Hawaiian Ruby Cardinalfish

Apogon erythrinus

The Hawaiian Ruby Cardinalfish is a small, red marine fish endemic to the Hawaiian Islands and Johnston Island. It reaches a maximum length of 4.0 cm and has a somewhat transparent appearance. Its distinguishing features include a long second dorsal fin spine that extends to the base of the third ray of the second dorsal fin and 14 pectoral fin rays. Unlike some related species, it lacks pigment on the dorsal surface of the caudal peduncle. These fish are nocturnal and shy, often found hiding in holes and crevices within their reef habitats. This species is typically observed in shallow waters, ranging from 1 to 48 meters deep, with most sightings occurring at depths shallower than 40 meters. The Hawaiian Ruby Cardinalfish is associated with reef environments, where it remains close to the bottom and seldom strays far from its shelter. As mouthbrooders, they exhibit distinct pairing during courtship and spawning, which is a notable aspect of their reproductive behavior. Divers encountering the Hawaiian Ruby Cardinalfish will likely find it near the bottom of reef areas, especially during night dives. Its conservation status is listed as Least Concern by the IUCN.