Trigonostigma (Rasbora) heteromorpha, most familiar of the Harlequin Rasboras

Trigonostigma (Rasbora) heteromorpha (Harlequin Rasbora). There are about sixty species in the genus Rasbora, but in the LFS, if you just say "Rasbora," this is probably the Rasbora you mean. "Heteromorpha," the "different shape," refers to the body shape, which is higher and deeper than the classic streamlined rasbora form, so different in fact that in 1993 researcher Maurice Kottelat  proposed the new genus, Trigonostigma ("triangle blotch") for the  "Harlequin" Rasboras,which now number four.
 
You may already be aware that R. heteromorpha is one of several lookalike species. A Thai species, R. espei, was recognized as a distinct subspecies by Hermann Meinken in 1967, when it was first imported into Germany by Heinrich Espe of Bremen; after it was considered a genuine species by Maurice Kottelat and Kai-Erik Witte in 1999, it has appeared increasingly often in the aquarium trade. Its dark blotch is less triangular than T. heteromorpha's, with a sharply incurving under edge and a long narrow produced point that stretches right to the base of the tailfin. It is flushed a more intense rosy-orange than heteromorpha, even on the fins, and when the two are (inadvertently) kept together, as I currently have them, it's the more dramatic and desirable species. But half-grown and showing little color in the LFS tanks, it's easy to mistake the one for the other.
 
A smaller "Harlequin," maxing out at about an inch, is R. hengeli, which is mostly transparent, with a rosy-orange bar above the body blotch. It comes from the Tembesi River drainage, a tributary of the Batang Hari River in central Sumatra. In 1999 Dr Kottelat and his co-worker Kai-Erik Witte added still another new Harlequin to this genus, Trigonostigma somphongsi. European and Japanese aquarists seemed to be adopting the new genus names faster than Americans: after about a decade I fell into line.
 
Rasbora heteromorpha, as they then were, were described in 1904 and first imported into Europe (to Germany) in 1906. For some decades they had a reputation for being difficult to spawn. Their homewaters are extremely soft, with pH dropping below 6.0. Luckily for us, they have become acclimatized to harder water over many captive generations, and they have lost their fussiness. Still, the more reverse osmosis or de-ionized water you can use, and the lower the pH drops as a result, the more viable eggs a spawning is likely to produce. Broad-leaved plants seem to be an inducement, for, unlike the egg-scattering Rasbora, the female tips upside-down to deposit a couple of eggs at a time on the underside of a leaf. Even without a male a female may signal her readiness as in this YouTube video by Polyacanthus. They aren't prolific, but left alone in a large planted aquarium with plenty of hiding places for the young, T. heteromorpha will maintain their school. Our captive T. heteromorpha remain smaller than the wild populations.
 
The common harlequin rasbora T. heteromorpha is found locally quite abundant only in swamp forest habitats in the Malay peninsula and Thailand and in the northern half of Sumatra, typically in warm peaty shaded "blackwaters." Luckily for the future of the species, it survives also in more "developed" and disturbed areas. The sustainable collection of this species in the swamp forest has been described in the University of Singapore's revised and more accessible website. The specimens are literally scooped out individually or in groups. Although this is a schooling species, the dense vegetation and uneven terrain of the swamp forest makes collection generally difficult. School children love to earn pocket money by catching fish in their spare time. Middlemen pay by lots of a hundred or per piece. During the low water season, when the fish population is concentrated in shallow pools of water, in excess of a hundred thousand fishes can be obtained within a week. Shortly afterwards, the populations often collapse anyway, due to a shortage of water. So, for this species at least, University of Singapore biologists anticipate that the rapid loss of forests (especially swamp forests) will pose a greater threat to Trigonostigma  heteromorpha's survival than aquarium-trade collecting pressure.
 
Though T. heteromorpha are not very prolific breeders, their endurance in the trade, despite the extensive collecting efforts, is due in no small part to their very extensive distribution and the difficulty in collecting them en masse like other Rasboras. Even as schooling fish, harlequin rasboras are not present in huge numbers. That the species is still extant, even abundant in some places, means that the current fishing practices are sustainable.
 
As a shoal of T. heteromorpha find conditions to their liking, in soft clean water with some tannin and a pH below 7.0, pre-spawning behavior begins to appear: members of the shoal start to react to one another as individuals, darting, rushing and feinting, indulging in brief chases. Pairing is preceded by mutual fin-vibrating displays, side-by-side or head-to-tail, with all fins stiffly erect and shimmering. The bond is a fluctuating one: they are easily distracted by other members of the shoal, let alone curious passers-by in a community tank. 
 
Links.   Species profile of Trigonostigma heteromorpha at Tropical Fish Hobbyist. Trigonostigma heteromorpha at FishBase. Rasbora heteromorpha at Wikipedia. Swedish aquarist Ola Åhlander gives the essentials concerning Trigonostigma.