This species is also sold as the 'redline' rasbora which is something of a misnomer as in reality the colour of the lateral stripe that gives rise to the name can vary from deep orange to golden depending upon the collection locality of the fish as well as diet, condition, and even mood. Some specimens also have a second, darkish stripe beneath the other which seems to vary in length and intensity depending on a similar array of factors. Older fish tend to possess a greater degree of black edging to the scales, especially in the ventral portion of the body.
T. pauciperforatum is sometimes said to closely resemble and occur sympatrically with R. agilis but despite being listed as valid by some sources the latter is a junior synonym of the former. R. agilis had previously been used to refer to a fish described by Ahl as R. taeniata, was subsequently redescribed as R. gracilis by Kottelat in 1991, and is now known as Trigonopoma gracile.
It's a smaller, more delicate-looking fish than T. pauciperforatum, has relatively longer fins, and the lateral stripe is dark in colour. It was pictured in a 1971 book by Brittan labelled as R. agilis and this is probably where much of the confusion originated.
Trigonopoma is distinguished from related genera by possession of a triangular-shaped opercle, the dorsal half of which, above the opercle-hyomandibular junction, is greatly reduced forming an upward process in the middle of the dorsal outline.
Rainboth's 'Fishes of the Cambodian Mekong' characterised members of Rasbora by: possession of an unbranched, non-spiny first dorsal-fin ray; seven soft dorsal rays; origin of the dorsal-fin in the middle of the body; five branched anal-fin rays; a small mouth not extending below the eye; absence of barbels.
It's long been recognised as a polyphyletic lineage as noted by Kottelat (1999) among others, and in 2010 the results of a phylogenetic analysis by Liao et al. suggested a number of changes in order to improve the taxonomy. The authors found species of rasborin genera to represent a monophyletic grouping existing in six clades and erected four new genera containing former members of Rasbora in order to preserve monophyly of the existing groups.
The first two of these clades contain new groupings Kottelatia and Brevibora, respectively. The third comprises Boraras, Horadandia, Rasboroides, and Trigonostigma, plus new genera Trigonopoma and Rasbosoma. However the results for Boraras and Trigonostigma were found to be inconclusive in some respects and further work regarding their phylogenetic position was recommended.
The fourth clade includes Rasbora semilineata, R. borapetensis, R. rubrodorsalis, and an undescribed fish similar to R. beauforti. Clade five consists of R. daniconius, R. hubbsi, R. paucisqualis, R. wilpita (plus allies), R. kobonensis, R. ornata, and R. cf. daniconius.
Clade six is subdivided into two groupings. The first contains R. einthovenii, R. elegans, and R. cephalotaenia, and the second R. lateristriata, R. argyrotaenia, R. volzii, R. paviana, R. rasbora (plus an undescribed related fish), R. caudimaculata, and R. trilineata.
As this final clade contains R. cephalotaenia, the type species of Rasbora, its members retain the generic name as do members of clade five because they don't differ sufficiently to warrant erection of a new genus or genera.
Unfortunately many species weren't included in the analysis leading to confusion regarding the correct placement of the 40 or so other Rasbora species, in particular. As the genus had previously been split into various 'species groups' (groups of closely-related taxa) dating back to Brittan (1972, who referred to them as 'species complexes') Liao et al. proposed the following arrangement whilst noting it may be subject to change in the future:
R. semilineata species group: R. semilineata, R. borapetensis, R. rubrodorsalis.
R. trifasciata species group: R. trifasciata, R. amplistriga, R. bankanensis, R. dies, R. ennealepis, R. hubbsi, R. johannae, R. meinkeni, R. paucisqualis, R. rutteni, R. sarawakensis, R. taytayensis, R. tobana, R. tuberculata.
R. daniconius species group: R. daniconius, R. caverii, R. kobonensis, R. labiosa, R. ornata, R. wilpita.
R. einthovenii species group: R. einthovenii, R. cephalotaenia, R. elegans, R. jacobsoni, R. kalochroma, R. kottelati, R. nematotaenia, R. tubbi.
R. argyrotaenia species group: R. argyrotaenia, R. aprotaenia, R. aurotaenia, R. baliensis, R. borneensis, R. bunguranensis, R. dusonensis, R. evereti, R. hobelmani, R. hossi, R. lateristriata, R. laticlavia, R. leptosoma, R. philippina, R. septentrionalis, R. spilotaenia, R. steineri, R. tawarensis, R. tornieri, R. volzii.
R. sumatrana species group: R. sumatrana, R. atridorsalis, R. calliura, R. caudimaculata, R. dorsinotata, R. notura, R. paviana, R. rasbora, R. subtilis, R. trilineata, R. vulgaris.
Not classified: R. beauforti, R. chrysotaenia, R. gerlachi (validity in question), R. kalbarensis, R. reticulata, R. vulcanus (possibly members of a different, unnamed, genus) and R. zanzibarensis (identity in question).
Shortly afterwards a paper investigating systematics of the subfamily Danioninae was published (Tang et al. 2010) The results differed significantly and the four new genera of Liao et al., plus Boraras and Trigonostigma, were synonymised with Rasbora based on an incomplete knowledge of relationships within the group, an approach described as 'more conservative'.
Though neither conclusion can be deemed 100% the system of Liao et al. is followed here on SF pending future studies, if only because we prefer to retain the familiar genera Boraras and Trigonostigma. In addition, species described post-2010 are not included in the list above.
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