Dusky shark
Encyclopedia
The dusky shark is a species
of requiem shark
, family Carcharhinidae, occurring in tropical and warm-temperate continental seas worldwide. A generalist
apex predator
, the dusky shark can be found from the coast to the outer continental shelf
and adjacent pelagic waters, and has been recorded from a depth of 400 m (1,300 ft). Populations migrate
seasonally towards the poles in the summer and towards the equator
in the winter, traveling hundreds to thousands of kilometers. One of the largest members of its genus, the dusky shark reaches 4.2 m (14 ft) in length and 347 kg (765 lb) in weight. It has a slender, streamlined body and can be identified by its short round snout, long sickle-shaped pectoral fins, ridge between the first and second dorsal fin
s, and faintly marked fins.
Adult dusky sharks have a broad and varied diet, consisting mostly of bony fishes, sharks and rays, and cephalopod
s, but also occasionally crustacean
s, sea star
s, bryozoans, sea turtle
s, marine mammal
s, carrion
, and garbage
. This species is viviparous with a three-year reproductive cycle; females bear litters of 3–14 young after a gestation period
of 22–24 months, after which there is a year of rest before they become pregnant again. Females are capable of storing sperm
for long periods, as their encounters with suitable mates may be few and far between due to their nomadic lifestyle and low overall abundance. Dusky sharks are one of the slowest-growing and latest-maturing sharks, not reaching adulthood until around 20 years of age.
Because of its slow reproductive rate, the dusky shark is very vulnerable to human-caused population depletion. This species is highly valued by commercial fisheries
for its fins, used in shark fin soup
, and for its meat, skin, and liver oil
. It is also esteemed by recreational fishers
. The International Union for Conservation of Nature (IUCN) has assessed this species as Near Threatened
worldwide and Vulnerable
off the eastern United States, where populations have dropped to 15–20% of 1970s levels. The dusky shark is regarded as potentially dangerous to humans due to its large size, but there are few attacks
attributable to it.
French naturalist
Charles Alexandre Lesueur
published the first scientific description of the dusky shark in an 1818 issue of Journal of the Academy of Natural Sciences of Philadelphia. He placed it in the genus Squalus and gave it the specific epithet obscurus (Latin
for "dark" or "dim"), referring to its coloration. Subsequent authors have recognized this species as belonging to the genus Carcharhinus. Lesueur did not designate a type specimen, though he was presumably working from a shark caught in North American waters.
Many early sources gave the scientific name of the dusky shark as Carcharias (later Carcharhinus) lamiella, which originated from an 1882 account by David Starr Jordan
and Charles Henry Gilbert
. Although Jordan and Gilbert referred to a set of jaws that came from a dusky shark, the type specimen they designated was later discovered to be a copper shark (C. brachyurus). Therefore, C. lamiella is not considered a synonym
of C. obscurus but rather of C. brachyurus. Other common name
s for this species include bay shark, black whaler, brown common gray shark, brown dusky shark, brown shark, common whaler, dusky ground shark, dusky whaler, river whaler, shovelnose, and slender whaler shark.
(23-5.3 Ma) have been recovered from the Kendeace and Grand Bay formations in Carriacou, the Grenadines, the Moghra Formation in Egypt, Polk County, Florida
, and possibly Cerro La Cruz in northern Venezuela. Teeth dating to the Late Miocene
or Early Pliocene (11.6-3.6 Ma) are abundant in the Yorktown Formation and the Pungo River
, North Carolina, and from the Chesapeake Bay
region; these teeth differ slightly from the modern dusky shark, and have often been misidentified as belonging to the oceanic whitetip shark
(C. longimanus). Dusky shark teeth have also been recovered from the vicinity of two baleen whale
s in North Carolina, one preserved in Goose Creek Limestone dating to the Late Pliocene (c. 3.5 Ma), and the other in mud dating to the Pleistocene
-Holocene
(c. 12,000 years ago).
In 1982, Jack Garrick published a phylogenetic analysis of Carcharhinus based on morphology
, in which he placed the dusky shark and the Galapagos shark
(C. galapagensis) at the center of the "obscurus group". The group consisted of large, triangular-toothed sharks with a ridge between the dorsal fin
s, and also included the bignose shark
(C. altimus), the Caribbean reef shark
(C. perezi), the sandbar shark
(C. plumbeus), and the oceanic whitetip shark. This interpretation was largely upheld by Leonard Compagno
in his 1988 phenetic study, and by Gavin Naylor in his 1992 allozyme
sequence study. Naylor was able to further resolve the interrelationships of the "ridge-backed" branch of Carcharhinus, finding that the dusky shark, Galapagos shark, oceanic whitetip shark, and blue shark
(Prionace glauca) comprise its most derived
clade
.
to southern Brazil, including the Bahamas and Cuba. In the eastern Atlantic Ocean, it has been reported from the western and central Mediterranean Sea, the Canary Islands, Cape Verde, Senegal, Sierra Leone, and possibly elsewhere including Portugal, Spain, Morocco, and Madeira. In the Indian Ocean, it is found off South Africa, Mozambique, and Madagascar, with sporadic records in the Arabian Sea, the Bay of Bengal, and perhaps the Red Sea. In the Pacific Ocean, it occurs off Japan, mainland China and Taiwan, Vietnam, Australia, and New Caledonia in the west, and from southern California to the Gulf of California, around Revillagigedo, and possibly off northern Chile in the east. Records of dusky sharks from the northeastern and eastern central Atlantic, and around tropical islands, may in fact be of Galapagos sharks. Mitochondrial DNA
and microsatellite evidence suggest that Indonesian and Australian sharks represent distinct populations.
Residing off continental coastlines from the surf zone
to the outer continental shelf
and adjacent oceanic waters, the dusky shark occupies an intermediate habitat
that overlaps with its more specialized relatives, such as the inshore sandbar shark, the pelagic silky shark
(C. falciformis) and oceanic whitetip shark, the deepwater bignose shark, and the islandic Galapagos shark and silvertip shark
(C. albimarginatus). One tracking study in the northern Gulf of Mexico found that it spends most of its time at depths of 10–80 m (32.8–262.5 ft), while making occasional forays below 200 m (656.2 ft); this species has been known to dive as deep as 400 m (1,312.3 ft). It prefers water temperatures of 19–28 °C (66.2–82.4 F), and avoids areas of low salinity
such as estuaries.
The dusky shark is nomadic and strongly migratory
, undertaking recorded movements of up to 3800 km (2,361.2 mi);adults generally move longer distances than juveniles. Sharks along both coasts of North America shift northward with warmer summer temperatures, and retreat back towards the equator
in winter. Off South Africa, young males and females over 0.9 m (3 ft) long disperse southward and northward respectively (with some overlap) from the nursery area off KwaZulu-Natal
; they join the adults several years later by a yet-unidentified route. In addition, juveniles spend spring and summer in the surf zone and fall and winter in offshore waters, and as they approach 2.2 m (7.2 ft) in length begin to conduct a north-south migration between KwaZulu-Natal in the winter and the Western Cape
in summer. Still-larger sharks, over 2.8 m (9.2 ft) long, migrate as far as southern Mozambique. Off Western Australia
, adult and juvenile dusky sharks migrate towards the coast in summer and fall, though not to the inshore nurseries occupied by newborns.
