One of the most popular shark species found in the Indian Ocean, Pacific Ocean, and the Caribbean sea is the Grey Reef Shark (Carcharhinus amblyrhynchos). This shark belongs to the family Carcharinidae and requiem shark species. This shark occurs as far west as South Africa, and as far east as Easter Island.
As a reef shark, this shark has a typical broad, round snout and large eyes. They are active predators with fast swimming skills. Their food consists mainly of cephalopods and bony fishes. The grey reef shark has dominance over most species on the reef. This is due to their known aggressive behavior toward others.
Most grey reef sharks prefer to choose a particular area in the reef which they always return to. In other words, they exhibit home range living within their habitat.
Even while these sharks are rather social than territorial, caution must be taken around them. Of course, they have several attack cases on humans. Grey reef sharks display many interesting behaviors from socializing, feeling of threat, to the method of attack that is worth learning about. Hence, the essence of this page.
Differentiating the Grey Reef Shark from Similar Species
Reef sharks have a typical body shape and structure. However, to distinguish the grey reef shark from similar species there are factors to look out for. These include a plain or white-tipped first dorsal fin, where the other fins have dark tips. The wide and black rear margin on the tail fin. Most of all, there is no ridge between the dorsal fins. Most grey reef sharks grow within the length of 6.2 feet (1.9 m).
Names of Grey Reef Sharks
Around the world, different people call the grey reef shark by various common names. These include bronze whaler, grey whaler shark, black-vee whaler, longnose blacktail shark, graceful whaler shark, grey shark, and Fowler’s whaler shark.
The first scientific description of this species was in 1856 by a Dutch ichthyologist Peter Bleeker. He first described it as Carcharias (Prionodon) amblyrhynchos. Later on, the authors moved this species to the genus Carcharhinus. The type specimen used for this description was a 4.9-foot (1.5 m)-long female captured from the Java sea.
Older works of literature may refer to this species as C. menisorrah. Even more, most recent authors now regard the blacktail reef shark (C. wheeleri) which is indigenous to the western Indian Ocean as the same with the grey reef shark. The initial distinguishing factors from the grey reef includes the white tip on the first dorsal fin, one tooth fewer on each side of the upper tooth row, and a shorter snout. In all, vertebral counts, morphological characters, and tooth shapes concluded the silvertip shark (C. albimarginatus) to be the most closely related to the grey reef shark.
Habitat and Distribution Range
Grey reef sharks are generally coastal, shallow-water dwellers. They mostly occur at depths not greater than 200 feet (60 m). Although, they have been known to dive to great depths up to 3,300 feet (1000 m). These sharks live over the continental and insular shelves. They prefer the leeward sides of coral reefs with rough topography and clear water.
Often, grey reef sharks swim close to the drop-offs at the outer edges of the reef. They prefer to stay near reef channels with strong currents in particular. These sharks may be found less often in lagoons. Though, on occasions, they may set out into the open ocean covering several miles.
The grey reef shark is indigenous to the Indian and Pacific Oceans. In the Indian Ocean, this species occurs from South Africa to India, including the Red sea, the Maldives, Madagascar and nearby islands. And, in the Pacific Ocean, they occur from southern China to northern Australia and Newzealand. This includes the Philippines, Gulf of Thailand, and Indonesia.
Reports of this species in various Pacific Islands abound. The islands include Easter Island, the Chagos Archipelago, Christmas Island, the Mariana Island, the Marquesas Islands, the Hawaiian Island, the Solomon Island, Palau, Pitcairn Island, Micronesia, Tuamotu Archipelago, Guam, and Kiribati.
Grey Reef Shark Description
The grey reef shark has a moderately stout and streamlined body, typical of reef sharks. They also have long, blunt snout with large eyes. This shark has an average-sized first dorsal fin with no ridge running between it and the second dorsal fin. The pectoral fins are narrow with the shape of a sickle.
There is a dominant grey color on the backside, and sometimes with a bronze sheen. However, the underside is mostly white. On the rear margin of the caudal fin is a characteristic broad, black band. While the pectoral fins, anal fin, second dorsal fin, and pelvic fin have dusky to black tips. Grey reef shark individuals that spend more time in shallower waters tend to be darker in color as a result of tanning.
