Silky shark
Template:Short descriptionTemplate:Cs1 config Template:Featured article Template:Speciesbox The silky shark (Carcharhinus falciformis), also known by numerous names such as blackspot shark, gray 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 Template:Convert. The silky shark has a slender, streamlined body and typically grows to a length of Template:Convert. 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 of the silky shark was once thought to buffer the species against fishing pressures. However, data suggest that silky shark numbers are declining around the world, which prompted the IUCN to reassess its conservation status to Vulnerable in 2017.
TaxonomyEdit
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.<ref name="compagno"/><ref name="muller and henle"/> 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.<ref name="garrick et al"/>
The specific epithet falciformis is Latin for "sickle-shaped", which refers to the outline of the dorsal and pectoral fins.<ref name="bonfil"/> 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.<ref name="knickle"/> 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.<ref name="fishbase"/>
Phylogeny and evolutionEdit
Template:Cladogram 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).<ref name="cicimurri and knight"/><ref name="bourdon"/> Fossil teeth have also been found in Pliocene strata at the Cava Serredi quarry in Tuscany, Italy.<ref name="carnevale et al"/> 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.<ref name="bourdon"/>
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.<ref name="garrick"/> 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).<ref name="compagno2"/>
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).<ref name="naylor"/> Mine Dosay-Abkulut's 2008 ribosomal DNA analysis, which included the silky, blue, and bignose sharks, confirmed the closeness of those three species.<ref name="dosay-abkulut"/>
Distribution and habitatEdit
The silky shark has a cosmopolitan distribution in marine waters warmer than Template:Convert. In the Atlantic Ocean, it is found from Massachusetts (USA) to Spain in the north, and from southern Brazil to northern Angola in the south, including the Gulf of Mexico and the Caribbean Sea. In the Mediterranean Sea, it has been recorded first in the Alboran Sea, subsequently in Algerian waters, the Gulf of Gabes (Tunisia) and more recently in the Ligurian Sea.<ref>Atlas of Exotic Fishes in the Mediterranean Sea (Carcharhinus falciformis). 2nd Edition. 2021. 366p. CIESM Publishers, Paris, Monaco.https://ciesm.org/atlas/fishes_2nd_edition/Carcharhinus_falciformis.pdf</ref> 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.<ref name="bonfil"/><ref name="compagno"/> 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.<ref name="bonfil"/>
Primarily an inhabitant of the open ocean, the silky shark is most common from the surface to a depth of Template:Convert, but may dive to Template:Convert or more.<ref name="compagno"/> Tracking studies in the tropical eastern Pacific and northern Gulf of Mexico have found that cruising silky sharks spend 99% of their time within Template:Convert of the surface, and 80–85% of their time in water with a temperature of Template:Convert; the pattern was constant regardless of day or night.<ref name="kohin et al"/><ref name="hoffmayer et al"/> 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 Template:Convert.<ref name="bonfil et al"/> 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 Template:Convert per day, and covering distances up to Template:Convert.<ref name="kohler et al"/> 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.<ref name="strasburg"/><ref name="watson et al"/> In the northern Atlantic, most sharks follow the Gulf Stream northward along the U.S. East Coast.<ref name="kohler et al"/> In the Gulf of Aden, it is most common in late spring and summer.<ref name="bonfil"/>
DescriptionEdit
Template:Multiple image 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.<ref name="compagno"/><ref name="mceachran and fechhelm"/> 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.<ref name="randall and hoover"/>
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.<ref name="compagno"/><ref name="mceachran and fechhelm"/>
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.<ref name="garrick et al"/><ref name="knickle"/> 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.<ref name="compagno"/><ref name="knickle"/> The coloration quickly fades to a dull gray after death.<ref name="bane"/> One of the largest members of its genus, the silky shark commonly reaches a length of Template:Convert, with a maximum recorded length and weight of Template:Convert and Template:Convert, respectively.<ref name="fishbase"/> Females grow larger than males.<ref name="knickle"/>
Biology and ecologyEdit
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.<ref name="fowler et al"/> 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.<ref name="compagno"/> 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.<ref name="myrberg et al"/> This shark is often found around floating objects such as logs or tethered naval buoys.<ref name="perrine"/>
Younger silky sharks are known to form large, loosely organized aggregations, possibly for mutual defense.<ref name="martin"/> During migrations, over a thousand individuals may gather.<ref name="villegas and sesana"/> These groups are generally segregated by size, and in the Pacific perhaps also by sex.<ref name="knickle"/><ref name="strasburg"/><ref name="branstetter"/> 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.<ref name="myrberg et al"/> 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.<ref name="martin2"/>
