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Lifespan, longevity, and ageing

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Maximum longevity: 23 years (wild) Observations: Unverified estimates suggest these animals may live up to 25 years (http://www.fishbase.org/).
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Joao Pedro de Magalhaes
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de Magalhaes, J. P.
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Behavior

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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.

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.

Communication Channels: visual ; chemical

Other Communication Modes: pheromones

Perception Channels: visual ; tactile ; acoustic ; vibrations ; chemical ; electric

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
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Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Conservation Status

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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.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
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Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Life Cycle

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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.

Development - Life Cycle: indeterminate growth

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
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Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Benefits

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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.

Negative Impacts: injures humans (bites or stings)

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
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Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Benefits

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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.

Positive Impacts: body parts are source of valuable material; research and education

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
author
Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Associations

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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.

Commensal/Parasitic Species:

  • Gnathia trimaculata (Order Isopoda: Subphylum Crustacea)
  • Kroeyerina cortezensis (Subclass Copepoda: Subphylum Crustacea)
  • Dasyrhynchus variouncinatus (Class Cestoda: Phylum Platyhelminthes)
  • Phyllobothrium sp. (Class Cestoda: Phylum Platyhelminthes)
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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
author
Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Trophic Strategy

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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 holocanthus) have been found in the stomachs of silky sharks.

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.

Animal Foods: fish; mollusks; aquatic crustaceans

Primary Diet: carnivore (Piscivore , Eats non-insect arthropods, Molluscivore )

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
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Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Distribution

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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.

Biogeographic Regions: indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native )

Other Geographic Terms: cosmopolitan

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
author
Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Habitat

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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.

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.

Range depth: 500 to 18 m.

Average depth: 200 m.

Habitat Regions: temperate ; tropical ; saltwater or marine

Aquatic Biomes: pelagic ; reef ; coastal

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
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Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Life Expectancy

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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.

Range lifespan
Status: wild:
25 (high) years.

Average lifespan
Status: wild:
23 years.

Average lifespan
Status: wild:
23 years.

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
author
Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Morphology

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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).

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.

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.

Range mass: 178 to 346 kg.

Range length: 1.8 to 3.5 m.

Average length: 2.5 m.

Other Physical Features: endothermic ; heterothermic ; bilateral symmetry

Sexual Dimorphism: female larger

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
author
Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Associations

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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.

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.

Known Predators:

  • humans (Homo sapiens)
  • killer whales (Orcinus orca)
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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
author
Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Reproduction

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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.

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.

Mating System: polygynous

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.

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.

Breeding interval: Silky sharks breed every two years.

Breeding season: In tropical waters, silky sharks breed year-round. In the warm-temperate waters of the Gulf of Mexico, silky sharks breed during the summer months (June, July, and August).

Range number of offspring: 2 to 14.

Average gestation period: 12 months.

Range age at sexual or reproductive maturity (female): 7 to 12 years.

Average age at sexual or reproductive maturity (female): 8 years.

Range age at sexual or reproductive maturity (male): 6 to 10 years.

Average age at sexual or reproductive maturity (male): 6.5 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); ovoviviparous

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.

Parental Investment: female parental care ; pre-hatching/birth (Provisioning: Female, Protecting: Female)

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Frazelle, J. 2012. "Carcharhinus falciformis" (On-line), Animal Diversity Web. Accessed April 27, 2013 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_falciformis.html
author
Jessica Frazelle, Radford University
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Karen Francl, Radford University
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Kiersten Newtoff, Radford University
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Melissa Whistleman, Radford University
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Jeremy Wright, University of Michigan-Ann Arbor
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Diagnostic Description

provided by FAO species catalogs
fieldmarks: A large, dark, slim, oceanic gray 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 moderate-sized 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. Large, fairly slender sharks (up to about 3.3 m) with snout moderately long and rounded, internarial width 1.2 to 1.6 in preoral length. Eyes circular and moderately large, length 1.2-2.7%TL. Upper labial furrows short and inconspicuous. Hyomandibular line of pores just behind mouth corners not conspicuously enlarged.

Gill slits moderate-sized, 3rd 2.9-3.6% TL and less than 2/5 of first dorsal base.

