Ageneiogarra theunensis

Ageneiogarra: from the Ancient Greek ἀγένειος (agéneios), meaning 'beardless', plus the generic name Garra, in reference to the lack of barbels.

theunensis: named for the Nam Theun river, Laos, from where the type series was collected.

Known from the Nam Theun and Nam Gnouang river systems in Laos, both of which are tributaries within the Mekong watershed, plus the Nan River in Nan province, Thailand.

Type locality is 'Upper Nam Theun, Laos, 18°04'09"N, 105°29'44"E'.

Tends to inhabit swiftly-flowing sections of headwaters and tributaries rather than larger, lowland river channels. The most favourable habitats contain clear, oxygen-saturated water which, allied with the sun, facilitates the development of a rich biofilm carpeting submerged surfaces.

This species can be distinguished from Garra and Ageneiogarra species in Southeast Asia and China by the following combination of characters: plain brown body and fins, sometimes with 6-8 narrow, faintly distinct, longitudinal stripes; snout triangular in dorsal view, with small tubercules and without transverse groove; 45-47+2-3 = 48-50 lateral line scales; caudal peduncle 1.8-2-2 times as long as deep; 8½ branched dorsal-fin rays; large adult size up to 300 mm SL.

The genus Garra is a particularly enigmatic grouping with new taxa described on a regular basis, while many of the existing ones may represent cases of misidentification or synonyms of other species. Some of the revisions have also been called into question, which has added further confusion. A full generic review would be ideal but is unlikely to materialise given the extensive distribution of its members which range from southern China across much of southeast Asia, India and the Middle East as far as north/central Africa.

Instead a number of less-extensive works published in recent years have resulted in a gradual, but continuing, improvement in knowledge, and it remains possible that the genus will be broken up into smaller taxa since the current assemblage is almost certainly polyphyletic.

Garra species are usually included in the subfamily Labeoninae/Cyprininae or tribe Labeonini (name varies with author) which by recent thinking is further divided into four subtribes; Labeoina, Garraina, Osteochilina, and Semilabeoina (Yang et al., 2012)[5]. The putatively monophyletic Garraina comprises a number of genetic lineages including Garra sensu stricto (which also includes Horalabiosa, Phreatichthys and possibly other genera), a small clade comprising Garra cambodgiensis and G. fascicauda (thus rendering Garra polyphyletic), Paracrossocheilus, and Gonorhynchus (which includes Akrokolioplax).

Two species, G. micropulvinus and Ageneiogarra imberba, are placed in the Semilabeoina assemblage. The generic name Ageneiogarra Garman, 1912 was suggested for both, and the Catalog of Fishes and FishBase now use it for A. imberba and this species. In addition, some genera which were previously considered to be close relatives of Garra species such as Discogobio, Discocheilus and Placocheilus, are now also placed in this subtribe.

All genera currently included in Garraina possess a lower lip modified to form a mental adhesive disc, allowing the fish to cling to surfaces in turbulent conditions. In most species the upper lip is almost entirely reduced and both the upper and lower jaw margins are keratinised, i.e., horny, and used to scrape food items from the substrate.

Garra species are distinguished from other Garraina members by the first two pectoral-fin rays usually being thickened, fleshy and unbranched, possession of 10-11 dorsal-fin rays, and a combination of internal characters. Some species have evolved particular environmental specialisms such as highly reduced eyes in hypogean forms or the ability to survive in thermal springs.