Grey Reef Shark
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The grey reef shark or gray reef shark (''Carcharhinus amblyrhynchos'', sometimes misspelled ''amblyrhynchus'' or ''amblyrhinchos'') is a
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of requiem shark, in the
family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...
Carcharhinidae. One of the most common reef sharks in the
Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...
, it is found as far east as Easter Island and as far west as
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
. This species is most often seen in shallow water near the drop-offs of
coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in group ...
s. It has the typical "reef shark" shape, with a broad, round snout and large eyes. It can be distinguished from similar species by the plain or white-tipped first
dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...
, the dark tips on the other fins, the broad, black rear margin on the tail fin, and the lack of a ridge between the dorsal fins. Most individuals are less than long. The grey reef shark is a fast-swimming, agile
predator Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...
that feeds primarily on free-swimming
bony fish Osteichthyes ( ; ), also known as osteichthyans or commonly referred to as the bony fish, is a Biodiversity, diverse clade of vertebrate animals that have endoskeletons primarily composed of bone tissue. They can be contrasted with the Chondricht ...
es and
cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...
s. Its aggressive demeanor enables it to dominate many other shark species on the reef, despite its moderate size. Many grey reef sharks have a home range on a specific area of the reef, to which they continually return. However, they are social rather than
territorial A territory is an area of land, sea, or space, belonging or connected to a particular country, person, or animal. In international politics, a territory is usually a geographic area which has not been granted the powers of self-government, ...
. During the day, these sharks often form groups of five to 20 individuals near coral reef drop-offs, splitting up in the evening as the sharks begin to hunt. Adult females also form groups in very shallow water, where the higher water temperature may accelerate their growth or that of their unborn young. Like other members of its family, the grey reef shark is viviparous, meaning the mother nourishes her
embryo An embryo ( ) is the initial stage of development for 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 sp ...
s through a
placenta The placenta (: placentas or placentae) 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 ...
l connection. Litters of one to six pups are born every other year. The grey reef shark was the first shark species known to perform a threat display, a stereotypical behavior warning that it is prepared to attack. The display involves a "hunched" posture with characteristically dropped pectoral fins, and an exaggerated, side-to-side swimming motion. Grey reef sharks often do so if they are followed or cornered by divers to indicate they perceive a threat. This species has been responsible for a number of attacks on humans, and should be treated with caution, especially if it begins to display. It has been caught in many fisheries and is susceptible to local population depletion due to its low reproduction rate and limited dispersal. As a result, the
International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the stat ...
has assessed this species as endangered.


Taxonomy and phylogeny

Dutch
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, 35,800 species of fish had been described as of March 2 ...
Pieter Bleeker Pieter Bleeker (10 July 1819 – 24 January 1878) was a Dutch medical doctor, Ichthyology, ichthyologist, and Herpetology, herpetologist. He was famous for the ''Atlas Ichthyologique des Indes Orientales Néêrlandaises'', his monumental work on ...
first described the grey reef shark in 1856 as ''Carcharias (Prionodon) amblyrhynchos'', in the
scientific journal In academic publishing, a scientific journal is a periodical publication designed to further the progress of science by disseminating new research findings to the scientific community. These journals serve as a platform for researchers, schola ...
''Natuurkundig Tijdschrift voor Nederlandsch-Indië''. Later authors moved this species to the genus ''Carcharhinus''. The
type specimen In biology, a type is a particular wikt:en:specimen, specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally associated. In other words, a type is an example that serves to ancho ...
was a -long female from the
Java Sea The Java Sea (, ) is an extensive shallow sea on the Sunda Shelf, between the Indonesian islands of Borneo to the north, Java to the south, Sumatra to the west, and Sulawesi to the east. Karimata Strait to its northwest links it to the South Ch ...
. Other
common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often con ...
s used for this shark around the world include black-vee whaler, bronze whaler, Fowler's whaler shark, graceful shark, graceful whaler shark, grey shark, grey whaler shark, longnose blacktail shark, school shark, and shortnose blacktail shark. Some of these names are also applied to other species. In older literature, the scientific name of this species was often given as ''C. menisorrah''. The blacktail reef shark (''C. wheeleri''), native to the western Indian Ocean, is now regarded as the same species as the grey reef shark by most authors. It was originally distinguished from the grey reef shark by a white tip on the first dorsal fin, a shorter snout, and one fewer upper tooth row on each side. Based on morphological characters,
vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...
l counts, and tooth shapes, Garrick (1982) concluded the grey reef shark is most closely related to the silvertip shark (''C. albimarginatus''). This interpretation was supported by a 1992 allozyme
phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
analysis by Lavery.