Usually 15/15 rows of anteroposterior teeth in each jaw half but varying from 14-16/13-17; upper teeth with fairly narrow, strongly serrated, erect to moderately oblique cusps, well-delimited from crown feet, feet with heavy serrations or small cusplets; lower teeth with erect, narrow, smooth-edged cusps and transverse roots.

A narrow interdorsal ridge present. 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%TL. First dorsal fin moderate-sized and falcate, with narrowly to broadly rounded apex and posterior margin curving ventral 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, height 1.3-2.2% TL, inner margin long and 1.6-3.0 (usually over 2.0) times height; origin of second dorsal over or slightly behind anal origin. 199-215 total vertebral centra, 98-106 precaudal centra.

Colour dark gray or gray 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.

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FAO species catalogue Vol.4. Sharks of the world. An Annotated and Illustrated Catalogue of Shark Species Known to Date Part 2 - Carcharhiniformes.Compagno, L.J.V. 1984. FAO Fish. Synop., (125) Vol.4, Part 2
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Food and Agriculture Organization of the UN
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Size

provided by FAO species catalogs
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, 1975).
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FAO species catalogue Vol.4. Sharks of the world. An Annotated and Illustrated Catalogue of Shark Species Known to Date Part 2 - Carcharhiniformes.Compagno, L.J.V. 1984. FAO Fish. Synop., (125) Vol.4, Part 2
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Food and Agriculture Organization of the UN
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Brief Summary

provided by FAO species catalogs
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.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 an active, quick-moving, aggressive shark, but defers to the more sluggish but stubbornly persistent 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 whitetip 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 seawards from them. This is one of the three commonest 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, mullet, 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 'net-eater shark' in the tropical Eastern Pacific.

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FAO species catalogue Vol.4. Sharks of the world. An Annotated and Illustrated Catalogue of Shark Species Known to Date Part 2 - Carcharhiniformes.Compagno, L.J.V. 1984. FAO Fish. Synop., (125) Vol.4, Part 2
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Food and Agriculture Organization of the UN
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Benefits

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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. Catch statistics for this species are reported to FAO only by Sri Lanka in area 51 (Western Indian Ocean). The catches started in 1960 with 5,000 t and since then the trend has been positive reaching a peak of 25,400 t in 1994 then slightly decreasing to 21,000 t in 1996. The total catch reported for this species to FAO for 1999 was 20 810 t. The countries with the largest catches were Sri Lanka (20 700 t) and Liberia (110 t). The meat of the silky shark has been utilized fresh and dried-salted for human consumption, while hides have been processed for leather, fins have figured in the oriental sharkfin trade, and its liver has been extracted for liver oil (which has a high vitamin A content in this species). The silky shark is fished directly or as a sometimes important bycatch throughout its range. There are a few major multispecies shark fisheries that catch large numbers of silky sharks, mainly in Mexico and Sri Lanka (Bonfil 1994). It is also taken in the coastal fisheries of Taiwan and in larger numbers in the Taiwanese shark fisheries in waters of Indonesia and Papua-New Guinea (Chenet al. 1996). In addition, it is relatively common as a bycatch in tuna longline and purse seine fisheries (mostly juveniles are caught in the latter), especially when the gear is set near continental or insular shelves. Bonfil (1994) estimated that some 1 million silky sharks were caught as bycatch in tuna longline fisheries in the Central and South Pacific at the beginning of the 1990s. However, there is large uncertainty surrounding these calculations and there are no estimates of numbers discarded alive and numbers actually killed. In addition, estimates of population sizes or indices of abundance are not available for any stock of silky sharks. FAO reports catches of silky sharks in Sri Lankan fisheries starting in 1986. These figures average to about 11,000 t/y. However, only about 75% of these catches reported for Sri Lanka are actually attributable to silky sharks (Bonfil 1994). Silky sharks are thought to be overexploited as juveniles in the shelf nursery areas of the Campeche Bank (Bonfil 1990, 1996, 1997). Due to the lack of estimates of total catches and the size of the populations of this species, the status of the stocks is unknown. Conservation Status : The silky shark has a mid-range intrinsic rebound potential (Smith et al. 1998). Its wide distribution and high abundance in most tropical shelves of the world suggests that presently there are no major concerns over the conservation of this species at the global level. However, there is a strong need to monitor the abundance of heavily fished stocks. The silky shark is preliminarily considered a species ofLower Risk/Least Concern for the IUCN Red List (Bonfil in press a). However, this classification is awaiting IUCN Shark Specialist Group consensus. According to Compagno (1984), silky sharks are among "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". There are no published observations of trends in abundance of silky sharks anywhere in the world. Some intensive localised fisheries (e.g. Mexico, Sri Lanka) could eventually result in local depletion if not monitored and controlled, although such cases are not thought to pose a threat to the species at large given the likely enormous size of the world population. The silky shark is at present relatively free of threats in the form of habitat destruction because it does not live inshore nor does it utilise coastal lagoons as pupping or nursery areas like other shark species.
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FAO species catalogue Vol.4. Sharks of the world. An Annotated and Illustrated Catalogue of Shark Species Known to Date Part 2 - Carcharhiniformes.Compagno, L.J.V. 1984. FAO Fish. Synop., (125) Vol.4, Part 2
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Diagnostic Description

