Coral Reefs: Diversity on Display
Grey Reef Shark
Best known for its spectacular agonistic display, the Grey Reef Shark (Carcharhinus amblyrhynchos) is pugnacious all out of proportion to its modest size. The agonistic display in this species ranges from mild to extreme, depending upon degree of escape route restriction. This display is elicited in approach-withdrawal conflict situations and appears to be ritualized from movements and postures associated with overpowering prey. It features rolling or spiraling during laterally exaggerated swimming while lowering the pectoral fins, raising the snout, and arching the back. The display is highly stereotyped and seems to communicate that the shark ‘feels’ threatened — warning the receiver to “come no closer”. If the receiver of this signal does not move away, the displaying shark may launch a lightning-fast, slashing attack.
Just the Facts:
Habitat: Intertidal, Sandy Plains, Coral Reefs, Deep Sea
Depth: Intertidal - 920 ft (280 m)
Distribution: Central Pacific, Southern African, Madagascaran, Arabian, Indian?, South East Asian, Western Australian, Southeast Australian/New Zealand, Northern Australian, Japanese
Despite such seemingly anti-social behavior, Grey Reef Sharks are apparently highly social with members of their own species. Day-time grouping behavior in Grey Reef Sharks has been investigated at Enewetak Atoll, Marshall Islands, and found to consist of three basic patterns. Polarized schools of 27 or more individuals occur close to the bottom over level areas, with clusters of closely-spaced, parallel-oriented sharks sometimes coalescing to form loose aggregations that together perform a circular milling pattern. These schools seem to become larger as the day wears on, suggesting that they break apart at night and gradually reform the following day. Loose aggregations of 5 to 20 individuals occur near reef drop-offs — places where, on the seaward side of the Atoll, the bottom falls away abruptly from 15 to 200 feet (95 to 60 metres). Lastly, lone individuals were usually observed over shallower reefs and lagoon pinnacles; these solitary animals are not merely patrolling the reef aimlessly, for they often make rapid investigatory approaches toward divers that sometimes culminate in an agonistic display.
Grouping behavior in Grey Reef Sharks has also been studied at Johnston Atoll, in the Central Pacific. Here, between March and late May, researchers discovered that loose aggregations of about 30 sharks form during the hottest part of each day. All the sharks are mature females, suggesting that these aggregations may have something to do with reproduction. If most or all of these sharks are pregnant, aggregating in warm water may increase the rate of embryonic development. Alternatively, warm water at the aggregation area may increase the growth rate of female sharks, thereby accelerating the onset of maturity and possibly increasing the reproductive fitness of individuals. Perhaps these female-only aggregations may simply be a designated place to avoid amorous, nippy males during the reproductive season. Or, since many species of sharks segregate by sex during much of the year, these aggregation areas may function as a central landmark to foraging routes, much like Scalloped Hammerheads (Sphyrna lewini) use seamounts as refuging areas. Time and further research may shed additional light on why these aggregations of female Grey Reef Sharks occur annually at Johnston Atoll.
Activity rhythms and home range of Grey Reef Sharks have been studied via sonic telemetry and direct observation at Rangiroa, French Polynesia. During the day, these sharks typically swim slowly in groups, milling about a well-defined “core area” in a relatively deep part of their collective home range. At dusk, they move away from the relatively small core area and range widely over shallower parts of the reef, presumably foraging. Telemetry data also suggests that Grey Reef Sharks are more dispersed at night, but this could not be confirmed by direct observation. For one Tiputa Pass individual tracked continuously for 72 hours, its home range was about 0.3 square miles (0.8 square kilometres) — an area slightly smaller than the average nocturnal activity space of Whitetip Reef Sharks (Triaenodon obesus). This smaller activity space of Grey Reef Sharks may reflect the richer feeding to be had at the depths and parts of the reef favored by this species. Dominated by Grey Reef Sharks, Whitetip Reef Sharks may be relegated to foraging over suboptimal parts of the reef at night, imposing a need to travel farther afield to secure sufficient food. As in human society, high social rank among sharks has its privileges.
Grey Reef Sharks from Rangiroa Atoll show circadian rhythms that are quite regular from day-to-day and, once learned, are quite predictable. Their daily routines are apparently keyed to periodic environmental cues. Movement patterns of Grey Reefs seem to be markedly influenced by the tidal cycle, at least in areas of strong tidal currents and water clarity fluctuations. Although there are slight regional differences in daily activity patterns of these sharks from different areas of Rangiroa, their circadian rhythms are fundamentally similar because they all respond in the same way to the twice-daily pulse of the tides: moving shoreward during each ebb tide and seaward during each flood.
Moving inshore and offshore counter to the tides may be related to Grey Reef Shark feeding. By swimming against tidal currents, these sharks may be better able to detect the scent of dead or injured animals inside lagoons or to take predatory advantage of additional cover provided by turbid, moving water. Although less speedy than the open-ocean Shortfin Mako (Isurus oxyrinchus), the Grey Reef Shark is much more maneuverable in ‘hot pursuit. In the complex architecture of the coral reef, inhabited by uncountable millions of small and agile fishes, maneuverability may be more advantageous than flat-out sprinting speed. From its highly varied diet, there can be little doubt that the Grey Reef Shark is a formidable apex predator on Indo-Pacific coral reefs.
Regardless of their relative sizes, Grey Reef Sharks are dominant over most other sharks with which they share Indo-Pacific coral reefs. The highly stereotyped nature of the Grey Reef Shark’s agonistic display suggests this behavior is probably important in normal social encounters with sharks of the same and other species. There is, for example, a report from Rangiroa of a Grey Reef Shark displaying to a much larger hammerhead, which subsequently left the area. The ability to communicate, however minimally, across species boundaries is no small feat (our own species is only moderately successful at it). Its spectacular agonistic display enables the Grey Reef Shark to resolve disputes with other sharks without resorting to violence which could result in injuries to itself and thus interfere with its ability to feed or mate.