Grey reef sharks are fast-swimming, agile predators that feed primarily on free-swimming bony fishes and cephalopods. Their aggressive demeanor enables them to dominate many other shark species on the reef, despite their 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. 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 embryos through a placental connection. Litters of one to six pups are born every other year.
Grey reef sharks feed mainly on bony fishes, with cephalopods such as squid and octopus being the second-most important food group, and crustaceans such as crabs and lobsters 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.
Along with the blacktip reef shark (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.
There is a variety of biotic reef types, including oyster reefs and sponge reefs, but the most massive and widely distributed are tropical coral reefs. Although corals are major contributors to the framework and bulk material comprising a coral reef; the organisms most responsible for reef growth against the constant assault from ocean waves are calcareous algae, especially, although not entirely, coralline algae.