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A large, slim shark with a moderately long, flat and rounded snout, large eyes, small jaws, and oblique-cusped teeth with serrations; 2nd dorsal fin low and with greatly elongated rear tip (Ref. 5578). Grey or bluish-grey above, white below; no conspicuous fin markings (Ref. 5578). Only Carcharhinus species with an interdorsal ridge that has the dorsal fin origin behind the free rear tip of the pectoral fin (Ref. 26938).
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Recorder
Cristina V. Garilao
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Life Cycle

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Viviparous, placental (Ref. 50449). 2-15 pups (Ref. 6871, 37816) born at 57-87 cm TL (Ref. 9997); 1-16 pups born at 55-72 cm TL. Females appear to breed every year, but there appears to be no reproductive seasonality (Ref. 58048). Distinct pairing with embrace (Ref. 205).
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Migration

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Oceanodromous. Migrating within oceans typically between spawning and different feeding areas, as tunas do. Migrations should be cyclical and predictable and cover more than 100 km.
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Morphology

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Dorsal spines (total): 0; Dorsal soft rays (total): 0; Analspines: 0; Analsoft rays: 0
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Trophic Strategy

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Often found over deepwater reefs and near insular slopes. 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. Associated with schools of tuna, causing damage to nets and catches. Referred to as the `net-eater shark' in the tropical eastern Pacific. Also in Ref. 9137.
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Crispina B. Binohlan
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Biology

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Found abundantly near the edge of continental and insular shelves, but also in the open sea and occasionally inshore (Ref. 244). Often found in deepwater reefs and near insular slopes (Ref. 244). Littoral and epipelagic, in the open sea or near the bottom at 18-500 m (Ref. 58302). It is quick-moving and aggressive (Ref. 244). Solitary (Ref. 26340); often associated with schools of tuna (Ref. 244). Feeds mainly on fishes, but also squid, paper nautiluses, and pelagic crabs (Ref. 244; 37816). Viviparous (Ref. 50449). Regarded as dangerous to humans (Ref. 9997). Flesh utilized fresh and dried-salted for human consumption; its hide for leather; its fin for shark-fin soup; its liver for oil (Ref. 244). 2 to 14 young, 73 to 87 cm, are born per litter (Ref. 1602).
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Importance

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fisheries: highly commercial; price category: high; price reliability: reliable: based on ex-vessel price for this species
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分布

provided by The Fish Database of Taiwan
分布於全球各溫、熱帶水域。臺灣東北及東部海域可見其蹤跡。
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利用

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主要以流刺網及延繩釣捕獲,經濟價值高。肉質佳,可加工成各種肉製品;鰭可做魚翅;皮厚可加工成皮革;肝可加工製成維他命及油;剩餘物製成魚粉。
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描述

