The Australian blacktip shark (''Carcharhinus tilstoni'') is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

requiem shark Requiem sharks are sharks of the family Carcharhinidae in the order Carcharhiniformes. They are migratory, live-bearing sharks of warm seas (sometimes of brackish or fresh water) and include such species as the tiger shark, bull shark, le ...

, in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Carcharhinidae,

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...

to northern and eastern

Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...

. Favoring the upper and middle parts of the

water column A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...

, it can be found from the

intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species ...

to a depth of . Appearance-wise this species is virtually identical to the common blacktip shark (''C. limbatus''), from which it can be reliably distinguished only by its lower

vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...

number and by

genetic marker A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify individuals or species. It can be described as a variation (which may arise due to mutation or alteration in the genomic loci) that can be ...

s. Generally reaching in length, it is a fairly stout-bodied, bronze-colored shark with a long snout and black-tipped fins. Primarily piscivorous, the Australian blacktip shark forms large groups of similar size and sex that tend to remain within a local area. It exhibits

viviparity Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...

, meaning that the unborn young are provisioned through a

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...

l connection. There is a well-defined annual reproductive cycle with

mating In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...

occurring in February and March. Females bear one to six pups around January of the following year, after a 10-month

gestation period In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once ...

. The Australian blacktip shark is among the sharks most commonly caught by northern Australian

commercial fisheries 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 p ...

. It is mainly valued for its

meat Meat is animal flesh that is eaten as food. Humans have hunted, farmed, and scavenged animals for meat since prehistoric times. The establishment of settlements in the Neolithic Revolution allowed the domestication of animals such as chic ...

, which is sold as "

flake Flake or Flakes may refer to: People * Floyd H. Flake (born 1945), A.M.E. minister, university administrator, former U.S. representative * Jeff Flake (born 1962), American politician * Christian "Flake" Lorenz, German musician and member of th ...

". This species was an important catch of a

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the no ...

ese

gillnet Gillnetting is a fishing method that uses gillnets: vertical panels of netting that hang from a line with regularly spaced floaters that hold the line on the surface of the water. The floats are sometimes called "corks" and the line with corks is ...

fishery that operated from 1974 to 1986, and of the Australian Northern Shark Fishery that continues to the present day. As current fishing levels are not thought to threaten this shark's population, the

International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...

(IUCN) has assessed it as

Least Concern A least-concern species is a species that has been categorized by the International Union for Conservation of Nature (IUCN) as evaluated as not being a focus of species conservation because the specific species is still plentiful in the wild. ...

Taxonomy and phylogeny

The Australian blacktip shark was described by Australian

ichthyologist Ichthyology is the branch of zoology devoted to the study of fish, including bony fish ( Osteichthyes), cartilaginous fish ( Chondrichthyes), and jawless fish ( Agnatha). According to FishBase, 33,400 species of fish had been described as of O ...

Gilbert Percy Whitley Gilbert Percy Whitley (9 June 1903 – 18 July 1975) was a British-born Australian ichthyologist and malacologist who was Curator of Fishes at the Australian Museum in Sydney for about 40 years. He was born at Swaythling, Southampton, Engla ...

in a 1950 issue of the

scientific journal In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. Content Articles in scientific journals are mostly written by active scientists such ...

''Western Australian Naturalist''. He named it ''Galeolamna pleurotaenia tilstoni'' in honor of Richard Tilston, assistant surgeon at

Port Essington Port Essington is an inlet and historic site located on the Cobourg Peninsula in the Garig Gunak Barlu National Park in Australia's Northern Territory. It was the site of an early attempt at British settlement, but now exists only as a remo ...

Northern Territory The Northern Territory (commonly abbreviated as NT; formally the Northern Territory of Australia) is an Australian territory in the central and central northern regions of Australia. The Northern Territory shares its borders with Western Aust ...

