Opsarius shacra

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

shacra: presumably derived from a vernacular Bengali name for this species.

Recorded from many rivers draining the Himalayan foothills in Nepal and northern India within the Ganges and Brahmaputra river systems, and is also known from the lower Ganges in Bangladesh, where the river is known as the Padma.

Type locality is 'Kosi River, Uttar Pradesh, India'.

Inhabits well-oxygenated, medium-to-high gradient, moderate to fast-flowing rivers and tributaries with substrates of gravel, cobbles, larger boulders and exposed bedrock.

In the Tamor River, a tributary of the Kosi in Nepal, it has been collected alongside the congeners Barilius barila and B. bendelisis plus Schizothoraichthys labiatus, S. progastus, Schizothorax richardsonii, Neolissochilus hexagonolepis, Psilorhynchoides pseudecheneis and Garra annandalei.

Water parameters recorded in the Tamor between May and July 2009 were temperature 16 – 19 °C61 – 66 °F, pH 7.3-7.5, dissolved oxygen 9.7-10 ppm, alkalinity 17.1 mg/l, total hardness 28.5-34.2 mg/l, CO2 5 mg/l, and conductivity 37.7-56.7 µs/cm.

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 Barilius spp. are prodigious jumpers.

Barilius 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, other 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.

Unconfirmed, but sexually mature females should be heavier-bodied than males when gravid.

Unreported.

This species is poorly-known in the aquarium hobby but is exported on occasion.

It's told apart from congeners by the following combination of characters: one pair each of maxillary and rostral barbels present; tubercles on head and body poorly-developed; 59-70 lateral line scales; 12 vertical bars on each flank, occasionally absent; dorsal-fin insertion anterior to anal-fin insertion; dorsal-fin with black band in the upper third; 22-25 predorsal scales.

The generic placement of Barilius species 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.