One of the largest members of its genus, the dusky shark commonly reaches a length of 3.2 m (10.5 ft) and a weight of 160–180 kg (352.7–396.8 lb); the maximum recorded length and weight are 4.2 m (13.8 ft) and 347 kg (765 lb) respectively. Females grow larger than males. This shark has a slender, streamlined body with a broadly rounded snout no longer than the width of the mouth. The nostril
s are preceded by barely developed flaps of skin. The medium-sized, circular eyes are equipped with nictitating membrane
s (protective third eyelids). The mouth has very short, subtle furrows at the corners and contains 13-15 (typically 14) tooth rows on either side of both jaws. The upper teeth are distinctively broad, triangular, and slightly oblique with strong, coarse serrations, while the lower teeth are narrower and upright, with finer serrations. The five pairs of gill slit
s are fairly long.
The large pectoral fins measure around one-fifth as long as the body, and have a falcate (sickle-like) shape tapering to a point. The first dorsal fin
is of moderate size and somewhat falcate, with a pointed apex and a strongly concave rear margin; its origin lies over the pectoral fin free rear tips. The second dorsal fin is much smaller and is positioned about opposite the anal fin. A low dorsal ridge is present between the dorsal fins. The caudal fin is large and high, with a well-developed lower lobe and a ventral notch near the tip of the upper lobe. The dermal denticles are diamond-shaped and closely set, each bearing five horizontal ridges leading to teeth on the posterior margin. This species is bronzy to bluish gray above and white below, which extends onto the flanks as a faint lighter stripe. The fins, particularly the underside of the pectoral fins and the lower caudal fin lobe) darken towards the tips; this is more obvious in juveniles.
positioned at the highest level of the trophic web, the dusky shark is generally less abundant than other sharks that share its range. However, high concentrations of individuals, especially juveniles, can be found at particular locations. Adults are often found following ships far from land, such as in the Agulhas Current
. A tracking study off the mouth of the Cape Fear River
in North Carolina reported an average swimming speed of 0.8 km/h (0.497096954187545 mph). The dusky shark is one of the hosts of the sharksucker
(Echeneis naucrates). Known parasites of this species include the tapeworms Anthobothrium laciniatum, Dasyrhynchus pacificus, Platybothrium kirstenae, Rhynchobothrium ingens, Tentacularia coryphaenae, and Triloculatum triloculatum, the monogenea
ns Dermophthirius carcharhini and Loimos salpinggoides, the leech
Stibarobdella macrothela, the copepod
s Alebion sp., Pandarus cranchii, P. sinuatus, and P. smithii, the praniza larva
e of gnathiid
isopods, and the sea lamprey
(Petromyzon marinus).
Full-grown dusky sharks have no significant natural predators. Major predators of young sharks include the ragged tooth shark (Carcharias taurus), the great white shark
(Carcharodon carcharias), the bull shark
(C. leucas), and the tiger shark
(Galeocerdo cuvier). Off KwaZulu-Natal, the use of shark net
s to protect beaches has reduced the populations of these large predators, leading to a dramatic increase in the number of juvenile dusky sharks (a phenomenon called "predator release"). In turn, the juvenile sharks have decimated populations of small bony fishes, with negative consequences for the biodiversity
of the local ecosystem
.
that takes a wide variety of prey from all levels of the water column, though it favors hunting near the bottom. A large individual can consume over a tenth of its body weight at a single sitting. The bite force exerted by a 2 m (6.6 ft) long dusky shark has been measured at 60 kg (132.3 lb) over the 2 square millimetre area at the tip of a tooth. This is the highest figure thus far measured from any shark, though it also reflects the concentration of force at the tooth tip. Dense aggregations of young sharks, forming in response to feeding opportunities, have been documented in the Indian Ocean.
The known diet of the dusky shark encompasses pelagic fish
es, including herring and anchovies
, tuna and mackerel
, billfish
, jacks
, needlefish and flyingfish
, threadfin
s, hairtails, lancetfish
, and lanternfish
; demersal fishes, including mullets, porgies
, grunts
, and flatheads
, eel
s, lizardfish, cusk eels, gurnard
s, and flatfish
; reef fish
es, including barracuda
s, goatfish
, spadefish, grouper
s, scorpionfish
, and porcupinefish
; cartilaginous fishes, including dogfish
, sawshark
s, angel shark
s, catshark
s, thresher shark
s, smoothhounds, smaller requiem sharks, sawfish
, guitarfish
, skates, stingray
s, and butterfly ray
s; and invertebrate
s, including cephalopod
s, decapod crustaceans
, barnacle
s, and sea star
s. Very rarely, the largest dusky sharks may also consume sea turtle
s, marine mammal
s (mainly as carrion
), and human refuse.
In the northwestern Atlantic, around 60% of the dusky shark's diet consists of bony fishes, from over ten families with bluefish
(Pomatomus saltatrix) and summer flounder
(Paralichthys dentatus) being especially important. Cartilaginous fishes, mainly skates and their egg cases, are the second-most important dietary component, while the lady crab
(Ovalipes ocellatus) is also a relatively significant food source. In South African and Australian waters, bony fishes are again the most important prey type. Newborn and juvenile sharks subsist mainly on small pelagic prey such as sardine
s and squid
; older sharks over 2 m (6.6 ft) long broaden their diets to include larger bony and cartilaginous fishes. The run
of the southern African pilchard (Sardinops sagax), occurring off the eastern coast of South Africa every winter, is attended by medium and large-sized dusky sharks. Pregnant and post-partum females do not join, possibly because the energy cost of gestation leaves them unable to pursue such swift prey. One South African study reported that 0.2% of the sharks examined had preyed upon bottlenose dolphin
s (Tursiops truncatus).
Like other requiem sharks, the dusky shark is viviparous: the developing embryo
s are initially nourished by a yolk sac
, which is converted into a placenta
l connection to the mother once the yolk supply is exhausted. Mating
occurs during spring in the northwestern Atlantic, while there appears to be no reproductive seasonality in other regions such as off South Africa. Females are capable of storing masses of sperm
, possibly from multiple males, for months to years within their nidamental gland
s (an organ that secretes egg cases). This would be advantageous given the sharks' itinerant natures and low natural abundance, which would make encounters with suitable mates infrequent and unpredictable.
