In the waters of the Mekong River, there is a large fish species with a body shape resembling a shark. That fish is the iridescent shark, scientifically known as Pangasianodon hypophthalmus.
Despite bearing the name "shark," the fish is not a member of the cartilaginous fish family. Taxonomically, it belongs to the order Siluriformes and the family Pangasiidae, categorized as a type of catfish or patin.
The name "iridescent" refers to the layer of color on the juvenile fish's skin, which reflects a silvery sheen like pearls when exposed to light. Its shimmering effect gradually fades into a dark blackish-gray as the fish ages.
The Morphology of the Iridescent Shark
The body size of an adult Pangasianodon hypophthalmus is exceptionally large for a freshwater species. Its length can reach up to 1.3 meters, with a maximum body weight approaching 44 kilograms in its natural habitat.
The fish has an elongated body that is somewhat laterally compressed. Its dorsal fin grows high and curves sharply backward, mimicking the silhouette of a ocean shark's fin when swimming near the water surface.
The skin surface of the fish is smooth and completely scaleless. Its mouth area is equipped with two pairs of short barbels that function as sensory organs for navigation on muddy riverbeds.
As an adaptation to living in waters with low dissolved oxygen levels, the fish possesses a secondary respiratory organ in the form of a modified swim bladder. The organ system enables the fish to gulp oxygen directly from the air when water conditions deteriorate.
The pectoral fin structure features sharp spines for self-defense. The caudal fin is deeply forked, which provides powerful propulsion when the fish accelerates quickly through river currents.
Distribution in the Wilds of the Mekong River
Wild populations of Pangasianodon hypophthalmus are concentrated in two major river systems in Indochina: the Mekong River and the Chao Phraya River. Its distribution range encompasses the waters of Viet Nam, Laos, Cambodia, and Thailand.
The fish displays seasonal migratory behavior that is heavily dependent on changes in river water volume. At the onset of the rainy season, adult groups travel hundreds of kilometers upstream to spawn in flooded wetlands.
When the dry season arrives and water levels recede, the population moves back into the deeper main river channels. Feeding activity is omnivorous, which consists of small fish, zooplankton, and aquatic plant matter.
The fish is naturally timid and highly sensitive to nearby sound vibrations. Sudden movements or shadows on the water surface can trigger a panic response, causing groups of fish to swim erratically to escape.
While foraging in flooded wetlands, the fish ingests and transports plant seeds to new areas. Food remnants discarded during migration also help maintain the availability of organic material along the river channel.
Conservation Status of Local Ecosystems
The International Union for Conservation of Nature (IUCN) listed Pangasianodon hypophthalmus as an endangered species in 2011. Its wild population in the Mekong River has declined by more than half due to continuous human activity.
The primary driver of the decline is the series of hydroelectric dams built along the main river channel. The concrete structures block the swimming path of adult fish attempting to reach upstream spawning grounds during the rainy season.
River harvesting also occurs heavily for meat consumption. Meanwhile, wild fry and juveniles are caught to supply the aquarium trade or transferred to floating cages for grow-out farming.
In addition to being wild-caught, the fish has long been commercially farmed in aquaculture facilities. The farmed fish are exported worldwide as patin/dory fillets as well as aquarium trade stock.
Reference:
Vidthayanon, C., & Hogan, Z. (2011). Pangasianodon hypophthalmus. The IUCN Red List of Threatened Species 2011. https://dx.doi.org/10.2305/IUCN.UK.2011-1.RLTS.T180689A7649971.en
