Fish
Giant Grouper
Epinephelus lanceolatus · also called Queensland grouper, brindle bass, mother-of-pearl fish
The giant grouper is the heavyweight of the coral reef — a slab-sided, small-eyed, enormous-mouthed fish that can weigh as much as a small horse and hunts by turning its head into a vacuum.
Overview
Giant groupers live across the Indo-Pacific, from the Red Sea to the central Pacific, on coral and rocky reefs, in estuaries, and in harbours, from shallow lagoons to about 100 metres. Juveniles often start life in estuaries and even freshwater reaches of rivers before moving out to the reef. An adult is strongly site-attached: it adopts a particular cave, ledge, wreck, or jetty pylon as its home and returns to it every day, sometimes for many years.
Feeding by suction
Despite its bulk, the giant grouper is not a fast swimmer, and it does not run prey down. It hunts by ambush and suction: it drifts within range of a fish, lobster, crab, small shark, or juvenile turtle, then abruptly opens its jaws and flares its gill covers, and the sudden drop in pressure inside its mouth inhales the prey whole. The teeth are small and used only to grip; everything is swallowed.
Life history
Like other groupers, the giant grouper is a protogynous hermaphrodite — it matures as a female and the largest, oldest individuals later become male. It grows fast when young and can live for decades. Spawning is thought to occur in aggregations at particular reef sites.
Threats and conservation
The giant grouper is listed as Data Deficient, but its biology makes it highly vulnerable, and large adults have disappeared from most reefs that people can reach. It is slow to mature, long-lived, tied to one spot, and famously bold and curious around divers, which makes it a sitting target for spearfishing and line fishing. Juveniles are also taken for aquaculture, the live-reef-food-fish trade, and public aquariums. A number of countries now give the species full legal protection, and its recovery depends on protecting large adults and key spawning sites.
Giant Grouper: common questions
How big does a giant grouper get?
It is the largest bony fish that lives on coral reefs, reaching about 2.7 metres and 400 kilograms — roughly the size and weight of a grand piano. (Larger bony fish exist, such as the ocean sunfish and the marlins, but they are open-water species, not reef fish.) Historical claims of giant groupers over 3 metres and swallowing divers are almost certainly exaggerations, but a big adult could easily engulf a person's arm or a small child, and there are a few unverified accounts of aggressive encounters.
How does a fish this size catch prey?
By suction, not by chasing. A hunting giant grouper approaches within striking distance, then suddenly drops its lower jaw and flares its gill covers, expanding the volume of its mouth so fast that the water pressure inside plummets and prey is pulled straight in — the same "gape-and-suck" mechanism smaller groupers use, scaled up. Its teeth are small; it does not tear food but swallows it whole, including hard-shelled spiny lobsters and small sharks and rays.
Do giant groupers really change sex?
Yes. Like most groupers, the giant grouper is a protogynous hermaphrodite — individuals mature first as females, and some later transform into males, usually the largest and oldest fish. This means the biggest breeders in a population are males, and heavy fishing that removes large individuals can leave a population short of males and unable to reproduce effectively.
Is the giant grouper endangered?
It is currently listed as Data Deficient, meaning there is not enough information for a firm global assessment, but there is clear evidence of local decline. Large adults are now rare on most accessible reefs. Giant groupers are slow-growing, long-lived, site-attached, curious, and unafraid of divers, which makes them extremely easy targets for spearfishing and hook-and-line fishing, and juveniles are collected for aquaculture and the aquarium and food trades. Several countries, including Australia, now legally protect the species.