Trigonostigma somphongsi (MEINKEN, 1958)

Classification

Order: Cypriniformes Family: Cyprinidae

Distribution

Probably endemic to western Thailand although the location(s) at which it can be found remain a mystery and at one point it was thought to be extinct. It’s reckoned to occur somewhere in the Mae Klong basin and has been reported from Ratchaburi province through which a section of the river flows. Type locality is given simply as ‘Thailand’.

Alarmingly several native species including Balantiocheilos melanopterus and Yasuhikotakia sidthimunki have already disappeared from that collection area due to the construction of three large dams disrupting the annual flooding of the river and destroying their breeding cycles.

Habitat

Needs to be confirmed but all other members of the genus inhabit 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.

Maximum Standard Length

25 – 30 mm.

Aquarium SizeTop ↑

Aquarium base dimensions of 60 ∗ 30 cm or equivalent should be the smallest considered since this species should be maintained in numbers.

Maintenance

Choice of décor is not especially critical though it apparently shows better colouration in a heavily-planted set-up with a dark substrate. A more natural-looking arrangement might consist of a soft, sandy substrate with wood roots and branches placed such a way that plenty of shady spots and caves are formed.

The addition of dried leaf litter (beech, oak or Ketapang almond leaves are all suitable) would further emphasise the natural feel and with it the growth of beneficial microbe colonies as decomposition occurs. These can provide a valuable secondary food source for fry, whilst the tannins and other chemicals released by the decaying leaves will aid in simulating a blackwater environment. Leaves can be left in the tank to break down fully or removed and replaced every few weeks.

This species seems to do best under fairly dim lighting and plant species from genera such as Microsorum, Taxiphyllum, Cryptocoryne, and Anubias are recommended since they will grow under such conditions. A few patches of floating vegetation to diffuse the light even further may also prove effective.

Water Conditions

Temperature: 22 – 26 °C

pH: 5.0 – 7.0

Hardness: 18 – 143 ppm

Diet

Probably a micropredator feeding on small insects, worms, crustaceans, and other zooplankton in nature. In the aquarium it’s easily-fed but the best condition and colours offer regular meals of small live and frozen foods such as bloodworm, Daphnia, and Artemia, alongside good quality dried flakes and granules.

Behaviour and CompatibilityTop ↑

If it ever becomes available in large numbers it will almost certainly make an excellent community fish but for now the emphasis should be on captive reproduction and we therefore recommend maintaining it alone. It is a schooling species by nature so if the opportunity to purchase a group arises it should be taken.

Sexual Dimorphism

Mature females are usually deeper-bodied and noticeably larger than males. Once settled males develop a coppery-orange colouration whereas females remain yellowish.

Reproduction

Captive breeding of this species should be considered a priority for any hobbyist lucky enough to obtain it. By all accounts it is not too difficult, with the process occuring in an identical fashion to others in the genus, but the overall yield of eggs tends to be very small (A. Vassiere, pers. comm.).

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 a separate tank should also be set up. This 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 ideally be of 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, other broad-leaved plant or artificial alternative should also be included. Filtration is not really necessary but you can use a small, air-powered sponge filter if you prefer.

UK expert breeder Alan Vassiere has had success using a similar method with bare tank base, a small sponge filter and a single artificial plant. His stock tank is maintained at around 7°GH and a pH of 7.2, but for spawning the fish he uses reverse osmosis water adjusted to a pH of around 6.0 and small amounts of reverse osmosis containing alder cone extracts (made by simply boiling the water, pouring over a few alder cones and left overnight to infuse) are also added.

Some breeders of other Trigonostigma spp. 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 adults will eat any eggs they find so either they or the eggs themselves 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.

Thanks to Alan P. Vaissiere.

NotesTop ↑

T. somphongsi is exceptionally rare both in nature and the hobby and was considered potentially extinct for around a dozen years before a trio were imported to Germany as contaminants among a batch of Boraras urophthalmoides in 2006.

Since then it has been successfully bred and distributed to various experts, mostly in Thailand and Germany, in the hope of establishing a sustainable captive population. The possibility of a reintroduction programme remains some way off due to a lack of knowledge about the habitat of the species and the reasons behind its decline.

There are currently four described species of Trigonostigma and while T. espei, T. hengeli, and T. heteromorpha can be tricky to distinguish from one another at first glance the same cannot be said of T. somphongsi. The dark, wedge-like flank marking that characterises members of the genus is reduced to a thickish black stripe, it has the most elongated body profile, and exhibits the greatest degree of sexual dimorphism among the four.

The genus was erected by Kottelat and Witte (1999) in order to separate members from the larger Rasbora grouping on the basis of the following combination of characters: colour pattern comprising a reddish, pinkish or orange body (in life) with a conspicuous black stripe extending from below dorsal-fin origin to middle of caudal-fin base and usually broadened anteriorly so as to have a triangular or hatchet shape (except in T. somphongsi in which the stripe is only slightly broader anteriorly); reproductive strategy in which eggs are deposited on the underside of broad leaves or similar structures.

Other characters distinguishing the genus (but not unique to it) are: miniature size (maximum 32 mm SL in nature, though captive specimens may grow slightly larger); incomplete lateral line reduced to 6-9 pores; absence of barbels; 5 branched anal-fin rays; relatively deep body (not quantified).

Rasbora has 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.

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.

References

1. Kottelat, M. and K.-E. Witte, 1999 - Journal of South Asian Natural History 4(1): 49-56
   Two new species of Microrasbora from Thailand and Myanmar, with two new generic names for small southeast Asian cyprinid fishes (Teleostei: Cyprinidae).
2. Mayden, R. L., K. L. Tang, K. W. Conway, J. Freyhof, S. Chamberlain, M. Haskins, L. Schneider, M. Sudkamp, R. M. Wood, M. Agnew, A. Bufalino, Z. Sulaiman, M. Miya, K. Saitoh, and S. He, 2007 - Journal of Experimental Zoology, Molecular Development and Evolution 308B: 1–13
   Phylogenetic relationships of Danio within the order Cypriniformes: a framework for comparative and evolutionary studies of a model species.
3. Tang, K. L., M. K. Agnew, W. J. Chen., M. V. Hirt, T. Sado, L. M. Schneider, J. Freyhof, Z. Sulaiman, E. Swartz, C. Vidthayanon, M. Miya, K. Saitoh, A. M. Simons, R. M. Wood, and R. L. Mayden, 2010 - Molecular Phylogenetics and Evolution 57(1): 189-214
   Systematics of the subfamily Danioninae (Teleostei: Cypriniformes: Cyprinidae).