They are also found in mangrove areas, moving in and out with the tide and even in fresh water near the sea. They occur singly or in small groups. Adults often aggregate in reef channels at low tide. This is one of the three most common reef sharks in the Indo-Pacific, the two others are the grey reef shark, Carcharhinus amblyrhynchos and whitetip reef shark, Triaenodon obesus.
Typically a solitary animal, juvenile blacktip reef sharks will commonly conjugate in shallow regions during high tide. Vulnerable to larger predators, they will reside in shallower areas until larger in size. Blacktip reef sharks tend to be more active during dawn and dusk, but like most sharks they are opportunistic feeders. Their diet consists of crustaceans, squid, octopus, and bony fish.
My home in the coral reefs is being damaged by ocean acidification—which occurs when the ocean absorbs carbon and becomes acidified. I love living among thriving reefs, but increasing acidification degrades the physical structure of these reefs, putting my habitat and food supply at risk. This affects all the creatures living among the reef—not just my team of fellow blacktip reef sharks.
Although there are no active reef shark fisheries in the US Pacific, the reef sharks' disappearance could be caused by recreational fishing or illegal shark finning, which, combined, kill 26 million to 73 million sharks each year. Another possible explanation is that the reef sharks are starving. Their food sources, including coral reef fishes, are decreasing in number because of habitat destruction and human exploitation, and could be taking the sharks with them.
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.
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 6 m (20 ft), 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.
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One useful definition distinguishes reefs from mounds as follows: Both are considered to be varieties of organosedimentary buildups – sedimentary features, built by the interaction of organisms and their environment, that have synoptic relief and whose biotic composition differs from that found on and beneath the surrounding sea floor. Reefs are held up by a macroscopic skeletal framework. Coral reefs are an excellent example of this kind. Corals and calcareous algae grow on top of one another and form a three-dimensional framework that is modified in various ways by other organisms and inorganic processes. By contrast, mounds lack a macroscopic skeletal framework (see stromatolite). Mounds are built by microorganisms or by organisms that don't grow a skeletal framework. A microbial mound might be built exclusively or primarily by cyanobacteria. Excellent examples of biostromes formed by cyanobacteria occur in the Great Salt Lake in Utah, and in Shark Bay on the coast of Western Australia.