Description

The grey reef shark has a streamlined, moderately stout body with a long, blunt snout and large, round eyes. The upper and lower jaws each have 13 or 14 teeth (usually 14 in the upper and 13 in the lower). The upper teeth are triangular with slanted cusps, while the bottom teeth have narrower, erect cusps. The tooth serrations are larger in the upper jaw than in the lower. The first
dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...
is medium-sized, and no ridge runs between the second dorsal fin and it. The
pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...
s are narrow and falcate (sickle-shaped). The coloration is grey above, sometimes with a bronze sheen, and white below. The entire rear margin of the caudal fin has a distinctive, broad, black band. Dusky to black tips are on the pectoral, pelvic, second dorsal, and
anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...
s. Individuals from the western Indian Ocean have a narrow, white margin at the tip of the first dorsal fin; this trait is usually absent from Pacific populations. Grey reef sharks that spend time in shallow water eventually darken in color, due to tanning. Most grey reef sharks are less than long. The maximum reported length is and the maximum reported weight is .


Distribution and habitat

The grey reef shark is native to the Indian and
Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...
s. In the Indian Ocean, it occurs from
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
to
India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...
, including
Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...
and nearby islands, the
Red Sea The Red Sea is a sea inlet of the Indian Ocean, lying between Africa and Asia. Its connection to the ocean is in the south, through the Bab-el-Mandeb Strait and the Gulf of Aden. To its north lie the Sinai Peninsula, the Gulf of Aqaba, and th ...
, and the
Maldives The Maldives, officially the Republic of Maldives, and historically known as the Maldive Islands, is an Archipelagic state, archipelagic country in South Asia located in the Indian Ocean. The Maldives is southwest of Sri Lanka and India, abou ...
. In the
Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...
, it is found from Southern China to northern
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
and
New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
, including the
Gulf of Thailand The Gulf of Thailand (), historically known as the Gulf of Siam (), is a shallow inlet adjacent to the southwestern South China Sea, bounded between the southwestern shores of the Indochinese Peninsula and the northern half of the Malay Peninsula. ...
, the
Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...
, and
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...
.Bester, C
Biological Profiles: Grey Reef Shark
. ''Florida Museum of Natural History Ichthyology Department''. Retrieved on April 29, 2009.
This species has also been reported from numerous Pacific islands, including
American Samoa American Samoa is an Territories of the United States, unincorporated and unorganized territory of the United States located in the Polynesia region of the Pacific Ocean, South Pacific Ocean. Centered on , it is southeast of the island count ...
, the
Chagos Archipelago The Chagos Archipelago (, ) or Chagos Islands (formerly , and later the Oil Islands) is a group of seven atolls comprising more than 60 islands in the Indian Ocean about south of the Maldives archipelago. This chain of islands is the southernmo ...
, Easter Island,
Christmas Island Christmas Island, officially the Territory of Christmas Island, is an States and territories of Australia#External territories, Australian external territory in the Indian Ocean comprising the island of the same name. It is about south o ...
, the
Cook Islands The Cook Islands is an island country in Polynesia, part of Oceania in the South Pacific Ocean. It consists of 15 islands whose total land area is approximately . The Cook Islands' Exclusive Economic Zone (EEZ) covers of ocean. Avarua is its ...
