Ostracion cyanurus
A boxfish with a blue tail, found in the Red Sea and Arabian Peninsula.
The bluetail trunkfish, also known as the Arabian trunkfish or Red Sea boxfish, is a marine ray-finned fish in the family Ostraciidae. Its scientific name is *Ostracion cyanurus*, and it lives in the Western Indian Ocean.
German naturalist and explorer Eduard Rüppell first formally described this species in 1828. He based his description on a specimen collected from the Red Sea off Al Muwaylih, in Saudi Arabia’s Tabuk Province. The genus name *Ostracion* means “little box,” referring to the boxy body shape of its type species, *O. cubicum*. The specific name *cyanurus* combines “blue” and “tail,” a nod to the fish’s blue caudal fin. Because *Ostracion* is neuter, some argue the correct spelling should be *cyanurum*.
This fish reaches a maximum total length of 15 cm (5.9 in). It has nine soft rays in both the dorsal and anal fins, and ten in the caudal fin, eight of which are branched. Females have a slightly concave head profile when viewed from above, while males have a convex one. The lips and chin are fleshy and protrude. Adult males are normally dark blue on the flanks and lower body, with spots, and an unspotted greenish-yellow upper body. Females are yellow with blue spots and red cheeks.
*Ostracion cyanurus* occurs in the Western Indian Ocean, from the Red Sea and around the Arabian Peninsula into the Persian Gulf, as far east as eastern Iran. It inhabits areas with moderate coral growth at depths between 5 and 50 m (16–164 ft).
A solitary species, it stays close to shelter. The apparent lack of smaller males suggests it may be protogynous (starting life as female and later becoming male). When threatened, it releases a toxin called pahutoxin (or ostracotoxin) into the water as a defense.
This species is available in the aquarium trade.
Quick Facts
- Taxon
- Ostracion cyanurus
Facts from the source article.
Lore & Background
Ostracion cyanurus was first formally described in 1828 by the German naturalist and explorer Eduard Rüppell, with its type locality given as the Red Sea off Al Muwaylih in Tabuk Province, Saudi Arabia. The genus name Ostracion means 'little box', alluding to the body shape of its type species, O. cubicum. The specific name cyanurus means 'blue tail', referencing the blue caudal fin; because Ostracion is neuter, it has been argued that the specific name should be spelt as cyanurum.
Adult males are normally dark blue on the flanks and lower body with spots and an unspotted greenish-yellow upper body. Females are yellow with blue spots and red cheeks. The lips and chin are fleshy and stick out. Females have a slightly concave dorsal profile of the head, while that of males is convex. This species is solitary and remains near shelter. The apparent lack of smaller males may indicate it is protogynous. If it senses danger, it releases a toxin called pahutoxin, or ostracotoxin, into the water as a defence.
Reader's Guide
Ostracion cyanurus is a notable species within the family Ostraciidae due to its distinctive coloration and defensive toxin. Its presence in the aquarium trade makes it of interest to marine hobbyists, though its solitary nature and toxin release require careful handling. The species is found in the Western Indian Ocean, specifically in the Red Sea and around the Arabian Peninsula into the Persian Gulf as far east as eastern Iran, inhabiting areas with moderate coral growth at depths between 5 and 50 m. The taxonomic debate over the spelling of its specific name (cyanurus vs. cyanurum) reflects ongoing discussions in ichthyological nomenclature. Its maximum published total length of 15 cm and fin-ray counts (9 soft rays on dorsal and anal fins, 10 on caudal fin with 8 branched) provide standard identification metrics. The species' potential protogynous hermaphroditism, suggested by the apparent lack of smaller males, adds to its biological interest. The release of pahutoxin as a defence mechanism is a key survival trait shared with other boxfishes.
Did You Know?
- The specific name cyanurus means 'blue tail', referring to the blue caudal fin.
- It releases a toxin called pahutoxin or ostracotoxin when it senses danger.
- Females have a slightly concave dorsal head profile, while males have a convex one.
- The species is found at depths between 5 and 50 m in the Western Indian Ocean.
More in Pufferfish and Boxfish 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced