Hemibagrus filamentus

Hemibagrus: from the Greek hemi, meaning 'half' and the generic name Bagrus.

Type locality is given simply as 'Cambodia' but this species has been recorded throughout the middle and lower Mekong river system in Laos, Cambodia and Thailand.

Inhabits slowly-flowing or standing waters including main river channels and lakes, with riverine populations typically performing localised migration into flooded forest during the wet season.

A predator feeding primarily on crustaceans and smaller fishes, although there should be no need to use such live foods in captivity.

This species appears to be unknown in the aquarium trade but is a valued food fish within its native range.

Hemibagrus has been divided into a number of putative species groups which may or may not represent monophyletic assemblages, and following a major review by Ng and Kottelat (2013) H. filamentus is included in the H. nemurus group.

Members of this assemblage can be told apart from other congeners by possession of 43–46 vertebrae, a relatively short adipose-fin base measuring less than 20% SL, a uniform colour pattern with no black markings on the lateral line and usually a greyish caudal-fin in life.

Currently valid species are H. capitulum, H. filamentus, H. fortis, H. hoevenii, H. nemurus, and H. spilopterus.

H. filamentus can be distinguished from H. capitulum by possessing a longer, narrower head (head width 55–60 % HL vs. 59–70).

It differs from H. fortis in having a shorter, deeper adipose-fin (maximum height 2.3–3.2 times in the length of its base vs. 2.8–4.9) plus the anterior branch of each dorsal-fin ray is longer than the other branches giving the fin an overall jagged appearance (vs. branches of the dorsal-fin rays of almost equal length giving a rounded appearace).

It can be told apart from H. hoevenii in that the premaxillary tooth band is not exposed when the mouth is closed (vs. partially exposed) plus possession of rounded (vs. tapering) caudal-fin lobes, absence (vs. presence) of a thick dark margin around the caudal-fin, a rounded (vs. triangular) anal-fin, broader dorsal-fin membranes giving the fin a rounded (vs. triangular) appearance and filamentous extensions on the dorsal-fin that do not reach beyond the middle of the adipose-fin base (vs. filamentous extensions of the first two dorsal-fin rays that reach beyond the posterior of the adipose-fin base).

It differs from H. nemurus in having a longer, narrower head (head width 55–60 % HL vs. 60–69) and a smaller dorsal to adipose distance (7.7–10.9 % SL vs. 10.2–16.2), and from H. spilopterus in that the dorsal to adipose distance is smaller (7.7–10.9 % SL vs. 10.8–17.6), interorbital space flat (vs. convex), and the dorsal-fin rays (excluding filamentous extensions) reach beyond the adipose-fin origin (vs. not reaching or just reaching).

The genus Hemibagrus currently contains 40 nominal species which are distributed east of the Godavari River system in India and south of the Changjiang (Yangtze) drainage in China, with Southeast Asia a particular centre of diversity.

Many species are important food fishes and some are cultured for the purpose, or for sport angling.

Hemibagrus has previously been considered synonymous with Mystus but following Ng and Kottelat (2013) members can be diagnosed by their moderate to large adult size and strongly-depressed head shape with the interorbital region normally flat or slighly convex.

The grouping also shares a number of characters with the genera Sperata and Bagrus, and these three can be separated from other bagrids by the following: mesethmoid highly depressed (vs. not highly depressed), prominent (vs. reduced) dorsoposterior laminar extension of the mesethmoid, the first infraorbital with (vs. lacking) a posterolateral spine, enlarged (vs. moderate or small) premaxilla, and the metapterygoid with a long, free posterior margin (vs. contacting quadrate and hyomandibular).

Hemibagrus can be told apart from Sperata by possession of a a relatively short and slender (vs. enlarged and elongate) interneural and by absence (vs. presence) of a concave surface in the posterior portion of the posttemporal in which lies a portion of the swimbladder.

It's distinguished from Bagrus by possession of 7, very rarely 8 (vs. 8-10) soft dorsal-fin rays.