Geographic Range
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
- atlantic ocean
- pacific ocean
- Other Geographic Terms
- cosmopolitan
Habitat
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.
- Habitat Regions
- temperate
- tropical
- saltwater or marine
Physical Description
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.
- Other Physical Features
- endothermic
- heterothermic
- bilateral symmetry
- Sexual Dimorphism
- female larger
Development
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
Reproduction
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.
- Key Reproductive Features
- iteroparous
- seasonal breeding
- year-round breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- fertilization
- 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
-
provisioning
Lifespan/Longevity
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.
Behavior
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.
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.
- Key Behaviors
- natatorial
- diurnal
- nocturnal
- crepuscular
- motile
- migratory
- solitary
Home Range
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.
Communication and Perception
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.
- Other Communication Modes
- pheromones
Food Habits
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.
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.
- Primary Diet
-
carnivore
- piscivore
- eats non-insect arthropods
- molluscivore
- Animal Foods
- fish
- mollusks
- aquatic crustaceans
Predation
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.
Ecosystem Roles
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
.
- Gnathia trimaculata (Order Isopoda: Subphylum Crustacea)
- Kroeyerina cortezensis (Subclass Copepoda: Subphylum Crustacea)
- Dasyrhynchus variouncinatus (Class Cestoda: Phylum Platyhelminthes)
- Phyllobothrium sp. (Class Cestoda: Phylum Platyhelminthes)
Economic Importance for Humans: Positive
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
Economic Importance for Humans: Negative
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
Conservation Status
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.
Additional Links
Contributors
Jessica Frazelle (author), Radford University, Karen Powers (editor), Radford University, Kiersten Newtoff (editor), Radford University, Melissa Whistleman (editor), Radford University, Jeremy Wright (editor), University of Michigan-Ann Arbor.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Atlantic Ocean
-
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.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- 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.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- cosmopolitan
-
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.
- temperate
-
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).
- tropical
-
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
- saltwater or marine
-
mainly lives in oceans, seas, or other bodies of salt water.
- pelagic
-
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
- reef
-
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.
- coastal
-
the nearshore aquatic habitats near a coast, or shoreline.
- endothermic
-
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.
- heterothermic
-
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.
- bilateral symmetry
-
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.
- indeterminate growth
-
Animals with indeterminate growth continue to grow throughout their lives.
- polygynous
-
having more than one female as a mate at one time
- iteroparous
-
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).
- seasonal breeding
-
breeding is confined to a particular season
- year-round breeding
-
breeding takes place throughout the year
- sexual
-
reproduction that includes combining the genetic contribution of two individuals, a male and a female
- fertilization
-
union of egg and spermatozoan
- internal fertilization
-
fertilization takes place within the female's body
- ovoviviparous
-
reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.
- female parental care
-
parental care is carried out by females
- natatorial
-
specialized for swimming
- diurnal
-
- active during the day, 2. lasting for one day.
- nocturnal
-
active during the night
- crepuscular
-
active at dawn and dusk
- motile
-
having the capacity to move from one place to another.
- migratory
-
makes seasonal movements between breeding and wintering grounds
- solitary
-
lives alone
- visual
-
uses sight to communicate
- chemical
-
uses smells or other chemicals to communicate
- pheromones
-
chemicals released into air or water that are detected by and responded to by other animals of the same species
- visual
-
uses sight to communicate
- tactile
-
uses touch to communicate
- acoustic
-
uses sound to communicate
- vibrations
-
movements of a hard surface that are produced by animals as signals to others
- chemical
-
uses smells or other chemicals to communicate
- electric
-
uses electric signals to communicate
- carnivore
-
an animal that mainly eats meat
- piscivore
-
an animal that mainly eats fish
- molluscivore
-
eats mollusks, members of Phylum Mollusca
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