Potential predators of the silky shark include larger sharks and killer whales (Orcinus orca).<ref name="myrberg"/> Known parasites of this shark include the isopod Gnathia trimaculata,<ref name="ota and euichi"/> the copepod Kroeyerina cortezensis,<ref name="deets"/> and the tapeworms Dasyrhynchus variouncinatus and Phyllobothrium sp.<ref name="beveridge and campbell"/><ref name="whittaker et al"/> 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.<ref name="martin"/><ref name="stafford-deitsch"/>
FeedingEdit
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.<ref name="bonfil"/><ref name="compagno"/><ref name="cicimurri and knight"/> 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.<ref name="bonfil"/> 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.<ref name="martin"/>
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.<ref name="myrberg et al"/><ref name="martin"/> 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.<ref name="myrberg"/>
The bite force of a 2-m-long silky shark has been measured at Template:Convert.<ref name="evans and gilbert"/> 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".<ref name="compagno"/><ref name="bane"/> 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.<ref name="acevedo-gutierrez"/>
Life historyEdit
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.<ref name="gilbert and schlernitzauer"/>
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).<ref name="bonfil et al"/><ref name="branstetter"/> However, in some cases, the presence of reproductive seasonality may have been obscured by biases in data collection.<ref name="bonfil"/> Females give birth after a gestation period of 12 months, either every year or every other year.<ref name="iucn"/> The litter size ranges from one to 16 and increases with female size, with six to 12 being typical.<ref name="bonfil"/> 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 Template:Convert 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.<ref name="bonfil"/><ref name="martin"/><ref name="branstetter"/>
| Region | Length at birth | Male length at maturity | Female length at maturity |
|---|---|---|---|
| Northwestern Atlantic | Template:Convert<ref name="bonfil"/> | Template:Convert<ref name="fowler et al"/> | Template:Convert<ref name="fowler et al"/> |
| Eastern Atlantic | ? | Template:Convert<ref name="cadenat and blanche"/> | Template:Convert<ref name="bane"/><ref name="cadenat and blanche"/> |
| Indian | Template:Convert<ref name="bonfil"/> | Template:Convert<ref name="bonfil"/><ref name="stevens"/> | Template:Convert<ref name="bonfil"/><ref name="stevens"/> |
| Western Pacific | ? | Template:Convert<ref name="stevens2"/><ref name="stevens and mcloughlin"/> | Template:Convert<ref name="stevens2"/><ref name="joung et al"/> |
| Central Pacific | Template:Convert<ref name="joung et al"/> | Template:Convert<ref name="oshitani et al"/> | Template:Convert<ref name="strasburg"/><ref name="oshitani et al"/> |
| Eastern Pacific | Template:Convert<ref name="bonfil"/> | Template:Convert<ref name="iucn"/><ref name="bonfil"/> | Template:Convert<ref name="iucn"/><ref name="bonfil"/> |
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.<ref name="bonfil"/>
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.<ref name="bonfil et al"/> The highest reported growth rates are from sharks in the northern Gulf of Mexico, and the lowest from sharks off northeastern Taiwan.<ref name="joung et al"/> Males and females reach sexual maturity at ages of 6–10 years and 7–12+ years, respectively.<ref name="bonfil"/> Sharks from more temperate waters may grow slower and mature later than those in warmer regions.<ref name="joung et al"/> The maximum lifespan is at least 22 years.<ref name="fowler et al"/>
Human interactionsEdit
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.<ref name="knickle"/> 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.<ref name="stafford-deitsch"/><ref name="stafford-deitsch2"/> As of May 2009, the International Shark Attack File lists six attacks attributable to the silky shark, three of them unprovoked and none fatal.<ref name="isaf"/>
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.<ref name="bonfil"/><ref name="camhi et al"/> 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.<ref name="iucn"/><ref name="bonfil"/> The meat (sold fresh or dried and salted), skin, and liver oil may also be used,<ref name="compagno"/> as well as the jaws: this species is the predominant source of dried shark jaw curios sold to tourists in the tropics.<ref name="martin"/> Some sport fishers catch silky sharks.<ref name="knickle"/>
ConservationEdit
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.<ref name="fowler et al"/> 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,<ref name="iucn"/> and the validity of the methodologies used to assess declines in the Gulf of Mexico and the northwestern Atlantic have come under much debate.<ref name="burgess et al"/><ref name="baum et al"/><ref name="burgess et al2"/>
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.<ref name="iucn"/> 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.<ref name="bonfil"/> 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.<ref name="watson et al"/>
ReferencesEdit
External linksEdit
- Carcharhinus falciformis, Silky shark at FishBase
- Template:SealifePhotos
- Biological Profiles: Silky Shark Template:Webarchive at Florida Museum of Natural History Ichthyology Department
- Open Ocean: Silky Shark at ReefQuest Centre for Shark Research
- Species Description of Carcharhinus falciformis at www.shark-references.com
Template:Carcharhinidae Template:Taxonbar Template:Authority control