, the Marquesas Islands, the Tuamotu Archipelago,
Guam Guam ( ; ) is an island that is an Territories of the United States, organized, unincorporated territory of the United States in the Micronesia subregion of the western Pacific Ocean. Guam's capital is Hagåtña, Guam, Hagåtña, and the most ...
,
Kiribati Kiribati, officially the Republic of Kiribati, is an island country in the Micronesia subregion of Oceania in the central Pacific Ocean. Its permanent population is over 119,000 as of the 2020 census, and more than half live on Tarawa. The st ...
, the
Marshall Islands The Marshall Islands, officially the Republic of the Marshall Islands, is an island country west of the International Date Line and north of the equator in the Micronesia region of the Northwestern Pacific Ocean. The territory consists of 29 c ...
,
Micronesia Micronesia (, ) is a subregion of Oceania, consisting of approximately 2,000 small islands in the Northwestern Pacific Ocean. It has a close shared cultural history with three other island regions: Maritime Southeast Asia to the west, Poly ...
,
Nauru Nauru, officially the Republic of Nauru, formerly known as Pleasant Island, is an island country and microstate in the South Pacific Ocean. It lies within the Micronesia subregion of Oceania, with its nearest neighbour being Banaba (part of ...
,
New Caledonia New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...
, the Marianas Islands,
Palau Palau, officially the Republic of Palau, is an island country in the Micronesia subregion of Oceania in the western Pacific Ocean. The Republic of Palau consists of approximately 340 islands and is the western part of the Caroline Islands ...
, the
Pitcairn Islands The Pitcairn Islands ( ; Pitkern: '), officially Pitcairn, Henderson, Ducie and Oeno Islands, are a group of four volcanic islands in the southern Pacific Ocean that form the sole British Overseas Territories, British Overseas Territory in the ...
,
Samoa Samoa, officially the Independent State of Samoa and known until 1997 as Western Samoa, is an island country in Polynesia, part of Oceania, in the South Pacific Ocean. It consists of two main islands (Savai'i and Upolu), two smaller, inhabited ...
, the
Solomon Islands Solomon Islands, also known simply as the Solomons,John Prados, ''Islands of Destiny'', Dutton Caliber, 2012, p,20 and passim is an island country consisting of six major islands and over 1000 smaller islands in Melanesia, part of Oceania, t ...
,
Tuvalu Tuvalu ( ) is an island country in the Polynesian subregion of Oceania in the Pacific Ocean, about midway between Hawaii and Australia. It lies east-northeast of the Santa Cruz Islands (which belong to the Solomon Islands), northeast of Van ...
, the
Hawaiian Islands The Hawaiian Islands () are an archipelago of eight major volcanic islands, several atolls, and numerous smaller islets in the Pacific Ocean, North Pacific Ocean, extending some from the Hawaii (island), island of Hawaii in the south to nort ...
, and
Vanuatu Vanuatu ( or ; ), officially the Republic of Vanuatu (; ), is an island country in Melanesia located in the South Pacific Ocean. The archipelago, which is of volcanic origin, is east of northern Australia, northeast of New Caledonia, east o ...
. Generally a coastal, shallow-water species, grey reef sharks are mostly found in depths less than . However, they have been known to dive to . They are found over continental and insular shelves, preferring the leeward (away from the direction of the current) sides of coral reefs with clear water and rugged topography. They are frequently found near the drop-offs at the outer edges of the reef, particularly near reef channels with strong currents, and less commonly within
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...
s. On occasion, this shark may venture several kilometers out into the open ocean.


