Opsarius signicaudus

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

signicaudus: from the Latin signum, meaning 'flag, sign', and cauda, meaning 'tail'.

Known only from headwater tributaries within the Khwae Yai and Khwae Noi branches of the Mae Klong River in western Thailand.

Type locality is 'Sangklaburi District, Maeklong basin, Kanchanaburi Province, Thailand'.

Inhabits headwaters and similar well-oxygenated, low-to-medium gradient, moderate to fast-moving rivers and streams with substrates of gravel, cobbles, larger boulders and exposed bedrock.

Adults tend to be found in deeper, flowing habitats with juveniles occupying nursery zones in shallow water or slower-flowing zones behind large rocks.

Sympatric species in Sangklaburi district include Sangklaburi included Opsarius koratensis, Poropuntius melanogrammus, Mystacoleucus obtusirostris, Garra spp., Rasbora cf. rasbora, Rasbora paucisqualis, Microdevario kubotai, Crossocheilus siamensis, Labiobarbus sp., Paracanthocobitis zonalternans, Schistura spp., Nemacheilus pallidus, Yasuhikotakia morleti, Glyptothorax spp., Batasio tigrinus, Xenentodon sp., Parambassis siamensis and Tetraodon sp. (Tejavej, 2012).

Requires a large aquarium with minimum surface area of 120 ∗ 45 cm48 ∗ 18 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.

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

Nuptial male individuals develop an enlarged chest as they grow, possess more numerous tubercules on the head, are more intensely-coloured than females.

Females may also grow a little larger and are thicker-bodied than males, especially when gravid.

Unrecorded.

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

It can be distinguished from congeners by the following combination of characters: presence of a large, elongated blotch at the caudal-fin base (formed by the large caudal spot at the fleshy end of the caudal-fin base plus the last vertical bar on the flank) extending around 1/3 of the distance from the caudal-fin origin beyond the last scale on the caudal base to the margin of the caudal fork; anal-fin origin located opposite the 2nd–4th branched dorsal-fin rays; eight scale rows above the lateral line; dark pigment on the dorsal fin mostly concentrated along the margins of the rays; 36 or more precaudal lateral-line scales; small dentary tubercles; rostral barbels small, short and not present in all specimens.

O. dogarsinghi and O. bernatziki are the only two Southeast Asian congeners to possess a large blotch at the caudal-fin base but in the former the blotch is vertically-orientated and hardly extends onto the fin itself (vs. laterally-elongate and extending onto the basal fin rays) and long (vs. short) barbels, while the latter has 33 or less (vs. 36-39 ) scales in the lateral row.

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.