Silky sharks (Carcharhinus falciformis) are large pelagic sharks that got their name from the Silky look of their skin. They are species of requiem shark, belonging to the family of Carcharhinus. These amazing sharks occur worldwide in warm temperate and tropical waters.
Also, they have many common names, which include grey whaler shark, blackspot shark, olive shark, sickle shark, ridgeback shark, sickle silk shark, and sickle-shaped shark.
Scientific Classification
Kingdom – Animalia
Phylum – Chordata
Class – Chondrichthyes
Order – Carcharhiniformes
Family – Carcharhinidae
Genus – Carcharhinus
Species – C. falciformis
Name Development
German biologists Jakob Henle and Johannes Muller first described the silky shark as *Carcharias (Prionodon) falciformis*. This description appeared in their 1839 book titled *Systematische Beschreibung der plagiostomen*.
Subsequent authors assigned this species to the genus *Carcharhinus*. Of course, there was no historical recognition of adult silky sharks as *C. falciformis*. This is because Henle and Muller’s type specimen was a female fetus 53 cm long from Cuba. Hence adults were described as *Carcharhinus floridanus*, a separate species by William Schroeder, Stewart springer, and Henry Bigelow, in 1943.
In 1964, Richard Backus, Jack Garrick, Robert Gibbs, Jr, synonymized *C. falciformis*with *C. floridanus*.
The specific epithet falciformis means “Sickle-shaped” in Latin, and it refers to the outline of the dorsal and pectoral fins of these sharks. The popular common name “Silky sharks” comes from the smooth texture of their skin.
Compared to other sharks, this skin appearance is unique, and it is a result of their tiny, densely packed dermal denticles.
Historical Development of Silky Sharks
Fossilized teeth of Silky sharks have been found In North Carolina. This is from the vicinity of two baleen whales, one in Goose creek limestone dating to the late Pliocene (circa 3.5 million years ago –Mya). And the other, in mud, dating to the Pleistocene –Holocene (circa 12,000 years ago).
There have also been fossil teeth in Pliocene strata at the Cava serrendi quarry in Tuscany, Italy. Carcharhinus elongatus, which is an earlier representative of silk shark lineage with smooth-edged teeth is known from Oligocene deposit in the Ashley formation of South Carolina and the old church formation of Virginia. Also, a set of poorly described Eocene (54 -34 Mya) teeth resembling that of silky sharks are known from Egypt.
First efforts to solve the evolutionary relationships of these sharks were inconclusive. Based on morphology, Jack Garrick in 1982 proposed the blackspot shark (C. sealei) as their closest relative. Leonard Compagno in 1988 assigned silky sharks phylogenetically to an informal “transitional group” also containing blacknose sharks (C.acronotus), the nervous shark (C. cautus), the blacktip reef shark (C. melanopterus), the copper shark (C.brachyurus) and the night shark (C. signatus).
Groups
A phylogenetic analysis conducted in 1992 by Gavin Naylor using the allozyme sequence data, recently discovered that silky sharks are part of a group of large sharks with a ridge between the first and second dorsal fins. One branch within the group includes bignose shark *(C. altimus)* and sandbar shark *(C. plumbeus)*.
Silky sharks are the basal member of the other branch and the sister taxon to a clade comprising Galapagos shark (C. galapagensis), Oceanic whitetip shark (C. longimanus), the Caribbean reef shark (C. perezi), Blue shark (Prionace glauca), and dusky shark (C. obscurus). Mine Dosay-Abkulut’s 2008 ribosomal DNA analysis that includes the silky, blue and bignose sharks confirmed the closeness of the three species.
Distribution Range
Silky sharks are highly migratory. In Atlantic Ocean, they occur from the U.S. state of Massachusetts to Spain in the north and from south brazil to north Angola in the south that includes Gulf of Mexico, Meditteranean sea and Caribbean sea.
These sharks occur throughout Indian Ocean, as far south as Mozambique in the west, western Australia in the east, Persian gulf and the red sea.
In the Pacific ocean, in the north, their range runs from southern china and japan to southern Baja California and the Gulf of California. And, in the south, they run from Australia, Sydney, to northern New Zealand to northern chile.
Due to life history differences, there are four distinct populations of silky sharks in oceans worldwide: in the north Atlantic, west and central Pacific, in the eastern Pacific and the Indian ocean.
Natural habitat
Although Silky sharks are pelagic, they are not totally an open ocean species. Thus, they also occur from depth as shallow as 56 ft (18 meters). Silky sharks are active, swift sharks that prefer warmer water (about 23 degrees Celsius). They are commonly found near edges of continental shelves and over deepwater reefs where their food source is abundant.
