The halfblack triplefin (Enneapterygius hemimelas), also known as the half-black triplefin, blackbelly triplefin, or the green-tail threefin, is a species of triplefin blenny in the genus Enneapterygius. It was originally described by R. Kner and F. Steindachner in 1867. It is a non-migratory tropical blenny known from coral reefs in the western Pacific Ocean, and has been described from the Ryukyu Islands to eastern Australia. It has been recorded swimming at a depth range of 0–30 metres (0-98.4 feet).
The halfblack triplefin is described as a "relatively large" member of the Enneapterygius hemimelas species group.Fricke, R., 1994 [ref. 21827] Tripterygiid fishes of Australia, New Zealand and the southwest Pacific Ocean (Teleostei). Theses Zoologicae v. 24: i-ix + 1-585.
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