With a gestation period
estimated at up to 22–24 months and a one-year resting period between pregnancies, female dusky sharks bear at most one litter of young every three years. The litter size ranges from 3 to 16, with 6 to 12 being typical, and does not correlate with female size. Sharks in the western Atlantic tend to produce slightly smaller litters than those from the southeastern Atlantic (averaging 8 versus 10 pups per litter). Depending on region, birthing may occur throughout the year or over a span of several months: newborn sharks have been reported from late winter to summer in the northwestern Atlantic, in summer and fall off Western Australia, and throughout the year with a peak in fall off southern Africa. Females move into shallow inshore habitats such as lagoon
s to give birth, as such areas offer their pups rich food supplies and shelter from predation (including from their own species), and leave immediately afterward. These nursery areas are known along the coasts of KwaZulu-Natal, southwestern Australia, western Baja California, and the eastern United States from New Jersey to North Carolina.
Newborn dusky sharks measure 0.7–1 m (2.3–3.3 ft) long; pup size increases with female size, and decreases with litter size. There is evidence that females can determine the size at which their pups are born, so as to improve their chances of survival across better or worse environmental conditions. Females also provision their young with energy reserves, stored in a liver that comprises one-fifth of the pup's weight, which sustains the newborn until it learns to hunt for itself. The dusky shark is one of the slowest-growing shark species, reaching sexual maturity
only at a substantial size and age (see table). Various studies have found growth rates to be largely similar across geographical regions and between sexes. The annual growth rate is 8–11 cm (3.1–4.3 in) over the first five years of life. The maximum lifespan is believed to be 40–50 years or more.
lists it as responsible for six attacks on people and boats, three of them unprovoked and one fatal. However, attacks attributed to this species off Bermuda and other islands were probably in reality caused by Galapagos sharks. Shark net
s used to protect beaches in South Africa and Australia entangle adult and larger juvenile dusky sharks in some numbers. From 1978 to 1999, an average of 256 individuals were caught annually in nets off KwaZulu-Natal; species-specific data is not available for nets off Australia. Young dusky sharks adapt well to display in public aquarium
s.
The dusky shark is one of the most sought-after species for shark fin trade, as its fins are large and contain a high number of internal rays (ceratotrichia). In addition, the meat is sold fresh, frozen, dried and salted, or smoked, the skin is made into leather, and the liver oil is processed for vitamin
s. Dusky sharks are taken by targeted commercial fisheries operating off eastern North America, southwestern Australia, and eastern South Africa using multi-species longlines and gillnets. The southwestern Australian fishery began in the 1940s and expanded in the 1970s to yield 500–600 tons per year. The fishery utilizes selective demersal gillnets that take almost exclusively young sharks under three years old, with 18–28% of all newborns captured in their first year. Demographic models suggest that the fishery is sustainable, provided that the mortality rate of sharks over 2 m (6.6 ft) long is under 4%.
In addition to commercial shark fisheries, dusky sharks are also caught as bycatch
on longlines meant for tuna
and swordfish
(and usually kept for its valuable fins), and by recreational fishers. Large numbers of dusky sharks, mostly juveniles, are caught by sport fishers off South Africa and eastern Australia. This shark was once one of the most important species in the Florida trophy shark tournaments, before the population collapsed.
. Stocks off the eastern United States are severely overfished; a 2006 stock assessment survey by the U.S. National Marine Fisheries Service (NMFS) showed that its population had dropped to 15–20% of 1970s levels. In 1997, the dusky shark was identified as a "Species of Concern" by the NMFS, meaning that it warranted conservation concern but there was insufficient information for listing on the U.S. Endangered Species Act
(ESA). Commercial and recreational retention of dusky sharks was prohibited in 1998, but this has been of limited effectiveness due to high bycatch mortality on multi-species gear. In addition, some 2,000 dusky sharks were caught by recreational fishers in 2003 despite the ban. In 2005, North Carolina implemented a time/area closure to reduce the impact of recreational fishing. To aid conservation efforts, molecular techniques using polymerase chain reaction
(PCR) have been developed that can identify whether marketed shark parts (e.g. fins) are from prohibited species like the dusky shark, versus similar allowed species such as the sandbar shark.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of requiem shark
Requiem shark
Requiem sharks are a family, Carcharhinidae, of sharks in the order Carcharhiniformes, containing migratory, live-bearing sharks of warm seas such as the tiger shark, the blue shark, the bull shark, and the milk shark.The name may be related to the French word for shark, "requin", itself of...
, family Carcharhinidae, occurring in tropical and warm-temperate continental seas worldwide. A generalist
Generalist and specialist species
A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources . A specialist species can only thrive in a narrow range of environmental conditions or has a limited diet. Most organisms do not all fit neatly into either...
apex predator
Apex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...
, the dusky shark can be found from the coast to the outer continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
and adjacent pelagic waters, and has been recorded from a depth of 400 m (1,300 ft). Populations migrate
Fish migration
Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...
seasonally towards the poles in the summer and towards the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
in the winter, traveling hundreds to thousands of kilometers. One of the largest members of its genus, the dusky shark reaches 4.2 m (14 ft) in length and 347 kg (765 lb) in weight. It has a slender, streamlined body and can be identified by its short round snout, long sickle-shaped pectoral fins, ridge between the first and second dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
s, and faintly marked fins.
Adult dusky sharks have a broad and varied diet, consisting mostly of bony fishes, sharks and rays, and cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s, but also occasionally crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s, sea star
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...
s, bryozoans, sea turtle
Sea turtle
Sea turtles are marine reptiles that inhabit all of the world's oceans except the Arctic.-Distribution:...
s, marine mammal
Marine mammal
Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...
s, carrion
Carrion
Carrion refers to the carcass of a dead animal. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters include vultures, hawks, eagles, hyenas, Virginia Opossum, Tasmanian Devils, coyotes, Komodo dragons, and burying beetles...
, and garbage
Waste
Waste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...
. This species is viviparous with a three-year reproductive cycle; females bear litters of 3–14 young after a gestation period
Gestation period
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.-Duration:...
of 22–24 months, after which there is a year of rest before they become pregnant again. Females are capable of storing sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
for long periods, as their encounters with suitable mates may be few and far between due to their nomadic lifestyle and low overall abundance. Dusky sharks are one of the slowest-growing and latest-maturing sharks, not reaching adulthood until around 20 years of age.
Because of its slow reproductive rate, the dusky shark is very vulnerable to human-caused population depletion. This species is highly valued by commercial fisheries
Commercial fishing
Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often pursue fish far into the ocean under adverse conditions...
for its fins, used in shark fin soup
Shark fin soup
Shark fin soup is a popular soup item of Chinese cuisine usually served at special occasions such as weddings and banquets, or as a luxury item in Chinese culture. The shark fins provide texture while the taste comes from the other soup ingredients.There is controversy over the practice of shark...
, and for its meat, skin, and liver oil
Shark liver oil
Shark liver oil is obtained from sharks that are caught for food purposes and are living in cold, deep oceans. The liver oil from sharks has been used by fishermen for centuries as a folk remedy for general health...
. It is also esteemed by recreational fishers
Recreational fishing
Recreational fishing, also called sport fishing, is fishing for pleasure or competition. It can be contrasted with commercial fishing, which is fishing for profit, or subsistence fishing, which is fishing for survival....