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體呈紡錘型,軀幹較修長。頭寬扁。尾基上下方各具一凹窪。吻平扁中長。眼圓,瞬膜發達。前鼻瓣退化;無口鼻溝或觸鬚。口裂弧形,口閉時上下頜緊合,不露齒;上頜齒側扁,三角形,邊緣具鋸齒,齒尖直立或外斜;下頜齒較窄而直立或略傾斜,邊緣略具鋸齒,基底寬。噴水孔缺如。背鰭2個,背鰭間明顯存在隆脊,第一背鰭中大,起點於胸鰭內角的後方,後緣凹入,下角尖突;第二背鰭小,起點稍後於臀鰭起點,後緣微凹,後角尖突;胸鰭大型,鐮刀形,後緣凹入,外角鈍尖,內角鈍圓,鰭端伸達第一背鰭基底後端;尾鰭寬長,尾椎軸上揚,下葉前部顯著三角形突出,中部低平延長,與後部間有一深缺刻,後部小三角形突出,尾端鈍尖。體背側暗灰色、灰褐色或藍灰色,腹側灰白;各鰭與體同色。
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棲地

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主要棲息於大陸棚或島嶼斜坡緣以及開放水域,亦常被發現於深海,偶可發現於沿海的大型鯊類。善游,移動速度快;通常獨游,但會與鮪魚類集結成群。主要以魚類、烏賊及大洋性蟹類為食。性兇猛,對人類有潛在性的危險。胎生,一胎可產下2-14尾幼鯊,剛出生之幼鯊體長可達57-87公分。
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Silky shark

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The silky shark (Carcharhinus falciformis), also known by numerous names such as blackspot shark, grey whaler shark, olive shark, ridgeback shark, sickle shark, sickle-shaped shark and sickle silk shark, is a species of requiem shark, in the family Carcharhinidae, named for the smooth texture of its skin. It is one of the most abundant sharks in the pelagic zone, and can be found around the world in tropical waters. Highly mobile and migratory, this shark is most often found over the edge of the continental shelf down to 50 m (164 ft). The silky shark has a slender, streamlined body and typically grows to a length of 2.5 m (8 ft 2 in). It can be distinguished from other large requiem sharks by its relatively small first dorsal fin with a curving rear margin, its tiny second dorsal fin with a long free rear tip, and its long, sickle-shaped pectoral fins. It is a deep, metallic bronze-gray above and white below.

With prey often scarce in its oceanic environment, the silky shark is a swift, inquisitive, and persistent hunter. It feeds mainly on bony fishes and cephalopods, and has been known to drive them into compacted schools before launching open-mouthed, slashing attacks. This species often trails schools of tuna, a favored prey. Its sense of hearing is extremely acute, allowing it to localize the low-frequency noises generated by other feeding animals, and, by extension, sources of food. The silky shark is viviparous, meaning that the developing embryos are sustained by a placental connection to their mother. Significant geographical variation is seen in its life history details. Reproduction occurs year-round except in the Gulf of Mexico, where it follows a seasonal cycle. Females give birth to litters of up to 16 pups annually or biennially. The newborn sharks spend their first months in relatively sheltered reef nurseries on the outer continental shelf, growing substantially before moving into the open ocean.

The large size and cutting teeth of the silky shark make it potentially dangerous, and it has behaved aggressively towards divers. However, attacks are rare, as few humans enter its oceanic habitat. Silky sharks are valued for their fins, and to a lesser extent their meat, hide, liver oil, and jaws. Because of their abundance, they form a major component of commercial and artisanal shark fisheries in many countries. Furthermore, their association with tuna results in many sharks being taken as bycatch in tuna fisheries. Although slow-reproducing like most other sharks, the wide distribution and large population size of the silky shark was once thought to buffer the species against these fishing pressures. However, data now suggest that silky shark numbers are declining around the world, which prompted the IUCN to reassess its conservation status to Vulnerable in 2017.

