The congener B. urophthalmoides is often traded as B. brigittae, presumably in order to obtain a more favourable price. Though these are similar in that they're the only two members of the genus to possess a dark lateral body stripe B. brigittae is easily identifiable by its larger adult size, lack of distinct dark blotch at the caudal peduncle, comparitively short, often broken lateral stripe and overall brighter, more uniformly red patterning. It's sometimes seen on sale with the alternative vernacular names 'chili rasbora' or 'dwarf rasbora'.
B. merah is also similar-looking but it lacks colour in much of the body with the red pigmentation tending to be concentrated around the dark body markings which do not usually form an unbroken stripe. Striped male individuals do exist, however, and this has led to speculation that B. brigittae and B. merah are the same species (Körner, 2010)[8]. The same author also reported that in an 'eastern' population (from the area around Banjarmasin?) both sexes develop a striped pattern before maturity, while in the 'central' population (Palangkaraya, where fish are collected for the aquarium trade) only males develop the striped colour pattern, and only when sexually mature. Kottelat (2013) argued that the different ontogenies in colour pattern, and presumed geographical distribution of these two populations, supports the existence of two species, whilst also noting that Palankaraya is not close to the type locality of B. merah; the Sungei Jelai Bila river basin in Kalimantan Tengah, much further west.
Boraras was erected in 1993 in order to separate a small group of species from the larger Rasbora assemblage on the basis of differences in morphology and reproductive strategy.
In older literature they are therefore referred to as members of Rasbora and following Liao et al. (2010) the genus is a member of the rasborin sub-group within the subfamily Danioninae (the other sub-group contains the danionins).
This group is further subdivided into six clades (clusters of closely-related species) of which Boraras (or at least B. brigittae; the only species involved in the study) is included in 'clade three' alongside Horadandia atukorali, Rasboroides vaterifloris, Trigonostigma heteromorpha and three species previously included in Rasbora but moved into new genera by the authors; Trigonopoma gracile, T. pauciperforatum and Rasbosoma spilocerca.
Of these it was found to form a monophyletic group with Rasboroides meaning these two derived from the same common ancestor. The results for B. brigittae and T. heteromorpha were found to be inconclusive in some respects and further work regarding their phylogenetic position was recommended.
Shortly afterwards a paper investigating systematics of the subfamily Danioninae was published by Tang et al. (2010) Their results differed from those of Liao et al. and their conclusion was to synonymise the four new genera plus Boraras and Trigonostigma with Rasbora based on an incomplete knowledge of relationships within the group, an approach they describe as 'more conservative'. This has not been followed by the majority of authors, however, and the separate groupings are maintained in the extensive work by Kottelat (2013), which is followed here.
The small adult size in Boraras species evolved via a process known as miniaturisation, characterised by sexually mature adults with a significantly reduced size of less than 20 mm SL.
Among bony fishes cyprinids are one of the few groups in which this phenomenon occurs repeatedly with all Barboides, Danionella, Microdevario, Microrasbora, Horadandia, Boraras, Paedocypris, Sawbwa and Sundadanio species representing miniaturised taxa along with a few members of Danio, Laubuca and Rasbora. All show a preference for still or slow-moving waters, often in nutrient-poor habitats such as forest peat swamps.
The anatomical structure of miniaturised cyprinids can vary greatly, and there are two principle 'groupings' with some species possessing intermediate features to some degree. The first contains those fishes which though small are essentially proportionally dwarfed versions of their larger relatives, e.g., Barboides, Microdevario, Microrasbora, Boraras, etc.
The other includes those in which anatomical development stops at a point where adult still resemble a larval form of their larger ancestor, i.e., Danionella and Paedocypris.
The latter are usually referred to as 'developmentally truncated' or 'paedomorphic' and are thought to have evolved via a process known as 'progenetic paedomorphosis' i.e. paedomorphosis brought about by accelerated maturation. They typically exhibit a simplified skeletal structure along with species-specific morphological peculiarities.
Britz et al. (2009) consider that developmental truncation may have facilitated the development of such novelties 'by freeing large parts of the skeleton from developmental constraints, dissociating developmentally linked pathways and creating a greater potential for more dramatic changes'.
Boraras species from the Malay Archipelago (B. brigittae, B. maculatus and B. merah) tend to have a slimmer, more elongate body profile than those from Indochina (B. micros, B. urophthalmoides, B. naevus) which may assist when attempting to identify them.
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