Biology and ecology

Along with the
blacktip reef shark The blacktip reef shark (''Carcharhinus melanopterus'') is a species of requiem shark, in the family (biology), family Carcharhinidae, which can be easily identified by the prominent black tips on its fins (especially on the first dorsal fin and ...
(''C. melanopterus'') and the whitetip reef shark (''Triaenodon obesus''), the grey reef shark is one of the three most common sharks inhabiting Indo-Pacific reefs. They actively expel most other shark species from favored habitats, even species larger in size. In areas where this species co-exists with the blacktip reef shark, the latter species occupies the shallow flats, while the former stays in deeper water. Areas with a high abundance of grey reef sharks tend to contain few sandbar sharks (''C. plumbeus''), and ''vice versa''; this may be due to their similar diets causing competitive exclusion. The consumptive influence of grey reef sharks on reef fish communities is likely to vary depending on whether sharks forage within the reef environment, or on pelagic resources (like they have been observed to do at
Palmyra Atoll Palmyra Atoll (), also referred to as Palmyra Island, is one of the Line Islands, Northern Line Islands (southeast of Kingman Reef and north of Kiribati). It is located almost due south of the Hawaiian Islands, roughly one-third of the way be ...
). On the infrequent occasions when they swim in oceanic waters, grey reef sharks often associate with
marine mammal Marine mammals are mammals that rely on marine ecosystems for their existence. They include animals such as cetaceans, pinnipeds, sirenians, sea otters and polar bears. They are an informal group, unified only by their reliance on marine enviro ...
s or large pelagic fishes, such as sailfish (''Istiophorus platypterus''). One account has around 25 grey reef sharks following a large pod of
bottlenose dolphin The bottlenose dolphin is a toothed whale in the genus ''Tursiops''. They are common, cosmopolitan members of the family Delphinidae, the family of oceanic dolphins. Molecular studies show the genus contains three species: the common bot ...
s (''Tursiops'' sp.), along with 25 silky sharks (''C. falciformis'') and a single silvertip shark. Rainbow runners (''Elagatis bipinnulata'') have been observed rubbing against grey reef sharks, using the sharks' rough skin to scrape off parasites. Grey reef sharks are themselves prey for larger sharks, such as the silvertip shark. At Rangiroa Atoll in
French Polynesia French Polynesia ( ; ; ) is an overseas collectivity of France and its sole #Governance, overseas country. It comprises 121 geographically dispersed islands and atolls stretching over more than in the Pacific Ocean, South Pacific Ocean. The t ...
, great hammerheads (''Sphyrna mokarran'') feed opportunistically on grey reef sharks that are exhausted from pursuing mates. Known
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
s of this species include the
nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...
'' Huffmanela lata'' and several
copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...
species that attach to the sharks' skin, and juvenile stages of the
isopod Isopoda is an Order (biology), order of crustaceans. Members of this group are called isopods and include both Aquatic animal, aquatic species and Terrestrial animal, terrestrial species such as woodlice. All have rigid, segmented exoskeletons ...
s ''Gnathia trimaculata'' and ''G. grandilaris'' that attach to the
gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...
filaments and
septa SEPTA, the Southeastern Pennsylvania Transportation Authority, is a regional public transportation authority that operates bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly four million people througho ...
(the dividers between each gill).


Feeding

Grey reef sharks feed mainly on bony fishes, with
cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...
s such as
squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...
and
octopus An octopus (: octopuses or octopodes) is a soft-bodied, eight-limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like oth ...
being the second-most important food group, and crustaceans such as
crab Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...
s and
lobster Lobsters are Malacostraca, malacostracans Decapoda, decapod crustaceans of the family (biology), family Nephropidae or its Synonym (taxonomy), synonym Homaridae. They have long bodies with muscular tails and live in crevices or burrows on th ...
s making up the remainder. The larger sharks take a greater proportion of cephalopods. These sharks hunt individually or in groups, and have been known to pin schools of fish against the outer walls of coral reefs for feeding. Hunting groups of up to 700 grey reef sharks have been observed at Fakarava atoll in French Polynesia. They excel at capturing fish swimming in the open, and they complement hunting whitetip reef sharks, which are more adept at capturing fish inside caves and crevices. Their sense of smell is extremely acute, being capable of detecting one part tuna extract in 10 billion parts of sea water. In the presence of a large quantity of food, grey reef sharks may be roused into a feeding frenzy; in one documented frenzy caused by an underwater explosion that killed several snappers, one of the sharks involved was attacked and consumed by the others.


Life history

During mating, the male grey reef shark bites at the female's body or fins to hold onto her for copulation. Like other requiem sharks, it is viviparous; once the developing embryos exhaust their supply of
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 bec ...
, the
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
placenta The placenta (: placentas or placentae) 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 ...
l connection that sustains them to term. Each female has a single functional
ovary The ovary () is a gonad in the female reproductive system that produces ova; when released, an ovum travels through the fallopian tube/ oviduct into the uterus. There is an ovary on the left and the right side of the body. The ovaries are end ...
(on the right side) and two functional uteri. One to four pups (six in Hawaii) are born every other year; the number of young increases with female size. Estimates of the
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 i ...
range from 9 to 14 months. Parturition is thought to take place from July to August in the Southern Hemisphere and from March to July in the Northern Hemisphere. However, females with "full-term embryos" have also been reported in the fall off Enewetak. The newborns measure long. Sexual maturation occurs around seven years of age, when the males are long and females are long. Females on the
Great Barrier Reef The Great Barrier Reef is the world's largest coral reef system, composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, ...
mature at 11 years of age, later than at other locations, and at a slightly larger size. The lifespan is at least 25 years.