Taxonomy

 src=
Müller and Henle's illustration of a silky shark, accompanying their original species description

A scientific description of the silky shark was first published by the German biologists Johannes Müller and Jakob Henle under the name Carcharias (Prionodon) falciformis, in their 1839 Systematische Beschreibung der Plagiostomen. Subsequent authors have assigned this species to the genus Carcharhinus.[5][6] Because Müller and Henle's type specimen was a 53-cm-long female fetus from Cuba, adult silky sharks were historically not recognized as C. falciformis and were described as a separate species, Carcharhinus floridanus, by Henry Bigelow, William Schroeder, and Stewart Springer in 1943. Jack Garrick, Richard Backus, and Robert Gibbs Jr. synonymized C. floridanus with C. falciformis in 1964.[7]

The specific epithet falciformis is Latin for "sickle-shaped", which refers to the outline of the dorsal and pectoral fins.[3] The silky shark's common name comes from the fine texture of its skin compared to other sharks, a product of its tiny, densely packed dermal denticles.[8] It may also be referred to as blackspot shark (usually used for C. sealei), grey reef shark (usually used for C. amblyrhynchos), grey whaler shark, olive shark, reef shark, ridgeback shark, sickle shark, sickle silk shark, sickle-shaped shark, silk shark, and silky whaler.[9]

Phylogeny and evolution

     

Carcharhinus altimus

   

Carcharhinus plumbeus

       

Carcharhinus falciformis

     

Carcharhinus perezi

     

Carcharhinus galapagensis

   

Carcharhinus obscurus

   

Carcharhinus longimanus

   

Prionace glauca

          Phylogenetic relationships of the silky shark, based on allozyme sequences[10]

Fossilized teeth belonging to the silky shark have been found in North Carolina: from the vicinity of two baleen whales, one in mud dating to the Pleistocene-Holocene (circa 12,000 years ago) and the other in Goose Creek Limestone dating to the Late Pliocene (circa 3.5 million years ago – Mya), as well as from the Pungo River, dating to the Miocene (23–5.3 Mya).[11][12] Fossil teeth have also been found in Pliocene strata at the Cava Serredi quarry in Tuscany, Italy.[13] Carcharhinus elongatus, an earlier representative of its lineage with smooth-edged teeth, is known from Oligocene (34–23 Mya) deposits in the Old Church formation of Virginia, and the Ashley formation of South Carolina. A set of poorly described, Eocene (56–34 Mya) teeth resembling those of this species are known from Egypt.[12]

Initial efforts to resolve the evolutionary relationships of the silky shark were inconclusive; based on morphology, Jack Garrick in 1982 suggested the blackspot shark (C. sealei) as its closest relative.[14] In 1988, Leonard Compagno assigned it phenetically to an informal "transitional group" also containing the blacknose shark (C. acronotus), the blacktip reef shark (C. melanopterus), the nervous shark (C. cautus), the copper shark (C. brachyurus), and the night shark (C. signatus).[15]

More recently, Gavin Naylor's 1992 phylogenetic analysis, based on allozyme sequence data, found that the silky shark is part of a group containing large sharks with a ridge between the dorsal fins. One branch within this group contains the sandbar shark (C. plumbeus) and the bignose shark (C. altimus), while the silky shark is the basal member of the other branch and the sister taxon to a clade containing the Caribbean reef shark (C. perezi), Galapagos shark (C. galapagensis), oceanic whitetip shark (C. longimanus), dusky shark (C. obscurus), and blue shark (Prionace glauca).[10] Mine Dosay-Abkulut's 2008 ribosomal DNA analysis, which included the silky, blue, and bignose sharks, confirmed the closeness of those three species.[16]

Distribution and habitat

Underwater side view of a streamlined olive shark with a pointed snout and a small dorsal fin against blue water
The silky shark is typically encountered in open water.

The silky shark has a cosmopolitan distribution in marine waters warmer than 23 °C (73 °F). 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.[3][5] 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.[3]

Primarily an inhabitant of the open ocean, the silky shark is most common from the surface to a depth of 200 m (660 ft), but may dive to 500 m (1,600 ft) or more.[5] 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 (160 ft) of the surface, and 80–85% of their time in water with a temperature of 26–30 °C (79–86 °F); the pattern was constant regardless of day or night.[17][18] 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 (59 ft).[19] 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 (37 mi) per day, and covering distances up to 1,339 km (832 mi).[20] 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.[21][22] In the northern Atlantic, most sharks follow the Gulf Stream northward along the U.S. East Coast.[20] In the Gulf of Aden, it is most common in late spring and summer.[3]

Description

 src=
Upper teeth
 src=
Lower teeth
A large bronze-colored shark lying on the deck of a boat
A shark lying on its side with its white belly towards the viewer; it has long pectoral fins with dark tips
Distinctive features of the silky shark include its small first dorsal fin and large pectoral fins.

