Green humphead parrotfish

Lore & Background

The green humphead parrotfish inhabits reefs in the Indian and Pacific Oceans, from the Red Sea in the west to Samoa in the east, and from the Yaeyama Islands in the north to the Great Barrier Reef, Australia, in the south. Unlike typical wrasse, it has a vertical head profile, and unlike other parrotfishes, it is uniformly covered with scales except for the leading edge of the head, which is often light green to pink. Primary phase colouration is a dull gray with scattered white spots, gradually becoming uniformly dark green. This species does not display sex-associated patterns of color change common in the wrasse family Labridae. The adult develops a bulbous forehead sometimes described as an 'ossified ridge', and the tooth plates are exposed, being only partly covered by lips. This species is gregarious and usually occurs in small schools, but group size can be quite large on seaward and clear outer lagoon reefs, exceeding 75 individuals. Records from the 1970s suggest even larger schools of up to 250 individual fish.

Reader's Guide

The green humphead parrotfish is notable for its feeding behavior and ecological role. It feeds on benthic algae and live corals; it is alleged to ram its head against corals to facilitate feeding. Each adult fish ingests over five tons of structural reef carbonates per year, contributing significantly to the bioerosion of reefs; they are often regarded as a keystone species due to this behavior. These fish sleep among corals, in caves and shipwrecks at night, usually in large groups. Ritualized combat between male Bolbometopon are sometimes observed: the males charge at one another, and collide head-on with enough force to make 'loud jarring sounds'. The pair immediately continue combat through attempted biting of the other fish's body. The head humps facilitate these ramming contests, having been compared to the horns of bighorn sheep. More frequently though, contests between males take the form of parallel swimming, akin to land ungulates, where two males move side by side to assess one another. These behaviors are associated with spawning, occurring mostly on days with observed spawning bouts. The species is slow-growing and long-lived (up to 40 years), with delayed reproduction and low replenishment rates. They are sexually monochromatic, with no initial or terminal phase in the life cycle of the adults. The fish spawn pelagically near the outer reef slope or near promontories, gutters, or channel mouths during a lunar cycle, usually spawning just prior to the new moon. They make use of spawning aggregation sites. Newly settled juveniles are found in branching coral habitats (primarily Acropora) in sheltered lagoons. Small juveniles (<50mm) are often associated with damselfish. Larger juvenile green humphead parrotfish are found in lagoons, often in seagrass beds, and the adults are found in clear outer lagoons and sea-ward reefs up to a depth of 30 m (98 ft).

Did You Know?

• The green humphead parrotfish is the only species in the monotypic genus Bolbometopon.
• Fossil remains of Bolbometopon sp. are known from the Late Miocene of Sri Lanka.
• Males engage in head-on ramming contests that produce 'loud jarring sounds'.