. The International Union for Conservation of Nature (IUCN) has assessed this species as Near Threatened
Near Threatened
Near Threatened is a conservation status assigned to species or lower taxa that may be considered threatened with extinction in the near future, although it does not currently qualify for the threatened status...
worldwide and Vulnerable
Vulnerable species
On 30 January 2010, the IUCN Red List of Threatened Species identified 9694 Vulnerable species, subspecies and varieties, stocks and sub-populations.-References:...
off the eastern United States, where populations have dropped to 15–20% of 1970s levels. The dusky shark is regarded as potentially dangerous to humans due to its large size, but there are few attacks
Shark attack
A shark attack is an attack on a human by a shark. Every year around 60 shark attacks are reported worldwide, although death is quite unusual. Despite the relative rarity of shark attacks, the fear of sharks is a common phenomenon, having been fueled by the occasional instances of serial attacks,...
attributable to it.
Taxonomy
Naturalist
Naturalist may refer to:* Practitioner of natural history* Conservationist* Advocate of naturalism * Naturalist , autobiography-See also:* The American Naturalist, periodical* Naturalism...
Charles Alexandre Lesueur
Charles Alexandre Lesueur
Charles Alexandre Lesueur was a French naturalist, artist and explorer.Pictured here is the oil portrait by Charles Willson Peale of Charles-Alexandre Lesueur...
published the first scientific description of the dusky shark in an 1818 issue of Journal of the Academy of Natural Sciences of Philadelphia. He placed it in the genus Squalus and gave it the specific epithet obscurus (Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
for "dark" or "dim"), referring to its coloration. Subsequent authors have recognized this species as belonging to the genus Carcharhinus. Lesueur did not designate a type specimen, though he was presumably working from a shark caught in North American waters.
Many early sources gave the scientific name of the dusky shark as Carcharias (later Carcharhinus) lamiella, which originated from an 1882 account by David Starr Jordan
David Starr Jordan
David Starr Jordan, Ph.D., LL.D. was a leading eugenicist, ichthyologist, educator and peace activist. He was president of Indiana University and Stanford University.-Early life and education:...
and Charles Henry Gilbert
Charles Henry Gilbert
Charles Henry Gilbert was a pioneer ichthyologist and fishery biologist of particular significance to natural history of the western United States. He collected and studied fishes from Central America north to Alaska and described many new species...
. Although Jordan and Gilbert referred to a set of jaws that came from a dusky shark, the type specimen they designated was later discovered to be a copper shark (C. brachyurus). Therefore, C. lamiella is not considered a synonym
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
of C. obscurus but rather of C. brachyurus. Other common name
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...
s for this species include bay shark, black whaler, brown common gray shark, brown dusky shark, brown shark, common whaler, dusky ground shark, dusky whaler, river whaler, shovelnose, and slender whaler shark.
Phylogeny and evolution
Teeth belonging to the dusky shark are fairly well-represented in the fossil record, though assigning Carcharhinus teeth to species can be problematic. Dusky shark teeth dating to the MioceneMiocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
(23-5.3 Ma) have been recovered from the Kendeace and Grand Bay formations in Carriacou, the Grenadines, the Moghra Formation in Egypt, Polk County, Florida
Polk County, Florida
Polk County is located in central Florida between the Tampa Bay and Greater Orlando metropolitan areas. The county was established by the state government in 1861 on the eve of the American Civil War and named after former United States president James K. Polk. The county seat is Bartow and its...
, and possibly Cerro La Cruz in northern Venezuela. Teeth dating to the Late Miocene
Late Miocene
The Late Miocene is a sub-epoch of the Miocene Epoch made up of two stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch....
or Early Pliocene (11.6-3.6 Ma) are abundant in the Yorktown Formation and the Pungo River
Pungo River
The Pungo River is a river in eastern North Carolina, USA. It originally began in the Great Dismal Swamp in Washington County, North Carolina; the upper part of the river has since been supplanted by the Pungo River Canal, dug in the 1950s to improve drainage of local farmland. The river flows...
, North Carolina, and from the Chesapeake Bay
Chesapeake Bay
The Chesapeake Bay is the largest estuary in the United States. It lies off the Atlantic Ocean, surrounded by Maryland and Virginia. The Chesapeake Bay's drainage basin covers in the District of Columbia and parts of six states: New York, Pennsylvania, Delaware, Maryland, Virginia, and West...
region; these teeth differ slightly from the modern dusky shark, and have often been misidentified as belonging to the oceanic whitetip shark
Oceanic whitetip shark
The oceanic whitetip shark, Carcharhinus longimanus, is a large pelagic shark inhabiting tropical and warm temperate seas. Its stocky body is most notable for its long, white-tipped, rounded fins....
(C. longimanus). Dusky shark teeth have also been recovered from the vicinity of two baleen whale
Baleen whale
The Baleen whales, also called whalebone whales or great whales, form the Mysticeti, one of two suborders of the Cetacea . Baleen whales are characterized by having baleen plates for filtering food from water, rather than having teeth. This distinguishes them from the other suborder of cetaceans,...
s in North Carolina, one preserved in Goose Creek Limestone dating to the Late Pliocene (c. 3.5 Ma), and the other in mud dating to the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
-Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
(c. 12,000 years ago).
In 1982, Jack Garrick published a phylogenetic analysis of Carcharhinus based on morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
, in which he placed the dusky shark and the Galapagos shark
Galapagos shark
The Galapagos shark is a species of requiem shark, family Carcharhinidae, found worldwide. This species favors clear reef environments around oceanic islands, where it is often the most abundant shark species...
(C. galapagensis) at the center of the "obscurus group". The group consisted of large, triangular-toothed sharks with a ridge between the dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
s, and also included the bignose shark
Bignose shark
The bignose shark is a species of requiem shark, family Carcharhinidae. Distributed worldwide in tropical and subtropical waters, this migratory shark frequents deep waters around the edges of the continental shelf. It is typically found at depths of , though at night it may move towards the...
(C. altimus), the Caribbean reef shark
Caribbean reef shark
The Caribbean reef shark is a species of requiem shark, family Carcharhinidae. It is found in the tropical waters of the western Atlantic Ocean from Florida to Brazil, and is the most commonly encountered reef shark in the Caribbean Sea...
(C. perezi), the sandbar shark
Sandbar shark
The sandbar shark, Carcharhinus plumbeus, is a species of requiem shark, family Carcharhinidae, native to the Atlantic Ocean and the Indo-Pacific. It is distinguishable by its very high first dorsal fin and inter-dorsal ridge....