Behavior

Grey reef sharks are active at all times of the day, with activity levels peaking at night. At Rangiroa, groups of around 30 sharks spend the day together in a small part of their collective home range, dispersing at night into shallower water to forage for food. Their home range is about . At Enewetak in the Marshall Islands, grey reef sharks from different parts of the reef exhibit different social and ranging behaviors. Sharks on the outer ocean reefs tend to be nomadic, swimming long distances along the reef, while those around lagoon reefs and underwater pinnacles stay within defined daytime and nighttime home ranges. Where strong tidal currents occur, grey reef sharks move against the water, toward the shore with the ebbing tide and back out to sea with the rising tide. This may allow them to better detect the scent of their prey, or afford them the cover of turbid water in which to hunt.Martin, R.A
Coral Reefs: Grey Reef Shark
''ReefQuest Centre for Shark Research''. Retrieved on April 30, 2009.
Little evidence of territoriality is seen in the grey reef shark; individuals tolerate others of their species entering and feeding within their home ranges. Off Hawaii, individuals may stay around the same part of the reef up to three years, while at Rangiroa, they regularly shift their locations up to . Individual grey reef sharks at Enewetak become highly aggressive at specific locations, suggesting they may exhibit dominant behavior over other sharks in their home areas.


Sociality

Social aggregation is well documented in grey reef sharks. In the northwestern Hawaiian Islands, large numbers of pregnant females have been observed slowly swimming in circles in shallow water, occasionally exposing their dorsal fins or backs. These groups last from 11:00 to 15:00, corresponding to peak daylight hours. Similarly, at Sand Island off
Johnston Atoll Johnston Atoll is an Unincorporated territories of the United States, unincorporated territory of the United States, under the jurisdiction of the United States Air Force (USAF). The island is closed to public entry, and limited access for mana ...
, females form aggregations in shallow water from March to June. The number of sharks per group differs from year to year. Each day, the sharks begin arriving at the aggregation area at 09:00, reaching a peak in numbers during the hottest part of the day in the afternoon, and dispersing by 19:00. Individual sharks return to the aggregation site every one to six days. These female sharks are speculated to be taking advantage of the warmer water to speed their growth or that of their embryos. The shallow waters may also enable them to avoid unwanted attention by males. Off Enewetak, grey reef sharks exhibit different social behaviors on different parts of the reef. Sharks tend to be solitary on shallower reefs and pinnacles. Near reef drop-offs, loose aggregations of five to 20 sharks form in the morning and grow in number throughout the day before dispersing at night. In level areas, sharks form polarized schools (all swimming in the same direction) of around 30 individuals near the sea bottom, arranging themselves parallel to each other or slowly swimming in circles. Most individuals within polarized schools are females, and the formation of these schools has been theorized to relate to mating or pupping.


Threat display

The "hunch" threat display of the grey reef shark is the most pronounced and well-known agonistic display (a display directed toward competitors or threats) of any shark. Investigations of this behavior have been focused on the reaction of sharks to approaching divers, some of which have culminated in attacks. The display consists of the shark raising its snout, dropping its pectoral fins, arching its back, and curving its body laterally. While holding this posture, the shark swims with a stiff, exaggerated side-to-side motion, sometimes combined with rolls or figure-8 loops. The intensity of the display increases if the shark is more closely approached or if obstacles are blocking its escape routes, such as landmarks or other sharks. If the diver persists, the shark may either retreat or launch a rapid, open-mouthed attack, slashing with its upper teeth. Most observed displays by grey reef sharks have been in response to a diver (or
submersible A submersible is an underwater vehicle which needs to be transported and supported by a larger ship, watercraft or dock, platform. This distinguishes submersibles from submarines, which are self-supporting and capable of prolonged independent ope ...
) approaching and following it from a few meters behind and above. They also perform the display toward
moray eel Moray eels, or Muraenidae (), are a family (biology), family of eels whose members are found worldwide. There are approximately 200 species in 15 genera which are almost exclusively Marine (ocean), marine, but several species are regu ...
s, and in one instance toward a much larger great hammerhead (which subsequently withdrew). However, they have never been seen performing threat displays toward each other. This suggests the display is primarily a response to potential threats (i.e. predators) rather than competitors. As grey reef sharks are not territorial, they are thought to be defending a critical volume of "personal space" around themselves. Compared to sharks from French Polynesia or Micronesia, grey reef sharks from the Indian Ocean and western Pacific are not as aggressive and less given to displaying.