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 (protective third eyelids). Short, shallow furrows are present at the corners of the mouth.[5][23] 14-16 and 13–17 tooth rows are found on either side of the upper and lower jaws, respectively (typically 15 for both). 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.[24]

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.[5][23]

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.[7][8] 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 (except for the first dorsal) darken at the tips; this is more obvious in young sharks.[5][8] The coloration quickly fades to a dull gray after death.[25] One of the largest members of its genus, the silky shark commonly reaches a length of 2.5 m (8.2 ft), with a maximum recorded length and weight of 3.5 m (11 ft) and 346 kg (763 lb), respectively.[9] Females grow larger than males.[8]

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.[26] 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.[5] 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.[27] This shark is often found around floating objects such as logs or tethered naval buoys.[28]

Younger silky sharks are known to form large, loosely organized aggregations, possibly for mutual defense.[29] During migrations, over a thousand individuals may gather.[30] These groups are generally segregated by size, and in the Pacific perhaps also by sex.[8][21][31] 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.[27] 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.[32]

Potential predators of the silky shark include larger sharks and killer whales (Orcinus orca).[33] Known parasites of this shark include the isopod Gnathia trimaculata,[34] the copepod Kroeyerina cortezensis,[35] and the tapeworms Dasyrhynchus variouncinatus and Phyllobothrium sp.[36][37] 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), 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.[29][38]

Feeding

Several spindle-shaped, silvery fish with crescent-shaped tails
Tuna are a favored prey of the silky shark, which is often found trailing their schools.

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, filefishes, triggerfishes, and porcupinefishes. It may also take squid, paper nautilus, and swimming crabs, and fossil evidence indicates it scavenged on whale carcasses.[3][5][11] 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 (a bait ball) and trapping it against the surface, whereupon the sharks consumed the entire school.[3] 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.[29]

Studies conducted off the Florida coast and the Bahamas have shown that silky sharks are highly sensitive to sound, in particular low-frequency (10–20 Hz), 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.[27][29] 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.[33]

The bite force of a 2-m-long silky shark has been measured at 890 newtons (200 lbf).[39] 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".[5][25] 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.[40]

Life history

A shark, smaller than the adults previously shown, but otherwise similar, lying on the deck of a ship
A juvenile silky shark – this species gives birth to live, fully formed young.

Like other members of its family, the silky shark is viviparous: once the developing embryo exhausts its supply of yolk, the depleted yolk sac is converted into a placental connection through which the mother delivers nourishment. Relative to other viviparous sharks, the placenta of the silky shark is less similar to the analogous mammalian structure in that no interdigitation exists between the tissues of the fetus and mother. Furthermore, the fetal red blood cells are much smaller than maternal blood cells, which is opposite the pattern seen in mammals. Adult females have a single functional ovary (on the right side) and two functional uteri, which are divided lengthwise into separate compartments for each embryo.[41]

Silky sharks in most parts of the world are thought to reproduce year-round, whereas mating and birthing in the Gulf of Mexico take place in late spring or early summer (May to August).[19][31] However, in some cases, the presence of reproductive seasonality may have been obscured by biases in data collection.[3] Females give birth after a gestation period of 12 months, either every year or every other year.[1] The litter size ranges from one to 16 and increases with female size, with six to 12 being typical.[3] The pups are born in reef nursery areas on the outer continental shelf, where ample food supplies and protection from large pelagic sharks occur. The risk of predation has selected for fast growth in young sharks, which add 25–30 cm (9.8–11.8 in) to their length within their first year of life. After a few months (or by the first winter in the Gulf of Mexico), the now-subadult sharks migrate out from the nursery into the open ocean.[3][29][31]

The life history characteristics of the silky shark differ across its range (see table). Northwestern Atlantic sharks tend to be larger than those in the western-central Pacific at all ages, while eastern Pacific sharks tend to be smaller than sharks in other regions. Eastern Atlantic and Indian Ocean sharks seem to match or exceed the size of northwestern Atlantic sharks, but the figures are based on relatively few individuals and more data are needed.[3]

