Fourstripe Damselfish

The Fourstripe Damselfish, also known as the blacktail dascyllus, is a small reef fish in the family Pomacentridae distributed across the western Pacific and eastern Indian Ocean, from Indonesia and the Philippines to Papua New Guinea and northern Australia. It closely resembles its relative the Humbug Damselfish but is distinguished by its solid black tail fin, which together with its three body bars gives the appearance of four dark stripes. It is commonly found sheltering in groups among the branches of live coral, particularly Acropora and Pocillopora species, on shallow reef flats and lagoons.

Distinguishing features of the Fourstripe Damselfish:

• White to pale grey body crossed by three broad black vertical bars
• A solid black tail fin, unlike the pale or translucent tail of the closely related Humbug Damselfish
• Small, deep, laterally compressed body shape, reaching about 8 cm
• Black-tipped dorsal fin
• Group-living habit around a single coral colony The black tail is the single most reliable feature separating this species from the very similar Humbug Damselfish, which shares the three-bar pattern but has a clear or whitish tail.

Fourstripe Damselfish are found on coral reefs across the western Pacific and eastern Indian Ocean, including Indonesia, the Philippines, Papua New Guinea, and northern Australia. They typically inhabit shallow reef flats, lagoons, and sheltered reef slopes at depths of 1 to 12 meters, closely associated with live branching coral colonies that provide immediate refuge from predators. They favor warm, stable tropical water temperatures of roughly 24-29°C and are rarely found far from their host coral, into which the entire group retreats when disturbed.

Fourstripe Damselfish live in small, loosely structured social groups centered on a single coral colony, with a size-based dominance hierarchy among group members. They feed mainly on zooplankton picked from the water column just above their coral home, along with some algae grazing. When threatened, the whole group dives simultaneously into the protective branches of the coral, re-emerging once the danger passes. Breeding involves a male preparing and defending a nest site on the coral or adjacent substrate, where a female deposits eggs that the male guards and fans until hatching. Their close, permanent association with a single coral head makes them a useful indicator of live coral cover on a reef.