Human interactions

Grey reef sharks are often curious about divers when they first enter the water and may approach quite closely, though they lose interest on repeat dives. They can become dangerous in the presence of food, and tend to be more aggressive if encountered in open water rather than on the reef. There have been several known attacks on spearfishers, possibly by mistake, when the shark struck at the speared fish close to the diver. This species will also attack if pursued or cornered, and divers should immediately retreat (slowly and always facing the shark) if it begins to perform a threat display. Photographing the display should not be attempted, as the flash from a camera is known to have incited at least one attack. Although of modest size, they are capable of inflicting significant damage: during one study of the threat display, a grey reef shark attacked the researchers' submersible multiple times, leaving tooth marks in the plastic windows and biting off one of the propellers. The shark consistently launched its attacks from a distance of , which it was able to cover in a third of a second. As of 2008, the International Shark Attack File listed seven unprovoked and six provoked attacks (none of them fatal) attributable to this species. Although still abundant in pristine sites, grey reef sharks are susceptible to localized depletion due to their slow reproductive rate, specific habitat requirements, and tendency to stay within a certain area. The
IUCN The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status ...
has assessed the grey reef shark as endangered; this shark is taken by multispecies fisheries in many parts of its range and used for various products such as shark fin soup and fishmeal. Another threat is the continuing degradation of coral reefs from human development. There is evidence of substantial declines in some populations. Anderson ''et al.'' (1998) reported, in the Chagos Archipelago, grey reef shark numbers in 1996 had fallen to 14% of 1970s levels. Robbins ''et al.'' (2006) found grey reef shark populations in
Great Barrier Reef The Great Barrier Reef is the world's largest coral reef system, composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, ...
fishing zones had declined by 97% compared to no-entry zones (boats are not allowed). In addition, no-take zones (boats are allowed but fishing is prohibited) had the same levels of depletion as fishing zones, illustrating the severe effect of
poaching Poaching is the illegal hunting or capturing of wild animals, usually associated with land use rights. Poaching was once performed by impoverished peasants for subsistence purposes and to supplement meager diets. It was set against the huntin ...
. Projections suggested the shark population would fall to 0.1% of pre-exploitation levels within 20 years without additional conservation measures. One possible avenue for conservation is
ecotourism Ecotourism is a form of nature-oriented tourism intended to contribute to the Ecological conservation, conservation of the natural environment, generally defined as being minimally impactful, and including providing both contributions to conserv ...
, as grey reef sharks are suitable for shark-watching ventures, and profitable diving sites now enjoy protection in many countries, such as the Maldives.


References


External links


"''Carcharhinus amblyrhynchos'', Grey reef shark" at FishBase

"''Carcharhinus amblyrhynchos'' (Grey Reef Shark)" at IUCN Red List


* ttp://elasmo-research.org/education/ecology/coral-grey_reef.htm "Coral Reefs: Grey Reef Shark" at ReefQuest Centre for Shark Research
"Species description of ''Carcharhinus amblyrhynchos''" at Shark-References.com
* {{Good article
grey reef shark The grey reef shark or gray reef shark (''Carcharhinus amblyrhynchos'', sometimes misspelled ''amblyrhynchus'' or ''amblyrhinchos'') is a species of requiem shark, in the family (biology), family Requiem shark, Carcharhinidae. One of the most com ...
Taxa named by Pieter Bleeker
grey reef shark The grey reef shark or gray reef shark (''Carcharhinus amblyrhynchos'', sometimes misspelled ''amblyrhynchus'' or ''amblyrhinchos'') is a species of requiem shark, in the family (biology), family Requiem shark, Carcharhinidae. One of the most com ...