The overall growth rate of the silky shark is moderate compared to other shark species and similar for both sexes, though it varies significantly between individuals. One central Pacific study has found females growing much slower than males, but the results may have been skewed by missing data from large females.[19] The highest reported growth rates are from sharks in the northern Gulf of Mexico, and the lowest from sharks off northeastern Taiwan.[46] Males and females reach sexual maturity at ages of 6–10 years and 7–12+ years, respectively.[3] Sharks from more temperate waters may grow slower and mature later than those in warmer regions.[46] The maximum lifespan is at least 22 years.[26]

Human interactions

A brown shark half hauled out of the water by a fishing line coming from the corner of its mouth
A silky shark caught by a sport angler – this shark is heavily fished in many regions.

Given its formidable size and dentition, the silky shark is regarded as potentially dangerous to humans. However, it only rarely comes into contact with people due to its oceanic habits.[8] Its natural curiosity and boldness may lead it to repeatedly and closely approach divers, and it can become dangerously excited in the presence of food. The silky shark tends to be more aggressive if encountered on a reef than in open water. Cases of individual sharks persistently harassing divers and even forcing them out of the water have been reported.[38][48] As of May 2009, the International Shark Attack File lists six attacks attributable to the silky shark, three of them unprovoked and none fatal.[49]

Large numbers of silky sharks are caught by commercial and artisanal multispecies shark fisheries operating off Mexico, Guatemala, El Salvador, Costa Rica, the United States, Ecuador, Spain, Portugal, Sri Lanka, the Maldives, Yemen, and Côte d'Ivoire. Even greater numbers are caught incidentally by tuna longline and purse seine fisheries throughout its range, particularly those using fish aggregating devices. It is the most common shark caught as bycatch in the eastern Pacific and Gulf of Mexico tuna fisheries, and the second-most common shark caught as bycatch (next to the blue shark) overall.[3][50] The fins are valued as an ingredient in shark fin soup, with captured sharks often finned at sea and the rest of the body discarded. Fins from an estimated one-half to one and a half million silky sharks are traded globally per year; it is the second- or third-most common species auctioned on the Hong Kong fin market, which represents over half the global trade.[1][3] The meat (sold fresh or dried and salted), skin, and liver oil may also be used,[5] as well as the jaws: this species is the predominant source of dried shark jaw curios sold to tourists in the tropics.[29] Some sport fishers catch silky sharks.[8]

Conservation

As one of the most abundant and widely distributed sharks on Earth, the silky shark was once thought to be mostly immune to depletion despite heavy fishing mortality. In 1989 alone, some 900,000 individuals were taken as bycatch in the southern and central Pacific tuna longline fishery, seemingly without effect on the total population.[26] Fishery data on this shark are often confounded by under-reporting, lack of species-level separation, and problematic identification. Nevertheless, mounting evidence indicates the silky shark has, in fact, declined substantially worldwide, a consequence of its modest reproductive rate which is unable to sustain such high levels of exploitation. The total annual catch reported to the Food and Agriculture Organization fell steadily from 11,680 tons in 2000 to 4,358 tons in 2004. Regional assessments have found similar trends, estimating declines of some 90% in the central Pacific from the 1950s to the 1990s, 60% off Costa Rica from 1991 to 2000, 91% in the Gulf of Mexico from the 1950s to the 1990s, and 85% (for all large requiem sharks) in the northwestern Atlantic from 1986 to 2005. The silky shark fishery off Sri Lanka reported a drop from a peak catch of 25,400 tons in 1994 to only 1,960 tons in 2006, indicative of a local stock collapse. However, Japanese fisheries in the Pacific and Indian Oceans have recorded no change in catch rate between the 1970s and the 1990s,[1] and the validity of the methodologies used to assess declines in the Gulf of Mexico and the northwestern Atlantic have come under much debate.[51][52][53]