In the upper jaw are usually 14 teeth that are triangular with slanted cusps. While the lower jaws contain usually 13 teeth that have narrower and erect cusps. There are noticeably larger tooth serrations in the upper jaw than in the lower jaw.
Distribution range differences may occur, thus, individuals from the Western Indian Ocean usually have at the tip of their first dorsal fin a narrow, white margin. However, this trait is not present in individuals from the Pacific Ocean.
The maximum length recorded of a grey reef shark is 8.5 feet (2.6 m). While the maximum weight is 74 pounds (33.7 kg). In all, most grey reef sharks will only grow within a length of 6.2 feet (1.9 m).
Typical Behavior of the Grey Reef Shark
These sharks tend to be more active in the dark, though they are not dormant during the day. The activities of grey reef sharks somehow vary depending on their environment.
During the daytime at Rangiroa, a group of about 30 grey reef sharks stays together in their collective home range choosing a smaller part. However, at night, they go out into shallower water in search of food. Here also, these sharks shift their locations regularly up to 9.3 miles (15 km). Though, off Hawaii, individuals may remain in the same part of the reef longer up to 3 years.
In the Marshall Island (Enewetak), there is a noticeable varying social and ranging behavior observed in grey reef sharks from different parts of the reef. Thus, sharks around the lagoon reefs and underwater pinnacles stay within defined day and night home ranges. But, those on the outer ocean reef tend to be nomadic. As such, they usually swim along the reef through long distances as part of their regular activities.
For places experiencing strong tidal currents, these sharks would move against the currents. With the ebbing tide (the tide reflux toward the sea), grey reef sharks swim toward the shore. As the tide rises, they move back out to the sea. This behavior may enable them to smell their prey, or provides for them turbid water cover in which they can hunt.
Does Grey Reef Sharks Exhibit Territoriality?
Grey reef sharks show very little evidence of territorial behavior. They tolerate other individuals of their species to come within their home ranges and feed. At particular locations, individual grey reef sharks at Enewetak tend to become highly aggressive. This suggests their likeliness to show dominance over other sharks within their home ranges.
Social Behavior
Grey reef sharks exhibit social grouping in various senses. Observations of these species in the northwestern Hawaiian Islands showed a large number of females that are pregnant swimming slowly in circles in shallow water and exposing their dorsal fins and backs occasionally. These groups appear during the peak hours of the daylight, hence within 11:00 and 15:00.
There is a similar grouping observed at Sand Island off Johnston Atoll, here, females form groups in shallow water from March to June. The number of individuals involved in the grouping varies each year. Individual sharks start arriving at the grouping site at about 9:00 each day. They reach the peak numbers in the hottest part of the afternoon and disperse around 19:00. The individual shark comes back to the grouping site every 1 to 6 days.
The females are believed to be taking advantage of the warmer water to hasten their growth or possibly that of their embryos. Also, the shallow waters may help them stay away from unwanted attention by males.
Grey reef sharks exhibit varying social behaviors on different parts of the reef off Enewetak. Thus, on shallower reefs and pinnacles sharks tend to be solitary. Meanwhile, they form loose groups of 5 to 20 individuals near reef drop-offs which increases through the day till they disperse at night.
Are Grey Reef Sharks Schooling Species?
Grey reef sharks form polarized schools in level areas. That is a school where all members swim in the same direction. There are about 30 members in a school which usually occur near the bottom of the sea. They will arrange themselves parallel to each other or swim slowly in a circular motion.
Females are dominant within polarized schools. Researchers believe that polarized groups have more to do with mating and giving birth to their pups.
Threat Display
Grey reef sharks have the most recognized agonistic display. In other words, a display or defensive social interaction directed toward threats or competitors. This display of grey reef sharks is known as the “hunch threat display”. While conducting research on this display of grey reef sharks, the focus was on the sharks’ reaction toward approaching divers.
This display involves the shark raising its snout and dropping its pectoral fins. Then, it aches its back while curving its body laterally (to the sides). With this posture, the shark swims in a stiff and heightened side-to-side motion. Rolls or figure-8 loops usually follow this motion. And, when more closely approached, or it notices blocked escape routes caused by other sharks or landmarks, the shark increases the intensity of its display.
