Squarespot Anthias

The squarespot anthias (Pseudanthias pleurotaenia) is a small, brightly colored basslet in the family Serranidae (subfamily Anthiinae), a group often called "fairy basslets." It ranges across the tropical Indo-Pacific, from the Red Sea and East Africa to the western Pacific and southern Japan. Like other anthias, it is a protogynous hermaphrodite: individuals begin life as females and some later transform into males as they mature or when a dominant male is lost from a group. The species is common on outer reef slopes and drop-offs, where large aggregations hover in the water column. It is not currently considered threatened and is widely photographed by divers for its striking sexual dimorphism.

Distinguishing the squarespot anthias from related Pseudanthias species relies mainly on the male's coloration:

• Males: bright orange body with a rectangular ("square") magenta-purple patch centered on the flank, roughly bordered and squared off rather than blotchy.
• Females: uniform peach to orange-yellow, lacking the patch, and generally paler and smaller.
• Fins: deeply forked caudal fin, the upper lobe often drawn into a fine filament in mature males.
• Body: elongated, laterally compressed, torpedo-shaped, typical of schooling anthiines.
• Size: males slightly larger (up to 15 cm) than females (around 12 cm). Look-alikes such as the bicolor anthias lack the squared purple mark, instead showing a sharp two-tone color split.

Squarespot anthias inhabit clear tropical waters of the Indo-Pacific, from the Red Sea and East Africa eastward to Fiji and Micronesia, and north to southern Japan. They favor steep outer reef slopes, drop-offs, and current-swept walls at depths of roughly 10-60 meters, often congregating well above the substrate to intercept plankton drifting in the current. Water temperatures in their range typically stay between 24-29°C. Juveniles and females tend to occupy shallower reef edges, while large males patrol slightly deeper structure where they can oversee their harem. The species favors areas with abundant hard coral cover and strong water movement.

Squarespot anthias form large, loose harem-based groups consisting of one dominant male and multiple females, hovering together above reef structure to feed on passing zooplankton. When threatened, the entire aggregation dives quickly into crevices in the reef for shelter. As protogynous hermaphrodites, the largest female in a harem will change sex to become the group's new male if the resident male dies or disappears, a transition that can occur within days to weeks. Spawning occurs in pairs, typically around dusk, with pelagic eggs released into the water column. Ecologically, the species is an important link between plankton and larger reef predators, and its constant feeding activity helps regulate zooplankton density over the reef.