As of 2017, the silky shark is classified by the International Union for Conservation of Nature as a vulnerable species. The silky shark is listed on Annex I, Highly Migratory Species, of the United Nations Convention on the Law of the Sea, though this has yet to result in any management schemes. The species should benefit from bans on shark finning, which are being increasingly implemented by nations and supranational entities, including the United States, Australia, and the European Union.[1] Organizations such as the International Commission for the Conservation of Atlantic Tunas and the Inter-American Tropical Tuna Commission have also taken steps to improve fishery monitoring, with the ultimate goal of reducing shark bycatch.[3] However, given the highly migratory nature of the silky shark and its association with tuna, no simple way is known to reduce bycatch without also affecting the economics of the fishery.[22]

References

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Silky shark: Brief Summary

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The silky shark (Carcharhinus falciformis), also known by numerous names such as blackspot shark, grey whaler shark, olive shark, ridgeback shark, sickle shark, sickle-shaped shark and sickle silk shark, is a species of requiem shark, in the family Carcharhinidae, named for the smooth texture of its skin. It is one of the most abundant sharks in the pelagic zone, and can be found around the world in tropical waters. Highly mobile and migratory, this shark is most often found over the edge of the continental shelf down to 50 m (164 ft). The silky shark has a slender, streamlined body and typically grows to a length of 2.5 m (8 ft 2 in). It can be distinguished from other large requiem sharks by its relatively small first dorsal fin with a curving rear margin, its tiny second dorsal fin with a long free rear tip, and its long, sickle-shaped pectoral fins. It is a deep, metallic bronze-gray above and white below.

With prey often scarce in its oceanic environment, the silky shark is a swift, inquisitive, and persistent hunter. It feeds mainly on bony fishes and cephalopods, and has been known to drive them into compacted schools before launching open-mouthed, slashing attacks. This species often trails schools of tuna, a favored prey. Its sense of hearing is extremely acute, allowing it to localize the low-frequency noises generated by other feeding animals, and, by extension, sources of food. The silky shark is viviparous, meaning that the developing embryos are sustained by a placental connection to their mother. Significant geographical variation is seen in its life history details. Reproduction occurs year-round except in the Gulf of Mexico, where it follows a seasonal cycle. Females give birth to litters of up to 16 pups annually or biennially. The newborn sharks spend their first months in relatively sheltered reef nurseries on the outer continental shelf, growing substantially before moving into the open ocean.

The large size and cutting teeth of the silky shark make it potentially dangerous, and it has behaved aggressively towards divers. However, attacks are rare, as few humans enter its oceanic habitat. Silky sharks are valued for their fins, and to a lesser extent their meat, hide, liver oil, and jaws. Because of their abundance, they form a major component of commercial and artisanal shark fisheries in many countries. Furthermore, their association with tuna results in many sharks being taken as bycatch in tuna fisheries. Although slow-reproducing like most other sharks, the wide distribution and large population size of the silky shark was once thought to buffer the species against these fishing pressures. However, data now suggest that silky shark numbers are declining around the world, which prompted the IUCN to reassess its conservation status to Vulnerable in 2017.

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Description

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Found abundantly near the edge of continental and insular shelves, but also in the open sea and occasionally inshore. Recorded temperatures where it occurs range from 23-24°C. It is quick-moving and aggressive. Associated with schools of tuna. Primarily feeds on fishes, but also cephalopods, and even crabs. Used in many ways with its meat eaten fresh and dried-salted; its hide for leather; its fin for shark-fin soup; its liver oil as a rich source of vitamin A.
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Froese, R. & D. Pauly (Editors). (2021). FishBase. World Wide Web electronic publication. version (08/2021). North-West Atlantic Ocean species (NWARMS) North-West Atlantic Ocean species (NWARMS)
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Edward Vanden Berghe [email]
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Distribution

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Circumtropical. Western Atlantic: Massachusetts, USA to southern Brazil, including the Gulf of Mexico and the Caribbean
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Froese, R. & D. Pauly (Editors). (2021). FishBase. World Wide Web electronic publication. version (08/2021). North-West Atlantic Ocean species (NWARMS) North-West Atlantic Ocean species (NWARMS)
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Kennedy, Mary [email]
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Habitat

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Froese, R. & D. Pauly (Editors). (2021). FishBase. World Wide Web electronic publication. version (08/2021). North-West Atlantic Ocean species (NWARMS) North-West Atlantic Ocean species (NWARMS)
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Kennedy, Mary [email]
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Kennedy, Mary [email]