In the cases where the diver persists, the grey reef shark will tend to retreat or if not possible, it launches a rapid attack. Their attack usually involves an open mouth slashing with its upper teeth.
Threat display by great reef sharks as observed are mostly as a response to approaching divers or submersible. These sharks also display when followed behind or above. They have been observed displaying towards moray eels and once toward a larger great hammerhead individual which withdrew subsequently.
Do Grey Reef Sharks Show Threat Display Towards their Species?
In all the threat display observed from grey reef sharks, none was towards their species. Hence, these sharks do not perform threat display towards each other. This suggests that the threat display adaptation is a primary response to potential threats rather than competitors.
Grey reef sharks are thought to defend a vital volume of personal space surrounding them, as they are not territorial species. Individual grey reef sharks from the western Pacific and Indian Oceans exhibit threat display less often and not as aggressive when compared to those from French Polynesia or Micronesia.
Hunting and Feeding Habits
Grey reef sharks can hunt individually or in groups. These sharks are notable for pinning schools of fish against the outer coral reef walls for feeding. In French Polynesia (Fakarava atoll), a hunting group consisting of up to 700 grey reef sharks have been observed. These sharks do well at capturing fish swimming in the open. They tend to compliment whitetip reef sharks which are extremely proficient in capturing fish hiding in crevices and caves.
The sense of smell of grey reef sharks is highly exquisite that they can detect one part of tuna extract in 10 billion parts of seawater. These shark in the abundance of food may go into a feeding frenzy. An example documented is a frenzy that came as a result of an underwater explosion leading to the death of several snappers. Here, they attacked one of the sharks involved and others consumed it.
The food of grey reef sharks consists of most bony fishes. Afterward, come cephalopods which include squid and octopus as the second in the list. Others are crustaceans such as crabs and lobsters which make up the remaining parts of the diet. Larger individuals consume more of cephalopods.
Mating and Reproduction
Mating in grey reef sharks involves the males biting tye females on their fins or body in an attempt to make her submit to copulation. These sharks just like other requiem sharks are viviparous. Thus, the embryos survive from the supply of yolk, however, as the yolk exhausts, the yolk sac develops into a connection just like a placenta that keeps sustaining them until birth.
The female shark possesses two functional uteri with a single functional ovary positioned on the right side. They give birth to pups ranging from 1 to 6 in number. The larger the size of the female, the higher the number of pups. An estimated gestation period for this species ranges from 9 to 14 months. Parturition which is the process of delivering offspring according to researchers’ speculation takes place from March to July in the Northern Hemisphere. While in the Southern Hemisphere, it starts from July to August.
At birth, the pups measure about 18 – 24 inches (45 – 60 cm) in length. Males of this species mature sexually at the length of 4.3 – 4.9 feet (1.3 – 1.5 m), while females do so at 3.9 – 4.6 feet (1.2 – 1.4 m); They attain this length when they are around 7 years of age. On the Great Barrier Reef, females may take about 11 years to mature, and even later at other locations. Though, they seem quite larger in size. This species lifespan is about 25 years.
The Grey Reef Shark Interaction with the Environment
The Grey reef shark, the whitetip reef shark (Triaenodon obesus), and the blacktip reef shark (C. melanopterus) are the three most common sharks inhabiting the indo-pacific reefs. These sharks actively force out most other species of sharks from favored habitats including larger species than them.
In regions where grey reef sharks cohabit with blacktip reef sharks, the former tend to stay in deeper water, while the latter mostly occupy the shallow flats. Places with a high number of grey reef sharks contain only a few sandbar sharks (C. plumbeus) and vice versa. The reason may have to do with their similar diets, thus causing competitive exclusion.
Grey reef sharks tend to associate with marine mammals and larger pelagic fishes as observed in their infrequent visits to the open ocean waters. One of such association observed, involved about 25 grey reef sharks following a large pod of bottlenose dolphins (Tursiops sp.), swimming along with 25 silky sharks (C. falciformis), and a silvertip shark.
Another association is rainbow runners (Elagatis bipinnulata) rubbing themselves against grey reef sharks. They take advantage of the rough skin of grey reef sharks to scrape off parasites.
Grey Reef Sharks’ Predators
Larger shark species such as the silvertip shark prey on grey reef sharks. Also, observed at Rangiroa Atoll in French Polynesia, is the great hammerhead shark (Sphyrna mokarran) opportunistically feeding on exhausted grey reef sharks pursuing mates.