. The

type specimen In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralizes th ...

is a long female caught from Van Cloon Reef in

Joseph Bonaparte Gulf Joseph Bonaparte Gulf is a large body of water off the coast of the Northern Territory and Western Australia and part of the Timor Sea. It was named after Joseph Bonaparte, brother of Napoleon and King of Naples (1806-1808) and then Spain (1808 ...

. Later authors have regarded ''Galeolamna'' as a

synonym A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are al ...

of the genus ''

Carcharhinus ''Carcharhinus'' is the type genus of the family Carcharhinidae, the requiem sharks. One of 12 genera in its family, it contains over half of the species therein. It contains 35 extant and eight extinct species to date, with likely more species ...

''. This shark may also be referred to as blacktip whaler, Tilston's whaler shark, and Whitley's blacktip shark. After Whitley's initial description, ''C. tilstoni'' was generally regarded as synonymous with ''C. limbatus'', the common blacktip shark. In the 1980s, additional morphological and life history data again favored the recognition of ''C. tilstoni'' as a separate species, which was eventually confirmed by

allozyme Alloenzymes (or also called allozymes) are variant forms of an enzyme which differ structurally but not functionally from other allozymes coded for by different alleles at the same locus. These are opposed to isozymes, which are enzymes that pe ...

studies performed by Shane Lavery and James Shaklee. Several

molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

studies based on

mitochondrial DNA Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...

, and

nuclear DNA Nuclear DNA (nDNA), or nuclear deoxyribonucleic acid, is the DNA contained within each cell nucleus of a eukaryotic organism. It encodes for the majority of the genome in eukaryotes, with mitochondrial DNA and plastid DNA coding for the rest. I ...

have found that the Australian and common blacktip sharks form a closely related

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

with the

graceful shark The graceful shark or Queensland shark (''Carcharhinus amblyrhynchoides'') is a species of requiem shark, in the family Carcharhinidae, found in the tropical Indo-Pacific, from the Gulf of Aden to northern Australia. It is a midwater species tha ...

(''C. amblyrhynchoides'') and the

smoothtooth blacktip shark The smoothtooth blacktip shark (''Carcharhinus leiodon'') is a species of requiem shark in the family Carcharhinidae. It is known only from the type specimen caught from the Gulf of Aden, off eastern Yemen, and a handful of additional specimens ...

(''C. leiodon''). The interrelationships between them have not been fully resolved, but available data suggest that ''C. tilstoni'' and ''C. limbatus'' are not the most closely related species within the clade despite their similarity.

Description

Carcharhinus limbatus - Caraibische zwartpunthaai

Physically, the Australian blacktip shark can only reliably be distinguished from the common blacktip shark by the number of

e (174–182 total, 84–91 before the tail in ''C. tilstoni'', 182–203 total, 94–102 before the tail in ''C. limbatus''). It has a moderately robust, spindle-shaped body and a long, pointed snout. The anterior rims of the nostrils are slightly enlarged into low triangular flaps. The large, circular eyes are equipped with

nictitating membrane The nictitating membrane (from Latin '' nictare'', to blink) is a transparent or translucent third eyelid present in some animals that can be drawn across the eye from the medial canthus to protect and moisten it while maintaining vision. All ...

s. The furrows at the corners of the mouth are barely evident. There are 32–35 upper and 29–31 lower tooth rows; each upper tooth has a slender, upright cusp and fine serrations that become coarser near the base, while the lower teeth are narrower and more finely serrated. The five pairs of

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 and rays, as well as deep-branching vertebrates such as lampreys. In co ...

s are long. The long and narrow

pectoral fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

s are falcate (sickle-shaped) with pointed tips. The large first

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through c ...

is also falcate and originates over or slightly behind the rear of the pectoral fin bases. The second dorsal fin is moderately tall and positioned about opposite the

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as see ...