Generally, these sharks range from the surface down to at least 1,550 ft (500 meters). But, they can dive to water as deep as 12,400 ft (4000 meters). Research showed no strong tendency for gender segregation in silky sharks.
Meanwhile, they often travel with others of their size, indicating that size segregation is present within them. Silky sharks are highly mobile and migratory species, even though there are less data on their movements.
Research shows that more massive silky sharks move longer distances than the smaller ones. In the pacific ocean and other places, they spend summer at slightly higher latitudes mostly during warmer Nino years. Typically, young silky sharks stay mostly in coastal nurseries and their adults further offshore over deeper water. Silky sharks juveniles popularly relate with schools of tuna.
Description of Silky sharks
Silky sharks have a slim and streamline body. Their snout is relatively long and rounded with barely developed flaps of skin in front of their nostrils. They have round and medium-sized eyes that have nictitating membranes (third eyelid) that can protect the eyes when feeding. Silky sharks have five pairs of gill slits that are moderate in length.
Dentition
These sharks have short, shallow furrows at the corners of their mouth. Silky sharks have about 14 to 16 and 13 to 17 tooth rows on their upper and lower jaws. Their upper teeth are triangular and sharply serrated with a notch in their posterior edge which is erect at the center and more oblique towards the sides. The lower teeth are long and smooth-edged.
Fins
These migratory sharks have dorsal and pectoral fins that are unique and help to differentiate them from similar species. Their first dorsal fin is relatively small, measuring less than a tenth as high as Silky shark is long and originates behind the free rear tips of pectoral fins.
The second dorsal fin is tiny and smaller than the anal fin with a drawn-out free rear tip up to three times as long as the fin is tall. They have a narrow dorsal ridge run between the dorsal fins.
Silky sharks anal fins originate a little bit ahead of the second dorsal fin and have a deep notch in the posterior margin. Furthermore, their caudal fin is relatively high and has well developed lower lobe.
Dermal Denticles
Minute overlapping dermal denticles cover silky sharks’ skin. Each dermal denticle has a diamond shape and bears horizontal ridges leading to their posterior marginal teeth which increases in number as the shark age.
Coloration
Their back is metallic golden brown to dark grey with a snowy white color underneath their belly which continues onto the flank as a faint lighter stripe. The fins of silky shark except for the first dorsal, darken at the tips and is more evident in juveniles than in adults. The coloration fades to dull grey after death.
Size, Age and Growth
Silky sharks have a slender streamlined body and grow to a maximum length of 10 ft (3.3 m). Their males mature at 9 to 10 years of age (215 -230 cm). Females of Silky sharks are larger than the males, and they reach maturity at 12 years of age (230 -245 cm).
However, these numbers may vary by population. For instance, populations in the pacific and Indian oceans typically have a smaller size at maturation. The size of silky sharks pups at birth is 2 to 2.5 ft (70 – 85 cm). Silky sharks have a life span of 22 years or more.
Typical Behavior of Silky Sharks
Silky sharks are one of the three most pelagic sharks along with blue and oceanic whitetip sharks. They are among the most numerous large marine animals in the world. Compared to the two other sharks (blue and Oceanic whitetip sharks), they are less strictly pelagic. Although, the highest numbers occur in offshore waters with link to the land where food is more readily available than farther out in the open ocean.
Silky sharks are an active, curious and aggressive predator. When approaching something of interest, these sharks may seem inattentive, slowly circling, swinging their head from side to side sometimes. However, they can respond with startling swiftness to any shift in their immediate surroundings. Silky sharks can swim around floating objects like tettered naval buoys or logs.
Social Grouping
Juveniles of silky sharks also form large groups loosely organized possibly for mutual defense. During migrations, thousand of young silky sharks gather and generally segregated by size and in the pacific by sex. These sharks in a group have been observed to “tilt” showing their full lateral profile towards each other, also puff out their gills or gape their jaws.
Occasionally, silky sharks have also been seen suddenly charging straight up, veering away just before reaching the surface and gliding back down to deeper oceans. It is not sure yet the importance of this behavior.
Threat Display
When silky sharks feel threatened, they perform a threat display in which they arch their back, drop their tail, pectoral fins and lift their head. These sharks then proceed to swim in tight loops with a stiff, jerky motion, most times turning broadside towards the perceived threat. Silky sharks often intermingle with schools of scalloped hammerheads (Sphyrna lewini) and are known to follow marine mammals.
