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Lagoon Triggerfish Rhinecanthus aculeatus (Linnaeus, 1758)
Triggerfishes are named for the mechanism whereby the first dorsal spine can be locked in erect position by the small second spine; if one presses down on the second dorsal spine (the trigger), the first spine can be moved.
These fishes are deep-bodied with the eye high on the head, a long snout, and small mouth. The teeth in the jaws are close-set and chisel-like, the upper jaw with 8 in an outer row and 6 in an inner row which serves to buttress the outer; the gill opening is a short slit anterior to the upper part of the pectoral fin; the skin is tough and rough to touch, the scales non-overlapping.
Some species have small, forward-curved spines posteriorly on the side of the body. There are 2 dorsal fins, the first of 3 spines; there are no pelvic fins. Triggerfishes are usually solitary except when they form pairs at spawning time. A notable exception is the Black Durgon (Melichthys niger) which may form aggregations.
Balistid fishes normally swim by undulating the second dorsal and anal fins, using their tail only for rapid movement. When frightened, they may take refuge in a hole in the reef into which they can barely pass; they then erect their first dorsal spine and extend their pelvic bone to wedge themselves in position. At night they enter the same or similar hole to sleep.
Most feed on invertebrates with hard skeletal parts such as crabs, mollusks, and sea urchins; they readily reduce such prey to pieces with their powerful jaws and sharp teeth. The species of the genera Melichthys and Xanthichthys feed heavily on zooplankton.
Triggerfishes, in general, are not good aquarium fishes because they may attack other fishes in the tank. Also they soon consume any resident crustaceans.
The pelagic juvenile stage of the species of Melichthys reaches surprisingly large size, to more than 6 inches (15 cm). The transforming stage of the Pinktail Durgon (Melichthys vidua) was twice described as a new species in different genera, Pachynathus nycteris Jordan & Evermann and Oncobalistes erythropterus Fowler.
Triggerfishes lay demersal eggs which are aggressively guarded by the female parent. Two of the largest species have bitten divers (including the author) who made the mistake of venturing too close to the nest. These species are the Titan Triggerfish (Balistoides viridescens) and the Yellowspotted Triggerfish (Pseudobalistes fuscus); they occur throughout most of the Indo-Pacific region, but not the Hawaiian Islands.
The guarding females of Hawaiian species sometimes threaten by swimming aggressively toward an intruder, and they have the capability of inflicting a bite.
Triggerfishes are well known for making a grunting noise. This is produced by intercostal muscles moving two bones in the pectoral girdle; the swim bladder then serves as a resonating chamber for the sound.
The general Hawaiian name for triggerfishes is humuhumu.
Melichthys niger (Bloch, 1786) Humuhumu'ele'ele
Dark gray to dark blue-green with black longitudinal lines following scale rows; a narrow pale blue band at base of soft dorsal and anal fins; often displays bright blue lines from eye across top of head; dorsal soft rays 30-34; longitudinal ridges following scale rows posteriorly on body.
Largest, 12.5 inches (32 cm), from Laysan. Circumtropical; when common, may form aggregations. Very abundant at Johnston Island. Feeds mainly on algae (about 70% of diet) and zooplankton.
Melichthys vidua (Solander, 1844) humuhumu hi'ukole
Dark brown to nearly black, often with a yellowish cast; scaled basal part of caudal fin white, remaining fin pink; second dorsal and anal fins whitish with a black border; dorsal soft rays 31-35; slight longitudinal ridges following scales posteriorly on body.
Largest, 13.4 inches (34 cm). Indo-Pacific; feeds mainly on algae and detritus, occasionally on crustaceans, octopuses, sponges, and fishes.
Rhinecanthus aculeatus (Linnaeus, 1758) Humuhumu nukunuku apua'a
Pale greenish gray, shading to white below, with a large blackish area on body with radiating bands; blue and black lines across interorbital; a long orange-yellow streak extending from mouth; rows of black antrorse spines posteriorly on side of body; dorsal rays 23-26.
Reaches 12 inches (30 cm). Indo-Pacific and eastern south Atlantic; not common in Hawaii. Prefers shallow protected waters. Feeds on algae and invertebrates, including mollusks, crustaceans, polychaete worms, and heart urchins.
Rhinecanthus rectangulus (Bloch & Schneider, 1801) humuhumu nukunuku apua'a
Light brown, shading to white below, with an oblique black band extending below eye and broadening on body; a gold-bordered black triangle containing antrorse spines posteriorly on body, preceded by a parallel gold line; blue and black lines across interorbital; a red bar at pectoral base; dorsal rays 22-25. To 10 inches (25 cm).
Indo-Pacific; inshore on reefs. Feeds mainly on algae and small benthic invertebrates.
All information and pictures in this section are from John E. Randall's Shore Fishes of Hawai'i by permission of the author.
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