. There is no midline ridge between the dorsal fins. A crescent-shaped notch is present on the

caudal peduncle Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as see ...

just before the upper

caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

origin. The caudal fin is asymmetrical, with a strong lower lobe and a longer upper lobe with a ventral notch near the tip. The roughly diamond-shaped

dermal denticle A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as ...

s are placed closely together and slightly overlapping; each bears five to seven (three in juveniles) horizontal ridges leading to marginal teeth. The Australian blacktip shark is bronze above (gray after death) and whitish below, with a pale stripe on the flanks. Some individuals have black tips on all fins, while others have unmarked pelvic and anal fins. It typically reaches long; the maximum length and weight on record are and .

Distribution and habitat

Inhabiting the

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

, the Australian blacktip shark is found from

Thevenard Island Thevenard Island is located approximately off the coast of Onslow, Western Australia, Onslow in the Pilbara region of Western Australia.Gazetteer of Australia (1996). Belconnen, ACT: Australian Surveying and Land Information Group. Description ...

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to t ...

Sydney Sydney ( ) is the capital city of the state of New South Wales, and the most populous city in both Australia and Oceania. Located on Australia's east coast, the metropolis surrounds Sydney Harbour and extends about towards the Blue Mounta ...

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

. Within its range, it co-occurs with the common blacktip shark; the ratio between ''C. limbatus'' and ''C. tilstoni'' was once thought to be 1:300, but recent genetic studies have found it to be closer to 50:50. This species has been reported from the

to a depth of ; larger sharks tend to occur in deeper water. Though it occupies the entire

, it is most common close to the surface or in midwater. Genetic data suggest Australian blacktip sharks across northern Australia are all members of a single population. Tagging studies have found this species mostly moves only short distances along the coastline and rarely enters offshore waters. However, some individuals have been recorded traveling longer distances, up to .

Biology and ecology

The Australian blacktip shark is known to form large groups, segregated by size and sex. It preys mainly on teleost fishes, including ponyfishes, grunters, tunas, and

herring Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Ocean ...

Cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s are a secondary food source, which are particularly important around April. This species also occasionally consumes smaller sharks, including

snaggletooth shark The snaggletooth shark, or fossil shark (''Hemipristis elongata''), is a species of weasel shark in the family Hemigaleidae, and the only extant member of the genus ''Hemipristis''. It is found in the Indo-West Pacific, including the Red Sea, fr ...

s (''Hemipristis pristis''), spottail sharks (''C. sorrah''), and sharpnose sharks (''Rhizoprionodon''). Their diets shift with age: smaller individuals feed on proportionately more bottom-dwelling fish, while larger individuals feed on proportionately more midwater fish and cephalopods. Documented

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson h ...

s of this shark include the

tapeworm Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class Cestoda (the other subclass is Cestodaria). Larvae have six posterior hooks on the scolex (head), in contrast to the ten-hooked Cesto ...

s '' Fossobothrium perplexum'' and '' Platybothrium'' sp., and the

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have ...

'' Perissopus dentatus''. Like other requiem sharks, the Australian blacktip shark is

viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...

: once the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

s exhaust their

yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example ...

supply, the depleted

yolk sac The yolk sac is a membranous wikt:sac, sac attached to an embryo, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac' ...

develops into a

l connection through which the mother provides sustenance. Females bear litters of one to six pups (average three) every year.

Mating In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...

takes place in February and March, with the females storing the

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, ...

until

ovulation Ovulation is the release of eggs from the ovaries. In women, this event occurs when the ovarian follicles rupture and release the secondary oocyte ovarian cells. After ovulation, during the luteal phase, the egg will be available to be fertilize ...

in March and April. After a

gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pr ...

period of 10 months, the young are born around January of the following year. Near-term females move into shallow, coastal nurseries, such as

Cleveland Bay The Cleveland Bay is a breed of horse that originated in England during the 17th century, named after its colouring and the Cleveland district of Yorkshire. It is a well-muscled horse, with legs that are strong but short in relation ...

in northern Queensland, to give birth. Such nursery areas appear to be widespread, as newborns have been recorded throughout the range of the species. The newborns are relatively large, measuring about long. They grow rapidly in their first year, increasing in length by an average of . The growth rate subsequently slows, averaging per year by age five. Both sexes attain

sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definit ...