One record from the Red seas describes 25 silky sharks following a large group of bottlenose dolphins along with 25 grey reef sharks (C. amblyrhynchos) and a “lone” silvertip shark (C. albimarginatus). Young pilot fish *(Naucrates doctor)* in turn, follows these sharks while swimming against the pressure wave ahead of silky sharks. There are also, jacks that follow along attempting to snatch scraps of food and rubbing against silky sharks’ skin to scrape off parasites.
Feeding Habits
Silky sharks are opportunistic predators. Their primary food consists of Bony fishes from all levels of the water column, including mackerel, tuna, mullets, sardines, groupers, snappers, mackerel scads, sea catfish, sea chubs, lanternfishes, triggerfishes, file fishes, and porcupinefishes. They also feed on squid, swimming crabs, and paper nautilus. Fossil evidence indicates silky sharks scavenged on whale carcasses.
In the pacific, silky sharks have been documented to feed in large numbers, they were recorded “herding” a school of fish into a compact mass (bait ball) and trapping them against the surface where they consumed the entire school.
When attacking packed fish, Silky sharks charge through the ball and cut open-mouthed, capturing the prey fish at the corners of their jaws. Though multiple silky sharks may feed at once, they launch their attack independently.
Adaptations for Tracking Prey
Research conducted off the coast of Florida, and the Bahamas showed that silky sharks are highly sensitive to sound. This is mainly low frequency (10 – 20 Hz), irregular pulses. When these sounds were played underwater, it attracted sharks from hundreds of meters away. Silky sharks possibly orient to these sounds because they are similar to the noise produced by feeding animals such as dolphins or birds, thus indicating a promising source of food.
These studies have also established that a silky shark attracted by this sound will quickly withdraw if the sound changes suddenly in character or amplitude. This sudden change does not need to relate to a sound that a predator produces to evoke such a response. Hence, it comes as a reflex.
Bite Force
The bite force of a 2 m long silky sharks is measured at 200 Ibf (890 newtons). These species have a well-established association with tuna fish. Off Ghana, nearly all tuna school has silky sharks trailing behind them. In the east Pacific, Silky sharks inflict severe damage to tuna fishing gear and catch, which earned them the name “net-eating sharks” from fishery workers.
Silky Sharks and Bottlenose Dolphins
These sharks and bottlenose dolphins compete when both of them target the same school of fish. The amount eaten by dolphins reduces, depending on the number of Silky sharks available. If a large number of sharks are present, they remain inside the prey school.
At the same time, the dolphins gather themselves to the periphery to avoid accidental injury from silky sharks slashing attacks. However, if a large enough group of dolphins gathers, they can chase Silky sharks away from the prey items. No matter which one dominates, the two predators do not engage in overtly aggressive behavior towards one another.
Predators/Parasites of Silky Sharks
Predators of these sharks include large sharks like Tiger sharks, great white sharks, and killer whales (Orcinus orca). Known parasites of silky sharks include the copepod Kroeyerina cortezensis, the isopod Gnathia trimaculata, the tapeworms Dasyrhynchus variouncinatus and phyllobothrium sp.
Mating and Reproduction
Like every other member of their family, Silky sharks are viviparous, i.e. they give birth to live pups. Compared to other viviparous sharks, silky sharks placenta is less similar in that no interdigitation exists between the mother and the tissues of the fetus.
Also, the fetal red blood cells are much smaller than maternal blood cells which are in contrast to what is seen in mammals. Adult females silky sharks have a single functional ovary. And, on the other hand, two functional uteri. The uteri are divided lengthwise into different compartments for each embryo.
These sharks in most parts of the world are likely to reproduce year-round, whereas mating and birthing take place in late spring or early summer in the Gulf of Mexico (May to August). Their females give birth after a gestation period of 12 months, sometimes every year or every other year.
The risk of predation on juvenile silky sharks tends to be low because of their high growth rate. They add about 9.8 to 11.8 inches (25 to 30 cm) to their length in the first year. After some months, the now subadult sharks move out of their nursery site into the open ocean. Silky sharks pups are born in reef nursery areas on the outer continental shelf where there are food supplies and protection from predators.
Human Interaction
Threat to Humans
Due to their formidable size and dentition, silky sharks prove to be potentially dangerous to humans. However, they rarely come in contact with people due to their oceanic habits. These sharks are naturally curious and bold; these may cause them to approach divers and also become dangerously excited in the presence of food.
