling fish, most likely because of an increased likelihood of catching more prey, which reduces the amount of energy used in foraging. When food is limited, silky sharks act as opportunistic feeders, consuming a wide variety of prey from different habitats and depths in the open ocean. When food is abundant, they may be more selective in what they eat. (Cabrera-Chavez-Costa, et al., 2010)
Like most large sharks, adult silky sharks have very few predators. They may occasionally encounter a killer whale (Orcinus orca) or another large shark that might pose a threat. Juveniles and smaller adults can also fall prey to larger, more mature sharks. Individuals in these smaller size classes often form small groups to avoid predation. (Knickle, 2012; MarineBio.org, 2012; Martin, 2007; Michael, 2005)
One of the few regular predators of silky sharks is humans. Silky sharks are known to follow schools of tuna and are often caught as a by-catch in tuna fisheries. They are also harvested by directed pelagic shark fisheries, and taken by recreational fisherman. (Knickle, 2012; MarineBio.org, 2012)
Silky sharks are top-level predators, feeding at very high trophic levels. They feed on many species of fish and also serve as hosts to various parasites including isopods, copepods, and tapeworms. These parasites are commonly found in pelegic fish and in other members of the genus Carcharhinus. (Beveridge and Campbell, 1993; Deets, 1987; Knickle, 2012; MarineBio.org, 2012; Martin, 2007; Ota and Euichi, 2009)
Silky sharks have been the subject of many scientific studies surrounding the sensory biology of sharks. They are also among the most common bycatch species in the tuna fishing process, making up 70 to 80% of the pelagic longline catch off the coast of the Maldives and Sri Lanka. Many fishermen will remove their fins for sale in Asian markets, occasionally selling the meat and oils as well. Silky sharks are one of the most common sources of cleaned and dried shark jaws sold to tourists in tropical countries. (Knickle, 2012; MarineBio.org, 2012; Martin, 2007; Michael, 2005; Watson, et al., 2009)
Silky sharks can be dangerous to humans due to their large size and aggressive nature. They should be treated with extreme caution if encountered by divers, as they have been involved in documented attacks on humans. Such attacks are rare, however, as this species is typically found in the open ocean. (Knickle, 2012; MarineBio.org, 2012; Martin, 2007; Michael, 2005)
Silky sharks are considered near threatened on the IUCN Redlist and are vulnerable to over-fishing because of their long gestation period, low number of offspring, and slow growth rate. However, there has been very little sampling of silky shark populations in tropical waters. It is estimated that the population of silky sharks has decreased by 85% over the course of a 19 year period (1984-2005) and is continuing to decrease. These numbers are uncertain, however, due to the under-reporting of catch rates and lack of population monitoring. States and areas that allow fishing for this species have been encouraged to cooperate over its management to date no regulatory plans have been enacted. (Joung, et al., 2008; Knickle, 2012; MarineBio.org, 2012; Martin, 2007; Michael, 2005)
Jessica Frazelle (author), Radford University, Karen Francl (editor), Radford University, Kiersten Newtoff (editor), Radford University, Melissa Whistleman (editor), Radford University, Jeremy Wright (editor), University of Michigan-Ann Arbor.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
uses sound to communicate
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.
active at dawn and dusk
uses electric signals to communicate
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
union of egg and spermatozoan
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
Animals with indeterminate growth continue to grow throughout their lives.
fertilization takes place within the female's body
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
makes seasonal movements between breeding and wintering grounds
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
chemicals released into air or water that are detected by and responded to by other animals of the same species
an animal that mainly eats fish
having more than one female as a mate at one time
structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.
mainly lives in oceans, seas, or other bodies of salt water.
breeding is confined to a particular season
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
movements of a hard surface that are produced by animals as signals to others
uses sight to communicate
breeding takes place throughout the year
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