(C. plumbeus), and the oceanic whitetip shark. This interpretation was largely upheld by Leonard Compagno
Leonard Compagno
Leonard Joseph Victor Compagno is an international authority on shark taxonomy and the author of many scientific papers and books on the subject, best known of which is his 1984 catalogue of shark species produced for the Food and Agriculture Organization of the United Nations.-Career:*Ph.D,...
in his 1988 phenetic study, and by Gavin Naylor in his 1992 allozyme
Allozyme
Variant forms of an enzyme that are coded by different alleles at the same locus are called allozymes. These are opposed to isozymes, which are enzymes that perform the same function, but which are coded by genes located at different loci....
sequence study. Naylor was able to further resolve the interrelationships of the "ridge-backed" branch of Carcharhinus, finding that the dusky shark, Galapagos shark, oceanic whitetip shark, and blue shark
Blue shark
The blue shark is a species of requiem shark, family Carcharhinidae, that inhabits deep waters in the world's temperate and tropical oceans. Preferring cooler waters, blue sharks migrate long distances, for example from New England to South America. Although generally lethargic, they can move very...
(Prionace glauca) comprise its most derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
.
Distribution and habitat
The range of the dusky shark extends worldwide, albeit discontinuously, in tropical and warm-temperate waters. In the western Atlantic Ocean, it is found from Massachusetts and the Georges BankGeorges Bank
Georges Bank is a large elevated area of the sea floor which separates the Gulf of Maine from the Atlantic Ocean and is situated between Cape Cod, Massachusetts and Cape Sable Island, Nova Scotia ....
to southern Brazil, including the Bahamas and Cuba. In the eastern Atlantic Ocean, it has been reported from the western and central Mediterranean Sea, the Canary Islands, Cape Verde, Senegal, Sierra Leone, and possibly elsewhere including Portugal, Spain, Morocco, and Madeira. In the Indian Ocean, it is found off South Africa, Mozambique, and Madagascar, with sporadic records in the Arabian Sea, the Bay of Bengal, and perhaps the Red Sea. In the Pacific Ocean, it occurs off Japan, mainland China and Taiwan, Vietnam, Australia, and New Caledonia in the west, and from southern California to the Gulf of California, around Revillagigedo, and possibly off northern Chile in the east. Records of dusky sharks from the northeastern and eastern central Atlantic, and around tropical islands, may in fact be of Galapagos sharks. Mitochondrial DNA
Mitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
and microsatellite evidence suggest that Indonesian and Australian sharks represent distinct populations.
Residing off continental coastlines from the surf zone
Surf zone
As ocean surface waves come closer to shore they break, forming the foamy, bubbly surface we call surf. The region of breaking waves defines the surf zone. After breaking in the surf zone, the waves continue to move in, and they run up onto the sloping front of the beach, forming an uprush of...
to the outer continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
and adjacent oceanic waters, the dusky shark occupies an intermediate habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
that overlaps with its more specialized relatives, such as the inshore sandbar shark, the pelagic silky shark
Silky shark
The silky shark is a species of requiem shark, 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...
(C. falciformis) and oceanic whitetip shark, the deepwater bignose shark, and the islandic Galapagos shark and silvertip shark
Silvertip shark
The silvertip shark is a large species of requiem shark, family Carcharhinidae, with a fragmented distribution throughout the tropical Indian and Pacific Oceans. This species is often encountered around offshore islands and coral reefs, and has been known to dive to a depth of 800 m...
(C. albimarginatus). One tracking study in the northern Gulf of Mexico found that it spends most of its time at depths of 10–80 m (32.8–262.5 ft), while making occasional forays below 200 m (656.2 ft); this species has been known to dive as deep as 400 m (1,312.3 ft). It prefers water temperatures of 19–28 °C (66.2–82.4 F), and avoids areas of low salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
such as estuaries.
The dusky shark is nomadic and strongly migratory
Fish migration
Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...
, undertaking recorded movements of up to 3800 km (2,361.2 mi);adults generally move longer distances than juveniles. Sharks along both coasts of North America shift northward with warmer summer temperatures, and retreat back towards the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
in winter. Off South Africa, young males and females over 0.9 m (3 ft) long disperse southward and northward respectively (with some overlap) from the nursery area off KwaZulu-Natal
KwaZulu-Natal
KwaZulu-Natal is a province of South Africa. Prior to 1994, the territory now known as KwaZulu-Natal was made up of the province of Natal and the homeland of KwaZulu....
; they join the adults several years later by a yet-unidentified route. In addition, juveniles spend spring and summer in the surf zone and fall and winter in offshore waters, and as they approach 2.2 m (7.2 ft) in length begin to conduct a north-south migration between KwaZulu-Natal in the winter and the Western Cape
Western Cape
The Western Cape is a province in the south west of South Africa. The capital is Cape Town. Prior to 1994, the region that now forms the Western Cape was part of the much larger Cape Province...
in summer. Still-larger sharks, over 2.8 m (9.2 ft) long, migrate as far as southern Mozambique. Off Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
, adult and juvenile dusky sharks migrate towards the coast in summer and fall, though not to the inshore nurseries occupied by newborns.
Description
Nostril
A nostril is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation...
s are preceded by barely developed flaps of skin. The medium-sized, circular eyes are equipped with nictitating membrane
Nictitating membrane
The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...
s (protective third eyelids). The mouth has very short, subtle furrows at the corners and contains 13-15 (typically 14) tooth rows on either side of both jaws. The upper teeth are distinctively broad, triangular, and slightly oblique with strong, coarse serrations, while the lower teeth are narrower and upright, with finer serrations. The five pairs of gill slit
Gill slit
Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of Cartilaginous fish such as sharks, rays, sawfish, and guitarfish. Most of these have five pairs, but a few species have 6 or 7 pairs...
s are fairly long.
The large pectoral fins measure around one-fifth as long as the body, and have a falcate (sickle-like) shape tapering to a point. The first dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
is of moderate size and somewhat falcate, with a pointed apex and a strongly concave rear margin; its origin lies over the pectoral fin free rear tips. The second dorsal fin is much smaller and is positioned about opposite the anal fin. A low dorsal ridge is present between the dorsal fins. The caudal fin is large and high, with a well-developed lower lobe and a ventral notch near the tip of the upper lobe. The dermal denticles are diamond-shaped and closely set, each bearing five horizontal ridges leading to teeth on the posterior margin. This species is bronzy to bluish gray above and white below, which extends onto the flanks as a faint lighter stripe. The fins, particularly the underside of the pectoral fins and the lower caudal fin lobe) darken towards the tips; this is more obvious in juveniles.
Biology and ecology
As an apex predatorApex predator
Apex predators are predators that have no predators of their own, residing at the top of their food chain. Zoologists define predation as the killing and consumption of another organism...
positioned at the highest level of the trophic web, the dusky shark is generally less abundant than other sharks that share its range. However, high concentrations of individuals, especially juveniles, can be found at particular locations. Adults are often found following ships far from land, such as in the Agulhas Current
Agulhas Current
The Agulhas Current is the Western Boundary Current of the southwest Indian Ocean. It flows down the east coast of Africa from 27°S to 40°S. It is narrow, swift and strong...