Attacks on Humans
They seem to be more aggressive when encountered on the reef than in open water. There have been reported cases of individual sharks persistently harassing divers and sometimes even forcing them out of the water. In May 2009, the International Shark Attack File (ISAF) listed the sixth attack attributed to silky sharks, three unprovoked and non-fatal.
Human Threats to Silky Sharks
These sharks are caught in large numbers by commercial and artisanal multispecies shark fisheries operating off Guatemala, Mexico, Costa Rica, El Salvador, Ecuador, the United States, Portugal, Spain, the Maldives, Sri Lanka, Cote d”Ivoire and Yemen. They are the most common shark caught as bycatch in eastern pacific And Gulf of Mexico tuna fisheries, and the second most common shark caught as bycatch.
Importance to Humans
Silky sharks are of great importance to longline and gillnet fisheries in many parts of the world. In the Gulf of Mexico, they are often captured as bycatch in the tuna fishery but are also harvested by the directed shark fishery. In the Caribbean, they are sometimes fished majorly by longline but are not a common catch.
Likewise, in Sri Lanka and the Maldives, they are the most critical shark species comprising about 70 to 80 % of the pelagic longline catch. In Japan, silky sharks are commonly taken in the directed shark fishery but also in the swordfish as bycatch. In contrast, around the mid-Atlantic coasts of the states, Silky sharks are fished but not in significant numbers.
They are used for their oils and fins, and the fins are valued as an ingredient in shark fin soup. These sharks are the third most auctioned on the Hongkong fin market, which represents over half of the global trade.
Silky sharks are also taken by recreational fishermen. Apart from their importance to various fisheries, these sharks have been used in multiple scientific studies investigating their sensory biology.
Conservation
In 2017, the International Union for Conservation of Nature (IUCN) assessed the silky shark as a “Vulnerable (VU)” species. These species should benefit from bans on shark finning, which several nations and supranational entities including Australia, the United States, and the European Union are implementing.
Organizations such as the Inter-America Tropical Tuna and International commission for the Conservation of Atlantic Tunas have also taken steps to improve fishery monitoring, intending to reduce bycatch. Nevertheless, given the highly migratory nature of the silky shark and their association with tuna, there is no known simple way to reduce bycatch without affecting the economy of the fishery.
Cool Silky Shark Facts at a Glance
1. They Got Their Common Name from the Smooth Appearance of Their Skin
The skin of silky sharks has tiny dermal denticles overlapping each other covering their skin. This gives the very smooth, silky appearance, hence, their common name “silky sharks”.
2. Their Dorsal and Pectoral Fins are Falcate
Silky sharks’ dorsal and pectoral fins appear falcate, which is sickle-shaped. As a result, the specific epithet falciformis is a Latin word meaning “sickle-shaped”.
3. Silky Sharks Have Interdorsal Ridge between the First and Second Dorsal Fins
This shark is in the group of large sharks with a ridge running between their dorsal fins.
4. Silky Sharks are Not Strict Open Water Species
Unlike most pelagic species, silky sharks occasionally swim to shallow waters at depths of about 56 feet (18 m).
5. These Sharks are Highly Migratory Species
Silky are long-distance travelers with larger individuals covering more distance than smaller ones. During migration, they may form groups segregated by sex or size.
6. The Unique Coloration of this Species Fade Upon Death
Silky sharks have unique metallic golden brown to gray coloration on their backside with a white color underneath. This color fades to dark gray after the shark dies.
7. They Perform Threat Display to Warn of an Attack
When these sharks feel threatened, they perform some specific gestures before approaching the threat in a rather broadside manner. This certainly shows their readiness for an attack.
8. Silky Sharks are Active, Curious, and Aggressive
While these sharks may appear inattentive and swimming in rather slow motion, they respond swiftly to any sudden movement around them.
9. These Sharks Actively Respond to Sound
They have high sensitivity to low-frequency irregular sounds 10 to 20 Hz. This helps them while hunting for prey.
10. Larger Sharks Prey on this Species
Larger sharks such as the great white shark, tiger sharks, killer whales (Orcinus orca) serve as major predators to the silky shark species.
Further Reading
- “Discover Fishes: Carcharhinus falciformis, Silky Shark”, Florida Museum.
- “Silky Shark, Carcharhinus falciformis“, The MarineBio Conservation Society.
- “Carcharhinus falciformis, Silky Shark”, FishBase.
- “Silky Shark (Carcharhinus falciformis)“, the IUCN Red List of Threatened Species.
- “Silky Shark”, Wikipedia (accessed, April 20, 2020).
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