Henicorhynchus siamensis

siamensis: 'from Siam' [Thailand].

Recorded from the middle to lower Mekong River basin in Laos, Thailand, Cambodia, and Vietnam, plus the Chao Phraya and Mae Klong watersheds in central and western Thailand.

Type locality is 'Bangkok, Thailand.'

Common in the Mekong main channel and larger tributaries, but also recorded in smaller affluents and some impoundments and swamps, especially where aquatic vegetation grows thickly.

During the wet season it moves into inundated zones with standing water to spawn, and is abundant in the massive seasonal migrations of cyprinid fishes that occur in the mainstream Mekong river in December to February and May to July.

Wild fish are known to feed on periphyton, phytoplankton, and benthic algae.

H. siamensis can be distinguished from congeners by the following combination of characters: possession of 5½ lateral line scales; body relatively deep, depth fits 3.2-3.4 times in SL; head large and broad, width fits 5.5-6.7 times in SL; snout not or only weakly projecting; body plain silver in colour with no dark marking on caudal peduncle.

Species currently included in Gymnostomus have previously been placed in a variety of genera including Henicorhynchus, Cirrhinus, Bangana, and Labeo in the last few decades alone. However, Gymnostomus is a senior subjective synonym of Henicorhynchus, thus rendering the latter invalid, while Gymnostomus is easily distinguished from Cirrhinus by possession of 8-9 (vs. 10-15) branched dorsal-fin rays.

Some confusion remains, however, since a recent phylogenetic study (Yang et al., 2012) considered Henicorhynchus to be valid, and members were not recovered in the same evolutionary lineage as Indian Gymnostomus species, suggesting that they are not closely related. It is unclear whether further taxonomic changes will be required in the future in order to reflect this disparity, but here on SF we currently follow Kottelat (2013) since it was published more recently and has been widely accepted, thus Henicorhynchus is a synonym of Gymnostomus.

Gymnostomus species are considered members of the tribe Labeonini within the putative cyprinid subfamily Cyprininae or simply the subfamily Labeoninae (name varies with author). According to the most recent phylogenetic research, this grouping is further divided into four subtribes; Labeoina, Garraina, Osteochilina, and Semilabeoina (Yang et al., 2012). Among these, Gymnostomus is included in the Labeoina alongside Labeo, Bangana sensu stricto (which includes the genus Nukta), Cirrhinus sensu stricto, Cirrhinus microlepis (which is of a different genetic lineage to other Cirrhinus species), and Incisilabeo. As noted above, however, this refers only to the Indian species of Gymnostomus, with the Southeast Asian species placed in the Osteochilina assemblage.

Some Gymnostomus species are common in mainstream rivers at certain times of year and extremely important in local fisheries during these periods.