. A tracking study off the mouth of the Cape Fear River
Cape Fear River
The Cape Fear River is a long blackwater river in east central North Carolina in the United States. It flows into the Atlantic Ocean near Cape Fear, from which it takes its name. The overall water quality of the river is continuously measured and monitored by and conducted by the , , and the...
in North Carolina reported an average swimming speed of 0.8 km/h (0.497096954187545 mph). The dusky shark is one of the hosts of the sharksucker
Echeneis naucrates
The Echeneis naucrates or live sharksucker is a rare species of remora, from the family Echeneidae. The species generally occurs in all tropical seas around the world, mainly in warm waters. The species stays close to coral reefs and feeds primarily off hosts, such as sting rays, sharks, sea...
(Echeneis naucrates). Known parasites of this species include the tapeworms Anthobothrium laciniatum, Dasyrhynchus pacificus, Platybothrium kirstenae, Rhynchobothrium ingens, Tentacularia coryphaenae, and Triloculatum triloculatum, the monogenea
Monogenea
Monogenea are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes, class Monogenea.-Characteristics:Monogenea are very small parasitic flatworms mainly found on skin or gills of fish....
ns Dermophthirius carcharhini and Loimos salpinggoides, the leech
Leech
Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways...
Stibarobdella macrothela, the copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s Alebion sp., Pandarus cranchii, P. sinuatus, and P. smithii, the praniza larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e of gnathiid
Gnathiidae
Gnathiidae is a family of isopod crustaceans. They occur in a wide range of depths, from the littoral zone to the deep sea. The adults are associated with sponges and may not feed...
isopods, and the sea lamprey
Sea lamprey
The sea lamprey is a parasitic lamprey found on the Atlantic coasts of Europe and North America, in the western Mediterranean Sea, and in the Great Lakes. It is brown, gray, or black on its back and white or gray on the underside and can grow up to 90 cm long. Sea lampreys prey on a wide...
(Petromyzon marinus).
Full-grown dusky sharks have no significant natural predators. Major predators of young sharks include the ragged tooth shark (Carcharias taurus), the great white shark
Great white shark
The great white shark, scientific name Carcharodon carcharias, also known as the great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. It is known for its size, with the largest individuals known to have approached...
(Carcharodon carcharias), the bull shark
Bull shark
The bull shark, Carcharhinus leucas, also known as Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers...
(C. leucas), and the tiger shark
Tiger shark
The tiger sharks, Galeocerdo cuvier, is a species of requiem shark and the only member of the genus Galeocerdo. Commonly known as sea tigers, tiger sharks are relatively large macropredators, capable of attaining a length of over . It is found in many tropical and temperate waters, and is...
(Galeocerdo cuvier). Off KwaZulu-Natal, the use of shark net
Shark net
A shark net is a submerged net placed around beaches to reduce shark attacks on swimmers.Shark nets do not offer complete protection but work on the principle of "fewer sharks, fewer attacks". They reduce occurrence via shark mortality. Reducing the local shark populations is believed to reduce the...
s to protect beaches has reduced the populations of these large predators, leading to a dramatic increase in the number of juvenile dusky sharks (a phenomenon called "predator release"). In turn, the juvenile sharks have decimated populations of small bony fishes, with negative consequences for the biodiversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
of the local ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
.
Feeding
The dusky shark is a generalistGeneralist and specialist species
A generalist species is able to thrive in a wide variety of environmental conditions and can make use of a variety of different resources . A specialist species can only thrive in a narrow range of environmental conditions or has a limited diet. Most organisms do not all fit neatly into either...
that takes a wide variety of prey from all levels of the water column, though it favors hunting near the bottom. A large individual can consume over a tenth of its body weight at a single sitting. The bite force exerted by a 2 m (6.6 ft) long dusky shark has been measured at 60 kg (132.3 lb) over the 2 square millimetre area at the tip of a tooth. This is the highest figure thus far measured from any shark, though it also reflects the concentration of force at the tooth tip. Dense aggregations of young sharks, forming in response to feeding opportunities, have been documented in the Indian Ocean.
The known diet of the dusky shark encompasses pelagic fish
Pelagic fish
Pelagic fish live near the surface or in the water column of coastal, ocean and lake waters, but not on the bottom of the sea or the lake. They can be contrasted with demersal fish, which do live on or near the bottom, and reef fish which are associated with coral reefs.The marine pelagic...
es, including herring and anchovies
Clupeiformes
Clupeiformes is the order of ray-finned fish that includes the herring family, Clupeidae, and the anchovy family, Engraulidae. The group includes many of the most important food fish....
, tuna and mackerel
Scombridae
Scombridae is the family of the mackerels, tunas, and bonitos, and thus includes many of the most important and familiar food fishes. The family consists of about 55 species in 15 genera and two subfamilies...
, billfish
Billfish
The term billfish is applied to a number of different large, predatory fish characterised by their large size and their long, sword-like bill. Billfish include the sailfish and marlin, which make up the family Istiophoridae, and the swordfish, sole member of the family Xiphiidae...
, jacks
Carangidae
Carangidae is a family of fish which includes the jacks, pompanos, jack mackerels, and scads.They are marine fish found in the Atlantic, Indian and Pacific Oceans...
, needlefish and flyingfish
Beloniformes
The Beloniformes are an order of five families of freshwater and marine ray-finned fish: the Adrianichthyidae ; Belonidae ; Exocoetidae ; Hemiramphidae ; and the Scomberesocidae...
, threadfin
Threadfin
Threadfins are silvery grey perciform marine fish of the family Polynemidae. Found in tropical to subtropical waters throughout the world, the threadfin family contains nine genera and 33 species...
s, hairtails, lancetfish
Lancetfish
Lancetfishes are large oceanic predatory fishes in the genus Alepisaurus , the only living genus in the family Alepisauridae....
, and lanternfish
Lanternfish
Cooper Lanternfishes are small mesopelagic fish of the large family Myctophidae. One of two families in the order Myctophiformes, the Myctophidae are represented by 246 species in 33 genera, and are found in oceans worldwide. They are aptly named after their conspicuous use of bioluminescence...
; demersal fishes, including mullets, porgies
Sparidae
The Sparidae is a family of fish, included in the order Perciformes. The fish of the family are commonly called sea breams and porgies . The sheepshead, scup, and red sea bream are species in this family. They are found in shallow temperate and tropical waters and are bottom-dwelling carnivores....
, grunts
Sciaenidae
Sciaenidae is a family of fish commonly called drums, croakers, or hardheads for the repetitive throbbing or drumming sounds they make...
, and flatheads
Platycephalidae
Platycephalidae is a family of marine fishes, most commonly referred to as flatheads. They are relatives of the popular lionfishes, belonging to the order Scorpaeniformes....
, eel
Eel
Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...
s, lizardfish, cusk eels, gurnard
Gurnard
-Fish:*Sea robin, fish of the family Triglidae, including:**Bluefin gurnard**Tub gurnard**Red gurnard**Grey gurnard**Scaly gurnard**Eastern spiny gurnard**Spotted gurnard...
s, and flatfish
Flatfish
The flatfish are an order of ray-finned fish, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through and around the head during development...
