Iridescent Shark

The iridescent shark, despite its common name, is a large freshwater catfish in the family Pangasiidae rather than a true shark. It is native to the Mekong and Chao Phraya river basins of Southeast Asia, including Thailand, Laos, Cambodia, and Vietnam. The name refers to its shark-like tall dorsal fin and streamlined body shape, along with a shimmering iridescent sheen visible in younger individuals. In the wild it can grow well over a meter long, undertaking seasonal migrations along major river systems. It is a schooling species as a juvenile, becoming more solitary and much larger as it matures into adulthood.

Distinguishing traits include:

• Sleek, elongated, laterally compressed silvery-gray body
• Tall, triangular, shark-like dorsal fin
• Deeply forked caudal fin
• Juveniles display an iridescent blue-silver sheen with a dark lateral stripe, both of which fade with age
• Small barbels near the mouth
• Adults can exceed 1 m and lose the juvenile iridescence, becoming more uniformly gray The combination of a shark-like dorsal fin silhouette with a scaleless catfish body distinguishes it clearly from any true shark or other freshwater fish.

Iridescent sharks inhabit the main channels and large tributaries of the Mekong and Chao Phraya river systems in Southeast Asia, favoring warm freshwater generally between 22-28°C with moderate to strong current. Juveniles are often found in schools near riverbanks and floodplain margins, while adults move into deeper main-channel habitat as they mature. This species undertakes seasonal migrations tied to monsoon flooding, moving between feeding and spawning areas across its native river range in Thailand, Laos, Cambodia, and Vietnam.

Juvenile iridescent sharks are active schooling fish, moving together in open water for protection, while adults become increasingly solitary and bottom-oriented as they grow. The species is an omnivorous forager, feeding on plant material, small invertebrates, and organic debris throughout the water column depending on life stage. Seasonal migrations along the Mekong and Chao Phraya systems are linked to monsoon-driven water level changes and spawning activity. As one of the larger native fish in these rivers, it plays a role in the broader food web, and its populations are affected by river damming and habitat alteration across its native range.