Parasites
Parasites linked to this species include juvenile stages of isopods Gnathia trimaculata and G. grandilaris which attaches to the gill filaments and septa, the nematode Huffmanela lata, and many copepod species that attach to the skin of the shark.
Interaction with Humans
From a diver’s point of view, grey reef sharks are curious about divers entering the water the first time. Thus, they tend to approach quite closely. However, repeated dives will make them lose interest. In the presence of food, these sharks can be dangerous. They also show more signs of aggression when encountered in the open ocean than in the reefs.
Atacks on Humans
The International Shark Attack File (ISAF) as of 2008 listed six provoked and seven unprovoked attacks on humans by the grey reef shark. This shark has several reports of attacks on spearfishers which could probably be due to mistake. Of course, the shark may strike at the spearfish caught by the diver.
This shark also attacks if it feels threatened when pursued or cornered. And, once this shark starts its threat display, divers should take caution by retreating while always facing the shark.
Attempting to photograph the display is not encouraged as at least one attack has been incited by the flash from a camera. This shark despite its moderate size can still inflict significant damage. In a study of this shark’s threat display, the individual left the plastic glass of the researchers’ submersible with tooth marks and even bit off one of its propellers.
It is worthy to note that none of the attacks on humans by this species recorded by the ISAF is fatal.
Conservation Status
The IUCN assessed the Carcharhinus amblyrhynchos as “Near Threatened (NT)”. The threat comes from localized depletion mostly resulting from their slow reproductive rate and requiring a specific habitat for survival.
Multispecies fisheries also take this shark for various uses such as shark fin soup and fishmeal. Another major threat is habitat degradation resulting from human developments. All these caused the depletion in the population of grey reef sharks.
There are currently several conservation actions put in place to protect this species in places it occurs. Certainly, there is a need for the actions, else the population of grey reef sharks will drop drastically in the nearest future.
Cool Grey Reef Shark Facts at a Glance
1. The Common Name comes from the Skin Color
The upper side of this shark is predominantly grey in color which gave rise to its common name. Even more, individuals occupying shallow water have darker shades of this color due to tanning.
2. Can Attain a Speed of 25mph
While grey reef sharks tend to swim slowly most times, it can also reach a great speed of 25 miles per hour while pursuing prey or launching an attack.
3. Grey Reef Sharks are More Active at Night
This shark species is active all through the day, however, their activity level increases at night.
4. Their Favored Prey is Bony Fish
Grey reef sharks prefer meals of bony fish. They also feed on crabs and squids. These sharks are active predators and hunt down their prey to feed.
5. The Teeth is Adapted to Tearing of Flesh
Grey reef sharks are true carnivores. Thus, they have very sharp triangular, serrated teeth for tearing of flesh.
6. Their Sense of Smell is Excellent
The sense of this species is so powerful that they can smell a tuna fish in 10 billionth part of the seawater.
7. Grey Reef Sharks are More Likely to Attack During Feeding
In the abundance of food this shark species get excited and likely to attack if approached without caution. In fact, most attacks happened while this shark was feeding.
8. Shows One of the Most Developed Threat Display
When threatened, this shark tends to swim in a defined side-to-side motion making the figure-8 loop. Unless the threat retreats, the shark would definitely attack following this display.
9. They Practice Social Grouping
These sharks can form a group of 20 individuals during the day. However, they disperse as it gets dark, and they generally prefer a solitary life. In all, they hunt alone.
10. Males Show Aggressive Mating Ritual
To make the female submit to mating, the males tend to bite her in the fins or back for copulation to take place. Consider they have sharp teeth.
Further Reading
- “Carcharhinus amblyrhynchos, Grey reef shark”, FishBase.
- “Discover Fishes: Carcharhinus amblyrhynchos, Grey reef shark”, Florida Museum of Natural History.
- “Grey Reef Shark (Carcharhinus amblyrhynchos)“, IUCN Red List of Threatened Species.
- “Coral Reefs: Diversity on Display, Grey Reef Shark”, ReefQuest Centre for Shark Research.
- “Carcharhinus amblyrhynchos, Grey Reef Shark”, Shark-References.