Trigonostigma heteromorpha - Harlequin

Cyprinidae

Usually quoted as being native to southern Thailand, Peninsular Malaysia, Singapore and the Greater Sunda islands of Borneo and Sumatra. Thai populations seem to be restricted to the district of Narathiwat close to the border with Peninsular Malaysia. In the latter it is more widely-distributed and has been collected from areas of peat swamp forest in the states of Selangor, Kelantan, Terengganu, Pahang and Johor. In Singapore it is found in Nee Soon Swamp, one of the last remaining pockets of pristine freshwater forest swamp found in the central catchment area of the island.

The extent of its distribution in the Greater Sunda Islands is less clear. While it certainly occurs on the island of Bintan in Riau Islands province, Sumatra we have not been able to find definitive information regarding the mainland other than a vague reference to the city of Medan in the north (the species is said to range throughout northern Sumatra). Similarly there appears not to exist a single confirmed occurence from Borneo.

The fish are known to vary somewhat depending on collection locality, with those collected from Johor and southern Thailand having an overall smaller, slimmer appearance than the Singapore/Sunda Island populations. It is unconfirmed as to whether this is as a result of natural deviation or captive-bred fish being released by/escaping from fish farms. At any rate wild caught fish are increasingly rare in the hobby as it is being produced commercially in several countries.

Mostly inhabits gently flowing sections of forest streams and tributaries where submerged aquatic plants such as Cryptocoryne species grow thickly. The water is sometimes stained faintly brown/yellow due to the presence of tannins and other chemicals released by decomposing organic matter and the substrate scattered with fallen leaves, twigs and branches. Such environments characteristically contain soft, weakly acidic to neutral water and are often dimly-lit due to dense marginal vegetation and the forest canopy above.

A large specimen can measure around 2"/5cm.

Really needs a tank measuring at least 24" x 12" x 12"/60cm x 30cm x 30cm/56.6 litres. Although it is a small fish it requires plenty of swimming space and is best kept in larger groups.

Choice of decor is not especially critical although the fish tend to show better colouration when maintained in a well-furnished set-up with a dark substrate. It is therefore an excellent choice for the carefully-aquascaped planted tank. The addition of some floating plants and driftwood roots or branches to diffuse the light entering the tank also seems to be appreciated and adds a more natural feel. Filtration does not need to be particularly strong as it mostly hails from sluggish waters and may struggle if there is a fast current.

To see it at its best a biotope-style set-up can also make an interesting project. A soft, sandy substrate is probably the best choice to which can be added a few driftwood roots and branches, placed in such a way that plenty of shady spots are formed. If you can't find driftwood of the desired shape common beech or oak is safe to use if thoroughly dried and stripped of bark.

The addition of dried leaf litter (beech, oak or Ketapang almond leaves are all suitable; we like to use a mixture of all three) would further emphasise the natural feel and as well as offering even more cover for the fish brings with it the growth of microbe colonies as decomposition occurs. These tiny creatures can provide a valuable secondary food source for fry whilst the tannins and other chemicals released by the decaying leaves are thought to be beneficial for blackwater fish species such as this. Leaves can be left in the tank to break down fully or removed and replaced every few weeks.

Allow the wood and leaves to stain the water. A small net bag filled with aquarium-safe peat can also be added to the filter or hung over the edge of the tank to aid in the simulation of black water conditions. Alternatively obtain some genuine peat fibre and simply drop a few handfuls into the tank. This will become completely saturated with water after a few days and sink to the bottom where it can look really effective. Provided a good routine of water maintenance is practiced no adverse effects should occur using either peat or leaves in an aquarium.

Fairly dim lighting should be used to simulate the conditions the fish would encounter in nature. You could add some Asian plants that can survive under such conditions such as Microsorum pteropus, Taxiphyllum barbieri or Cryptocorynes. A few patches of floating vegetation would be really useful to diffuse the light entering the tank too.

70 - 82°F/21 - 28°C

Adaptable and happy within the range 5.0 - 7.5. A value towards the lower extreme is necessary if you want to breed the fish, though.

1 - 15°H

Stomach analyses of wild specimens have revealed it to be a micropredator feeding on small insects, worms, crustaceans and other zooplankton. In the aquarium it will accept dried foods of a suitable size but should not be fed these exclusively. Daily meals of small live and frozen fare such as Daphnia, Artemia and suchlike will result in the best colouration and encourage the fish to come into breeding condition.

This species is very peaceful indeed making it an ideal resident of the well-maintained community tank and an unreserved recommendation for those new to fishkeeping. As it places no special demands in terms of water chemistry it can be combined with many of the most popular fish in the hobby including other small cyprinids as well as tetras, livebearers, dwarf cichlids, catfish and loaches. A well-chosen community based around fish native to Peninsular Malaysia would also make an attractive display with some of the more commonly exported examples include Puntius lineatus , P. pentazona , Trigonopoma pauciperforatum , T. gracile , Rasbora einthovenii , Brevibora dorsiocellata  and Pangio spp. (kuhli) loaches. As always thorough research is essential when choosing tankmates and its small adult size must be a consideration. It also makes an ideal companion for shy anabantoids such as Sphaerichthys spp. or the more diminutive Betta spp.

It's a schooling species by nature and really should be kept in a group of at least 8-10 specimens. Maintaining it in decent numbers will not only make the fish less nervous but will result in a more effective, natural-looking display. Males will also display their best colours as they compete with one other for female attention.

