Opsarius ornatus

Opsarius: apparently from an Ancient Greek word meaning 'small fish'.

ornatus: from the Latin ornatus, meaning 'decorated'.

Unclear. Possibly restricted to a handful of small systems in Chumphon Province on the eastern side of the Isthmus of Kra, southern Thailand, with the majority of records deriving from the Tha Sae basin (Tejavej, 2012). This theory has been disputed (Kottelat, 2013)[8] and the species may have been described from the Chao Phraya river system, although if this is the case it has not been collected there since its description.

Populations inhabiting the Salween, Ataran, Ping and Mekong drainages which were formerly considered to represent O. ornatus are confirmed as O. infrafasciatus (Tejavej, 2012).

Type locality is 'Mé-Nam River, Thailand', with 'Mé-Nam' having been considered an older name for the Chao Phraya river in central and western Thailand prior to this species' redescription.

Collected exclusively from habitats containing flowing water, from sluggish streams to fast-moving rivers with substrates composed of sand, gravel or mud.

Sympatric species include Rasbora paucisqualis, Rasbora trilineata, Rasbora paviana, Hampala macrolepidota, Homaloptera sp., Nemacheilus masyae, Pangio sp., Xenenthodon sp., and Poropuntius genyognathus.

Requires a large aquarium with minimum surface area of 150 ∗ 60 cm60 ∗ 24 in or equivalent.

The aquarium should ideally be designed to resemble a flowing stream or river with a substrate of variably-sized rocks, sand, fine gravel, and some larger water-worn boulders. This can be further furnished with driftwood roots and branches if you wish but be sure to leave plenty of open swimming space.

While the majority of aquatic plants will fail to thrive in such surroundings hardy genera such as Microsorum, Bolbitis, or Anubias spp. can be grown attached to the décor.

This species is intolerant to the accumulation of organic wastes and requires spotless water at all times in order to thrive. It also does best if there is a high proportion of dissolved oxygen and moderate degree of water movement meaning external filters, powerheads, airstones, etc., should be employed as necessary.

As stable water conditions are obligatory for its well-being this fish should never be added to biologically-immature aquaria, and weekly water changes of 30-50% aquarium volume should be considered mandatory. A tightly-fitting cover is also essential as Opsarius spp. are prodigious jumpers.

A tightly-fitting cover is also essential since Barilius spp. are prodigious jumpers.

Opsarius spp. are predominantly surface-feeders preying on aquatic and terrestrial insects in nature, with some small fishes and benthic invertebrates probably taken as well.

In the aquarium good quality dried products can be offered but should be supplemented with regular meals of live and frozen fare such as chironomid larvae (bloodworm), Artemia, chopped earthworms, etc. Drosophila fruit flies and small crickets are also suitable provided they are gut-loaded prior to use.

Unsuitable for the general community due to its environmental requirements, and likely to outcompete or intimidate slow-moving and less bold species at meal times since it is an extremely fast swimmer and vigorous feeder. Much smaller fishes should also be omitted as they may be predated upon.

The best tankmates are similarly-sized, robust, pelagic cyprinids such as Dawkinsia, Barilius or larger Devario and Rasbora spp. while bottom-dwellers could consist of Garra, Crossocheilus, Botia and Schistura spp., for example. Many characids and loricariid catfishes should also work well, but be sure to perform thorough research before purchase.

Although gregarious by nature this is a shoaling rather than schooling species which develops a distinct pecking order and therefore should always be maintained in a group of five or more individuals. If only two or three are purchased the subdominant fish may be bullied incessantly whereas solitary specimens may become aggressive towards similar-looking species

Adult females are less-brightly-coloured, tend to grow a little larger and are thicker-bodied than males, especially when gravid.

Sexually mature males appear to possess small tubercules on the head, body and fins, plus reddish-orange to bronze pigmentation in the ventral portion of the body.

Unreported.

This species may have been exported for aquaria though probably not under the correct name.

Following its redescription by Tejavej (2012) it can be distinguished from congeners by the following combination of characters: lacking or possessing a small caudal spot; 37–43 (rarely 36) lateral line scales; 6–7 scale rows above lateral line; 17–20 (rarely 16 or 21) predorsal scales; 12-14 (rarely 15 or 16) circumpeduncular scales; anal-fin origin located beneath or posterior to the 6th branched dorsal-fin ray; head depth 17–21% SL; predorsal length <58 % SL; dark pigmentation on the dorsal-fin concentrated distally on the branched rays; rostral and maxillary barbels generally short and thin when present; dentary tubercles small.

The generic placement of species currently referred to Opsarius has been open to question since Howes (1980) concluded that the Barilius grouping, within which they were included at the time, was not monophyletic and identified two separate lineages. The first group contained B. barila, the type species, plus B. bendelisis, B. radiolatus, B. vagra, and B. shacra, while the second included all other species, and initially these were referred to the subdivisions 'group i' and 'group ii' within Barilius itself.

The composition of these groups is confusing since in a later work Howes (1983) included B. evezardi and B. modestus in 'group i' whilst omitting B. bendelisis, B. radiolatus, and B. shacra without explicitly stating why.

Rainboth (1991) assigned the members of Howe's 'group ii' to the revalidated generic name Opsarius based on the fact it was the oldest available with 'group i' species retaining the name Barilius due to the presence of the type species.

This system has been followed by some subsequent authors, e.g., Tang et al. (2010) and Collins et al. (2012) but not gain wider usage until Kottelat (2013), who included all former Barilius from Southeast Asia in Opsarius.