; reef fish
Reef fish
Coral reef fish are fish which live amongst or in close relation to coral reefs. Coral reefs form complex ecosystems with tremendous biodiversity. Among the myriad inhabitants, the fish stand out as particularly colourful and interesting to watch. Hundreds of species can exist in a small area of a...
es, including barracuda
Barracuda
The barracuda is a ray-finned fish known for its large size and fearsome appearance. Its body is long, fairly compressed, and covered with small, smooth scales. Some species could reach up to 1.8m in length and 30 cm in width...
s, goatfish
Goatfish
Goatfishes are tropical marine perciform fish of the family Mullidae. Seldom found in brackish waters, goatfish are most associated with the reefs of the Atlantic, Indian, and Pacific Oceans...
, spadefish, grouper
Grouper
Groupers are fish of any of a number of genera in the subfamily Epinephelinae of the family Serranidae, in the order Perciformes.Not all serranids are called groupers; the family also includes the sea basses. The common name grouper is usually given to fish in one of two large genera: Epinephelus...
s, scorpionfish
Scorpionfish
Scorpaenidae, the scorpionfish, are a family of mostly marine fish that includes many of the world's most venomous species. As the name suggests, scorpionfish have a type of "sting" in the form of sharp spines coated with venomous mucus. The family is a large one, with hundreds of members. They are...
, and porcupinefish
Porcupinefish
Porcupinefish are fish of the family Diodontidae, , also commonly called blowfish ....
; cartilaginous fishes, including dogfish
Squaliformes
Squaliformes is an order of sharks that includes about 97 species in seven families.Members of the order have two dorsal fins, which usually possess spines, no anal fin or nictitating membrane, and five gill slits. In most other respects, however, they are quite variable in form and size...
, sawshark
Sawshark
The sawsharks or saw sharks are an order of sharks bearing long blade-like snouts edged with teeth, which they use to slash and disable their prey...
s, angel shark
Angel shark
The angel sharks are an unusual genus of sharks with flattened bodies and broad pectoral fins that give them a strong resemblance to rays. The more than 16 known species are in the genus Squatina, the only genus in its family, Squatinidae, and order Squatiniformes. They occur worldwide in temperate...
s, catshark
Catshark
Catsharks are ground sharks of the family Scyliorhinidae, with over 150 known species. While they are generally known as catsharks, many species are commonly called dogfish....
s, thresher shark
Thresher shark
Thresher sharks are large lamniform sharks of the family Alopiidae. Found in all temperate and tropical oceans of the world, the family contains three species all within the genus Alopias.-Taxonomy:...
s, smoothhounds, smaller requiem sharks, sawfish
Sawfish
Sawfish, also known as the Carpenter Shark, are a family of rays, characterized by a long, toothy nose extension snout. Several species can grow to approximately . The family as a whole is largely unknown and little studied...
, guitarfish
Guitarfish
The guitarfish are a family, Rhinobatidae, of rays. The guitarfish are known for an elongated body with a flattened head and trunk and small ray like wings. The combined range of the various species is tropical, subtropical and temperate waters worldwide. They often travel in large...
, skates, stingray
Stingray
The stingrays are a group of rays, which are cartilaginous fishes related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes, and consist of eight families: Hexatrygonidae , Plesiobatidae , Urolophidae , Urotrygonidae , Dasyatidae , Potamotrygonidae The...
s, and butterfly ray
Butterfly ray
The butterfly rays are a group of rays forming the genus Gymnura and the family Gymnuridae. They are found in warm oceans worldwide, and occasionally in estuaries....
s; and invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s, including cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s, decapod crustaceans
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...
, barnacle
Barnacle
A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...
s, and sea star
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...
s. Very rarely, the largest dusky sharks may also consume sea turtle
Sea turtle
Sea turtles are marine reptiles that inhabit all of the world's oceans except the Arctic.-Distribution:...
s, marine mammal
Marine mammal
Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...
s (mainly as carrion
Carrion
Carrion refers to the carcass of a dead animal. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters include vultures, hawks, eagles, hyenas, Virginia Opossum, Tasmanian Devils, coyotes, Komodo dragons, and burying beetles...
), and human refuse.
In the northwestern Atlantic, around 60% of the dusky shark's diet consists of bony fishes, from over ten families with bluefish
Bluefish
The bluefish , called tailor in Australia, is a species of popular marine gamefish found in all climates. It is the sole species of the Pomatomidae family....
(Pomatomus saltatrix) and summer flounder
Summer flounder
The Summer Flounder is a marine flatfish that is found in the Atlantic Ocean off the East coast of the United States and Canada. It is especially abundant in waters from North Carolina to Massachusetts.-Description:...
(Paralichthys dentatus) being especially important. Cartilaginous fishes, mainly skates and their egg cases, are the second-most important dietary component, while the lady crab
Lady crab
Ovalipes ocellatus is a species of crab from eastern North America, known as the lady crab, calico crab or ocellated crab. It has a shell long and only slightly wider, which is covered in clusters of purple spots. It occurs from Canada to Georgia, and lives mainly on molluscs, such as the Atlantic...
(Ovalipes ocellatus) is also a relatively significant food source. In South African and Australian waters, bony fishes are again the most important prey type. Newborn and juvenile sharks subsist mainly on small pelagic prey such as sardine
Sardine
Sardines, or pilchards, are several types of small, oily fish related to herrings, family Clupeidae. Sardines are named after the Mediterranean island of Sardinia, around which they were once abundant....
s and squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
; older sharks over 2 m (6.6 ft) long broaden their diets to include larger bony and cartilaginous fishes. The run
Sardine run
The sardine run of southern Africa occurs from May through July when billions of sardines – or more specifically the Southern African pilchard Sardinops sagax – spawn in the cool waters of the Agulhas Bank and move northward along the east coast of South Africa...
of the southern African pilchard (Sardinops sagax), occurring off the eastern coast of South Africa every winter, is attended by medium and large-sized dusky sharks. Pregnant and post-partum females do not join, possibly because the energy cost of gestation leaves them unable to pursue such swift prey. One South African study reported that 0.2% of the sharks examined had preyed upon bottlenose dolphin
Bottlenose Dolphin
Bottlenose dolphins, the genus Tursiops, are the most common and well-known members of the family Delphinidae, the family of oceanic dolphins. Recent molecular studies show the genus contains two species, the common bottlenose dolphin and the Indo-Pacific bottlenose dolphin , instead of one...
s (Tursiops truncatus).
