Threadfin Trevally

The Threadfin Trevally, also known as the Indian threadfish or diamond trevally, is a striking member of the jack family (Carangidae) found throughout the Indo-Pacific, from East Africa and the Red Sea to Southeast Asia, Japan, and northern Australia. Closely related to the African Pompano, it shares the genus Alectis and its hallmark trait: juveniles possess extraordinarily long, trailing filaments extending from the dorsal and anal fins, which shorten dramatically as the fish matures. Adults develop a steep, blunt head profile and deep, laterally compressed body. It ranges from shallow coastal reefs to deeper offshore waters and holds no major conservation concern, though it is less frequently observed as it favors turbid coastal habitats.

• Deep, laterally compressed, near-diamond-shaped body
• Silvery flanks with a bluish-green sheen on the back
• Juveniles have extremely long, thread-like filaments trailing from the dorsal and anal fins
• Very steep, blunt forehead in adults
• Small, low-set mouth
• Deeply forked tail

Adults reach 40-70 cm, occasionally up to 1.65 m. Nearly identical to the African Pompano in juvenile form; adults are best separated by subtle fin-ray counts and geographic range, with Threadfin Trevally restricted to the Indo-Pacific.

Threadfin Trevally occur throughout the Indo-Pacific, from East Africa and the Red Sea across the Indian Ocean to Southeast Asia, Japan, and northern Australia. Juveniles are typically pelagic, drifting near the surface in open water or around floating debris, often in turbid coastal waters and river mouths with brackish influence. As they mature, adults move toward deeper coastal and offshore habitats, including sandy bottoms, reef edges, and continental shelf waters to depths of around 100 m. The species tolerates a range of salinities and water clarities, contributing to its broad distribution across varied Indo-Pacific coastal environments.

Threadfin Trevally show a pronounced shift in behavior and appearance with age: juveniles drift in open water or near floating structure, their long trailing filaments possibly serving as camouflage or a deterrent to predators by mimicking jellyfish tentacles or debris. As adults, they become more active, solitary or loosely grouped predators over sandy coastal bottoms and reef edges, feeding on small fish and benthic invertebrates. Spawning is believed to occur offshore in warmer months. Their reliance on turbid coastal and estuarine nursery habitat as juveniles links them ecologically to mangrove and river-mouth ecosystems before they transition to more open coastal adult habitats.