Blacktail Dartfish

The Blacktail Dartfish, scientifically known as Ptereleotris heteroptera, is a vibrant, small reef-associated fish found in warm tropical marine waters. Typically residing at depths of 5 to 15 meters (16 to 49 feet), but capable of reaching depths up to 80 meters (262 feet), this species is part of the colorful biodiversity of coral reefs.

The Blacktail Dartfish can grow up to a maximum length of 14 centimeters (about 5.5 inches). Its coloring ranges from light blue to pale bluish-gray, featuring a distinct dark spot on its caudal (tail) fin. Its dorsal fin structure includes 7 spines and 29 to 33 soft rays, while its anal fin is composed of 1 spine and 27 to 30 soft rays, along with 26 vertebrae. A standout feature is the iridescent blue band divided into two sections found along the upper edge of the operculum (the bony flap covering the gills). The caudal fin often has a yellowish hue, sometimes marked by an orangish streak extending from the spot. Additionally, the base of the pectoral fin is usually marked with a blue-edged salmon-pink spot.

These dartfish are typically found in lagoons, bays, reef passes, or along outer reef slopes, preferring areas with hard substrates as well as rubble or sand patches at the base of the reef. Adult Blacktail Dartfish are usually seen in pairs, sharing burrows under rubble or coral slabs, and are known for their monogamous behavior. They feed on zooplankton carried by ocean currents, venturing up to 3 meters (about 10 feet) above their burrows, while maintaining close proximity of 1 to 2 meters (3 to 6.5 feet) to each other. Juvenile dartfish, in contrast, form large schools that can consist of up to 500 individuals.

This species is widely distributed across the Indo-Pacific region, from the 🌊 Red Sea and East Africa to the Hawaiian Islands, and as far south as Lord Howe Island. They are also found in the Pacific archipelagos such as the Mariana, Caroline, and 🇲🇭 Marshall Islands, and are known to inhabit areas as north as the Ryukyu Islands (🇯🇵 Japan). The genus name "Ptereleotris" comes from Greek, with "pteron" meaning wing or fin, and "eleotris," referencing a type of fish from the Nile River.