at three to four years of age, at lengths of roughly for males and females, respectively. The

maximum lifespan Maximum life span (or, for humans, maximum reported age at death) is a measure of the maximum amount of time one or more members of a population have been observed to survive between birth and death. The term can also denote an estimate of the max ...

is estimated to be 20 years. Hybrids between the Australian blacktip shark and the common blacktip shark, comprising both F1 and backcrossed individuals, have been discovered all along the eastern coast of Australia. Despite the widespread incidence of hybridization, there is no evidence that the two parental lineages are merging and the mechanisms preventing

introgression Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Intr ...

are unclear. This is the first confirmed case of hybridization among

cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...

es.

Human interactions

Along with the spottail shark, the Australian blacktip shark is one of the most economically important sharks off northern Australia. It was historically fished for its meat and

fins A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fin ...

by a

ese

fishery Fishery can mean either the enterprise of raising or harvesting fish and other aquatic life; or more commonly, the site where such enterprise takes place ( a.k.a. fishing ground). Commercial fisheries include wild fisheries and fish farms, ...

, which began operating off northern Australia in 1974. The annual catch initially averaged around 25,000 tons (live weight), with approximately 70% consisting of Australian blacktip sharks, spottail sharks, and longtail tuna (''Thunnus tonggol''). Following the establishment of the Australian Fishing Zone (AFZ) in November 1979, the Australian government progressively excluded Taiwanese gillnet vessels from large portions of Australian waters and imposed

fishing quota Catch share is a fishery management system that allocates a secure privilege to harvest a specific area or percentage of a fishery's total catch to individuals, communities, or associations. Examples of catch shares are individual transferable quot ...

s. The fishery eventually became uneconomical when Australia restricted gillnet lengths to in May 1986, and Taiwanese vessels ceased fishing in Australian waters that year. At end of the fishery, Australian blacktip shark numbers were estimated to have been diminished by around 50%. Given its relatively high reproductive rate, its population has likely since recovered. Since 1980, the Australian blacktip shark has also been targeted by Australian commercial gillnetters and

longliner Longline fishing, or longlining, is a commercial fishing angling technique that uses a long ''main line'' with baited hooks attached at intervals via short branch lines called ''snoods'' or ''gangions''.caught incidentally by other

targeting

bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartil ...

es or

prawn Prawn is a common name for small aquatic crustaceans with an exoskeleton and ten legs (which is a member of the order decapoda), some of which can be eaten. The term "prawn"Mortenson, Philip B (2010''This is not a weasel: a close look at nature' ...

s. The contemporary northern Australian shark catch is estimated to be between 100 and 900 tons (live weight) annually, of which most are Australian blacktip and spottail sharks. The meat of this species is sold in Australia as "

", though it may contain high concentrations of mercury. The fins are exported to Asia, while the

cartilage Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck ...

, liver oil, and

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...

may also be used. Since fishing pressure on the Australian blacktip shark is now much lower than historical levels, the

(IUCN) has listed it under

. However, of potential concern are

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Gui ...

n fishers in the

Arafura Sea The Arafura Sea (or Arafuru Sea) lies west of the Pacific Ocean, overlying the continental shelf between Australia and Western New Guinea (also called Papua), which is the Indonesian part of the Island of New Guinea. Geography The Arafura S ...

, who are increasingly trespassing into the AFZ.

References

External links

"''Carcharhinus tilstoni'', Australian blacktip shark" at FishBase

"Species description of ''Carcharhinus tilstoni''" at Shark-References.com
{{Taxonbar, from=Q783306

Australian blacktip shark The Australian blacktip shark (''Carcharhinus tilstoni'') is a species of requiem shark, in the family Carcharhinidae, endemic to northern and eastern Australia. Favoring the upper and middle parts of the water column, it can be found from the ...

Fish of Australia Marine fish of Northern Australia Endemic fauna of Australia Taxa named by Gilbert Percy Whitley