Life history
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s are initially nourished by a yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
, which is converted into a placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
l connection to the mother once the yolk supply is exhausted. Mating
Mating
In biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation. In social animals, it also includes the raising of their offspring. Copulation is the union of the sex organs of two sexually reproducing animals for insemination and subsequent internal fertilization...
occurs during spring in the northwestern Atlantic, while there appears to be no reproductive seasonality in other regions such as off South Africa. Females are capable of storing masses of sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
, possibly from multiple males, for months to years within their nidamental gland
Nidamental gland
The nidamental gland is an internal organ found in some elasmobranchs and certain molluscs, including cephalopods and gastropods....
s (an organ that secretes egg cases). This would be advantageous given the sharks' itinerant natures and low natural abundance, which would make encounters with suitable mates infrequent and unpredictable.
With a gestation period
Gestation period
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.-Duration:...
estimated at up to 22–24 months and a one-year resting period between pregnancies, female dusky sharks bear at most one litter of young every three years. The litter size ranges from 3 to 16, with 6 to 12 being typical, and does not correlate with female size. Sharks in the western Atlantic tend to produce slightly smaller litters than those from the southeastern Atlantic (averaging 8 versus 10 pups per litter). Depending on region, birthing may occur throughout the year or over a span of several months: newborn sharks have been reported from late winter to summer in the northwestern Atlantic, in summer and fall off Western Australia, and throughout the year with a peak in fall off southern Africa. Females move into shallow inshore habitats such as lagoon
Lagoon
A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
s to give birth, as such areas offer their pups rich food supplies and shelter from predation (including from their own species), and leave immediately afterward. These nursery areas are known along the coasts of KwaZulu-Natal, southwestern Australia, western Baja California, and the eastern United States from New Jersey to North Carolina.
| Region | Male length and age at maturity | Female length and age at maturity |
|---|---|---|
| Northwestern Atlantic | 2.8 m (9.2 ft), 19 years | 2.84 m (9.3 ft), 21 years |
| Eastern South Africa | 2.8 m (9.2 ft), 19–21 years | 2.6–3 m (8.5–9.8 ft), 17–24 years |
| Indonesia | 2.8–3 m (9.2–9.8 ft), age unknown | 2.8 m (9.2 ft), age unknown |
| Western Australia | 2.65–2.8 m (8.7–9.2 ft), 18–23 years | 2.95–3.1 m (9.7–10.2 ft), 27–32 years |
Newborn dusky sharks measure 0.7–1 m (2.3–3.3 ft) long; pup size increases with female size, and decreases with litter size. There is evidence that females can determine the size at which their pups are born, so as to improve their chances of survival across better or worse environmental conditions. Females also provision their young with energy reserves, stored in a liver that comprises one-fifth of the pup's weight, which sustains the newborn until it learns to hunt for itself. The dusky shark is one of the slowest-growing shark species, reaching sexual maturity
Sexual maturity
Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...
only at a substantial size and age (see table). Various studies have found growth rates to be largely similar across geographical regions and between sexes. The annual growth rate is 8–11 cm (3.1–4.3 in) over the first five years of life. The maximum lifespan is believed to be 40–50 years or more.
Human interactions
The dusky shark is considered to be potentially dangerous to humans because of its large size, though little is known of how it behaves towards people underwater. As of 2009, the International Shark Attack FileInternational Shark Attack File
The International Shark Attack File is a global database of shark attacks. It began as an attempt to catalogue shark attacks on servicemen during World War II. The Office of Naval Research funded it from 1958 until 1968. During that time a panel of shark experts developed a standard system for...
lists it as responsible for six attacks on people and boats, three of them unprovoked and one fatal. However, attacks attributed to this species off Bermuda and other islands were probably in reality caused by Galapagos sharks. Shark net
Shark net
A shark net is a submerged net placed around beaches to reduce shark attacks on swimmers.Shark nets do not offer complete protection but work on the principle of "fewer sharks, fewer attacks". They reduce occurrence via shark mortality. Reducing the local shark populations is believed to reduce the...
s used to protect beaches in South Africa and Australia entangle adult and larger juvenile dusky sharks in some numbers. From 1978 to 1999, an average of 256 individuals were caught annually in nets off KwaZulu-Natal; species-specific data is not available for nets off Australia. Young dusky sharks adapt well to display in public aquarium
Public aquarium
A public aquarium is the aquatic counterpart of a zoo, housing living aquatic species for viewing. Most public aquariums feature tanks larger than those kept by home aquarists, as well as smaller tanks. Since the first public aquariums were built in the mid-19th century, they have become popular...
s.
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...
s. Dusky sharks are taken by targeted commercial fisheries operating off eastern North America, southwestern Australia, and eastern South Africa using multi-species longlines and gillnets. The southwestern Australian fishery began in the 1940s and expanded in the 1970s to yield 500–600 tons per year. The fishery utilizes selective demersal gillnets that take almost exclusively young sharks under three years old, with 18–28% of all newborns captured in their first year. Demographic models suggest that the fishery is sustainable, provided that the mortality rate of sharks over 2 m (6.6 ft) long is under 4%.
In addition to commercial shark fisheries, dusky sharks are also caught as bycatch
Bycatch
The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...
on longlines meant for tuna
Tuna
Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...
and swordfish
Swordfish
Swordfish , also known as broadbill in some countries, are large, highly migratory, predatory fish characterized by a long, flat bill. They are a popular sport fish of the billfish category, though elusive. Swordfish are elongated, round-bodied, and lose all teeth and scales by adulthood...
(and usually kept for its valuable fins), and by recreational fishers. Large numbers of dusky sharks, mostly juveniles, are caught by sport fishers off South Africa and eastern Australia. This shark was once one of the most important species in the Florida trophy shark tournaments, before the population collapsed.
Conservation
The International Union for Conservation of Nature (IUCN) has assessed this species as Near Threatened worldwide and Vulnerable in the northwestern Atlantic and the Gulf of Mexico. The American Fisheries Society has also assessed North American dusky shark populations as Vulnerable. Its very low reproductive rate renders the dusky shark extremely susceptible to overfishingOverfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....
. Stocks off the eastern United States are severely overfished; a 2006 stock assessment survey by the U.S. National Marine Fisheries Service (NMFS) showed that its population had dropped to 15–20% of 1970s levels. In 1997, the dusky shark was identified as a "Species of Concern" by the NMFS, meaning that it warranted conservation concern but there was insufficient information for listing on the U.S. Endangered Species Act
Endangered Species Act
The Endangered Species Act of 1973 is one of the dozens of United States environmental laws passed in the 1970s. Signed into law by President Richard Nixon on December 28, 1973, it was designed to protect critically imperiled species from extinction as a "consequence of economic growth and...
(ESA). Commercial and recreational retention of dusky sharks was prohibited in 1998, but this has been of limited effectiveness due to high bycatch mortality on multi-species gear. In addition, some 2,000 dusky sharks were caught by recreational fishers in 2003 despite the ban. In 2005, North Carolina implemented a time/area closure to reduce the impact of recreational fishing. To aid conservation efforts, molecular techniques using polymerase chain reaction
Polymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
(PCR) have been developed that can identify whether marketed shark parts (e.g. fins) are from prohibited species like the dusky shark, versus similar allowed species such as the sandbar shark.