Iridescent shark (Pangasianodon hypophthalmus): freshwater catfish used for food and aquaria
A Southeast Asian freshwater catfish commonly called the iridescent shark; valued in aquaculture and aquarium trade, native to the Mekong and Chao Phraya basins and marketed internationally as swai.
The iridescent shark is a large freshwater catfish (scientific name Pangasianodon hypophthalmus) native to major rivers of Southeast Asia. Despite its common name, it is not a true shark; the name refers to its streamlined, silvery body and shark-like profile. It has become globally known both as an inexpensive whitefish sold for food and as a juvenile ornamental fish in the aquarium trade.
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Physical characteristics
Iridescent sharks are distinguished by a smooth, laterally compressed body with a silvery, often slightly iridescent sheen. They have the whisker-like barbels typical of catfish, a forked tail and a rounded, sharklike head. In the wild they can grow to large sizes—commonly exceeding one metre (three feet) in length—though individuals sold for food are typically processed at smaller sizes.
Range, habitat and behaviour
Wild populations are native to river systems in mainland Southeast Asia, notably the Mekong Basin and the Chao Phraya system. They inhabit large rivers and floodplains and are known to migrate seasonally, feeding on a mix of plant material, invertebrates and small fish. Their schooling behaviour and sensitivity to river conditions make them indicators of freshwater ecosystem health.
Uses: aquaculture, market names and cuisine
Intensive farming of iridescent sharks expanded in the late 20th century, especially in Vietnam and Thailand. Farmed fillets are exported worldwide under a variety of common names; in the United States they are often marketed as "swai." The flesh is mild-flavoured and white, making it a popular and affordable substitute for other white-fleshed fishes in cooking.
Aquarium trade and husbandry
Young iridescent sharks are sold as aquarium fish because of their graceful, silvery appearance. Hobbyists should note that these fish grow rapidly and require very large, well-filtered tanks and to be kept in groups because they are gregarious. Common care notes include:
- Provide ample swimming space and stable water quality.
- Keep in groups; solitary specimens become stressed.
- Be prepared for eventual large adult size—many outgrow home aquaria.
Conservation, distinctions and notable facts
The iridescent shark should not be confused with marine sharks or with other pangasiid species such as basa (Pangasius spp.). Wild populations face pressures from river damming, habitat alteration and overfishing in some areas, though aquaculture has become the primary source for international markets. For more information on taxonomy and trade, see species resources, regional river studies at Southeast Asia basin reports, and aquaculture or market information at trade summaries.
Features
The pangasius has the scaleless, elongated body typical of the shark catfish (Pangasiidae). Adult animals reach a maximum weight of 44 kilograms at a length of up to 150 centimeters. They are dark grey with a lighter belly and dark grey to black fins. Very old and large animals become uniformly gray. As characteristic features of the genus Pangasianodon, the Pangasius has a terminal mouth (upper and lower jaws are therefore of equal length), a swim bladder with only one lobe and ventral fins with eight, rarely nine soft rays. In adults, the mouth is also toothless and the barbels on the lower jaw are missing. It differs from the other species of the genus, the Mekong Giant Catfish, by its much smaller size as well as the less broad head and the construction of the gill trap, which is well developed in the Pangasius and carries about 40 very short rays on the first arch between 15 longer rays.
The dorsal fin has, as in many catfish, a spiny hard ray followed by six branched soft rays. An adipose fin is present. The anal fin is elongated on the belly and has a black stripe in the middle, as do both lobes of the caudal fin. The small pelvic fins are located just in front of the anal fin.
Newly hatched individuals are yellowish and almost translucent with distinct barbels, while older juveniles resemble adults and have a black stripe along the lateral line organ and another just below. This pattern fades with age.
The chromosome set consists of 30 pairs (2n = 60).
Occurrence
The species inhabits the major river courses of the Mekong and Mae Nam Chao Phraya (Chao Phraya River) in Thailand, Vietnam, Laos and Cambodia. The Mekong River occurrence is believed to be divided into a southern population in Cambodia and Vietnam and a northern population in Laos and Thailand by the waterfalls in the Lao province of Champasak. Catches in the area of the falls, however, indicate some exchange between populations. More detailed genetic studies are only available on the southern population, within which there are three co-occurring, genetically distinct subpopulations, possibly separated by different spawning seasons. The spawning area of the southern Mekong population is in a relatively small area in northern Cambodia between Kratie and the Mekong Falls; that of the northern and Chao Phraya populations is unknown.
In Bangladesh, Singapore and the Philippines, naturalized stocks exist as a result of the introduction to aquaculture. Here the species is considered ecologically very problematic due to its voracity and possible competition with native fish.
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AlegsaOnline.com Iridescent shark (Pangasianodon hypophthalmus): freshwater catfish used for food and aquaria Leandro Alegsa
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