Mature females are usually rounder-bellied and often a little larger than the slightly more colourful males. An alternative method of sexing is to examine the shape of the dark wedge-like marking on the flanks of the fish. In males this tends to have a sharper, more well-defined outline whereas in females it has a 'rounded' appearance.

Like many small cyprinids it exhibits no parental care although it does utilise a different spawning method to most as the eggs are attached to broad plant leaves. When the fish are in good condition they will spawn often and in a densely-planted, mature aquarium it is possible that small numbers of young may start to appear without human intervention.

However if you want to increase the yield of fry a slightly more controlled approach is required. The adult group can still be conditioned together but one or more separate, say 18" x 10" x 10"/45cm x 25cm x 25cm/29.5 litre containers should also be set up. These should be very dimly lit with the base either left bare or covered with some kind of mesh of a large enough grade so that any eggs that fail to adhere to the plant can pass through but small enough so that the adults cannot reach them. The widely available plastic 'grass'-type matting can also be used and works very well. The water itself should be of slightly pH 5.0-6.0, 1-5°H with a temperature towards the upper end of the range suggested above. A decent-sized clump of Microsorium, Cryptocoryne or other broad-leaved plant should also be added. Filtration is not really necessary but you can use a small, air-powered sponge filter if you prefer, and the use of peat/peat filtration is also optional.

Some breeders report that older fish aged a year or more make the best subjects for spawning which may be the reason why this species is often said to be difficult to breed. Apparently it is trickier to initiate spawning behaviour in younger individuals and they are also are less fecund. At any rate the best way to condition them is by feeding small amounts of live and frozen foods 2 or 3 times a day in the weeks leading up to a spawning attempt. When the females appear full of eggs and the males are showing their best colours as they display to one another a large (40-50% of tank volume), cool water change should be performed and one or two pairs introduced to each spawning container a few hours later, preferably in the evening.

Spawning usually occurs in the morning hours and is preceded by a flurry of courtship activity by the male(s). Often a pair will perform a number of 'dry runs' over a chosen spawning surface and it may be several hours before any eggs are produced. Eventually the female will begin to lay small batches of eggs which are fertilised by the male before the next batch is laid. The spawning process is particularly interesting as often a female will choose the underside of a plant leaf and thus the pair will be observed in an inverted position as eggs and sperm are released. If the pair(s) fail to spawn immediately they can be left in situ but if no eggs have been observed after 3 or 4 days they should be returned to the main group and a different set of fish chosen. There is no need to feed the adults while they are in the spawning tank.

Post-spawning the fish will eat any eggs they find and should be removed as soon as possible. Incubation is temperature-dependant to an extent but usually takes between 24 and 48 hours with the young free-swimming in around a week. Initial food should be Paramecium or similar introducing Artemia nauplii and/or microworm once the fry are large enough to accept them.

Trigonostigma heteromorpha was first imported in the early 1900s and has gone on to become one of the most familiar, enduring species in the hobby. Unfortunately it has suffered as a result of its popularity to a certain extent with the mass-produced fish we see today lacking much of the colour seen in wild specimens and even exhibiting slight deformities in some cases. In recent years several garish, selectively-bred varieties have become available including 'blue', 'black' and 'golden' (leucistic) forms. These tend to show an even greater degree of physical abnormality and it is possible that hybridisation with other species has occured.

It can be confused with the similar-looking T. espei  and T. hengeli  although on close inspection they are actually quite easy to tell apart. The basic body colouration of Trigonostigma heteromorpha is pinkish, it has the deepest body shape of the three and the characteristic body marking is broader, more triangular in shape and darker in colour. T. espei  is generally a bright copper/reddish colour with a thinner, 'lambchop'-shaped body marking whereas T. hengeli  is a slightly smaller fish with an overall greyish/colourless body colouration and a flash of bright orange pigmentation along the upper and anterior edges of the 'lambchop' marking. T. somphongsi is the fourth, exceptionally rare, member of the genus but is very easy to distinguish from the others as the dark body marking is reduced to a thickish stripe.

The genus Trigonostigma was erected in 1999 in order to separate a small band of species from the larger Rasbora grouping on the basis of major differences in body shape and reproductive strategy. In older literature they are therefore referred to as members of Rasbora which remains a genus in need of further systematic review. It has long been recognised as a polyphyletic lineage as noted by Kottelat (1999) amongst others and it is likely that many more species will be reassigned to alternative, and possibly new, genera in the future. Indeed a 2007 study that aimed to assess the phylogenetic position of the genus Danio within the order Cypriniformes revealed that various species currently accepted as Rasbora are more closely related to Trigonostigma than to their congenerics.

Literature cited

1. Mayden, Richard L.; Tang, Kevin L.; Conway, Kevin W.; Freyhof, Jörg; Chamberlain, Sarah; Haskins, Miranda; Schneider, Leah; Sudkamp, Mitchell; Wood Robert M.; Agnew, Mary; Bufalino, Angelo; Sulaiman, Zohrah; Miya, Masaki; Saitoh, Kenji; He, Shunping. 2007 - J. Exp. Zool. (Mol. Dev. Evol.) 308B: 1–13.
   Phylogenetic relationships of Danio within the order Cypriniformes: a framework for comparative and evolutionary studies of a model species.
2. www.fishbase.org