Rasbora dusonensis

Cyprinidae

Occurs in the great Mekong, Chao Phraya and Mae Klong river drainages in western Thailand as well as parts of Cambodia, Laos, Peninsular Malaysia and southwestern China, although records from Vietnam are strangely lacking. Its distribution also extends southwards into the Greater Sunda Islands of Sumatra and Borneo with specific occurences numbering far too many to list here. The fish may vary in colour and patterning depending on collection locality (see notes).

Clearly an adaptable fish as it has been collected from various types of habitat including swiftly-flowing forest hill streams with substrates of boulders, smaller stones, sand or gravel and more sluggish blackwaters associated with areas of ancient peat swamp forest which are typically full of fallen leaves, twigs and branches. In the former the water tends to be clear with a pH between 6.5 and 7.5 whereas in the latter it is characteristically stained brown due to the release of tannins and other chemicals released by decomposing organic matter, very soft (negligible hardness) and acidic (pH 4.0 - 6.0). Such forested environments are often dimly-lit due to the tree canopy above. It also inhabits permanent bodies of water including pools, backwaters and reservoirs and is known to move into flooded areas during the wet season across some of its range.

Around 4"/10cm.

An active species and a group will need a tank measuring at least 48" x 18" x 18"/120cm x 45cm x 45cm/255 litres.

Choice of decor is not as critical as water quality and the amount of open swimming-space provided. We suggest keeping it in a large, well-planted aquarium or alternatively it would look superb in a set-up designed to resemble a flowing river with a substrate of variably-sized rocks and gravel, some large water-worn boulders and plenty of water movement. A rivertank manifold could also be constructed to provide naturalistic unidirectional flow. The tank can be further furnished with driftwood branches and aquatic plants for aesthetic value. While the vast majority of the latter will fail to thrive in such conditions hardy species such as Java fern, Bolbitis or Anubias can be grown attached to the decor.

In either situation the tank must have a very tightly-fitting cover as this species is an accomplished jumper and can escape through the tiniest of gaps. Like other members of the genus it is intolerant to the accumulation of organic wastes and requires spotless water at all times in order to thrive.

73 - 79°F/23 - 26°C

5.0 - 7.5

2 - 12°H

According to stomach analyses of wild specimens this species feeds mainly on invertebrates both aquatic and terrestrial, particularly insects. 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-furnished community tank. As always when selecting a compatible community of fish proper research is essential and its adult size must be a consideration. A community based around one of its native countries or river basins would also make a worthwhile project with some interesting alternatives. For example in the Mun River, Thatoom District, Thailand sympatric species include Yasuhikotakia lecontei , Y. modesta , Y. morleti , Syncrossus helodes , Crossocheilus siamensis, Cyclocheilichthys apogon, C. armatus , C. repasson, Epalzeorhynchos frenatum , Mystacoleucus marginatus , M. atridorsalis, Laubuca metallicus, Parambassis siamensis , Trichogaster trichopterus , T. pectoralis  and several other species of Rasbora. It should be noted that while these may be compatible with R. myersi in some cases they are not with one another.

It's a schooling species by nature and ideally should be kept in a group of at least 6 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 some interesting behaviour as they compete with one other for female attention.

Mature females are noticeably rounder-bellied and often a little larger than males.

We're not sure if it has been bred in the hobby although it should certainly be possible. Like most cyprinids this species is an egg-scattering, continuous spawner that exhibits no parental care. That is to say 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 fry may start to appear without human intervention.

However if you want to increase the yield of fry a slightly more controlled approach is required and we suggest starting with an approach that has proven successful for similar members of the genus. The adult group can still be conditioned together but one or more long, shallow, say 30" x 12" x 12"/75cm x 30cm x 30cm/71 litre containers should also be set up and half-filled with water. These should be very dimly lit and the base covered with some kind of mesh of a large enough grade so that the eggs can fall through it 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 for other species. The water itself should be of slightly acidic to neutral pH with a temperature towards the upper end of the range suggested above. An internal power filter can be added initially and this should be positioned so that the flow is directed down the full length of the tank.

When the adult fish are well-conditioned and the females appear full of eggs one or two pairs should then be introduced to each container. Spawning can be initiated by adding small amounts of cool water every few hours in such a way that the tank is gradually topped up and feeding small amounts of live and frozen foods. Several spawning events will probably occur before a female is spent of eggs.

The adults will likely eat any eggs they find and are best removed after a couple of days at which point the power filter should be switched for a mature sponge-type unit in order to avoid fry being sucked into the mechanism. Incubation in Rasbora eggs is temperature-dependant to an extent but usually takes between 18 and 48 hours with the young free-swimming 24 to 48 hours later. Initial food should be Paramecium or similar introducing Artemia nauplii and/or microworm once the fry are large enough to accept them.

This species is hardly ever imported in numbers and is most often found as a contaminant among other species such as R. caudimaculata . In his 1996 field guide to fish species of the Cambodian Mekong Walter Rainboth noted that there appear to be two different fish that closely resemble Brittan's original description of R. myersi (now considered a synonym of dusonensis. The first of these is slim and elongate and was "usually found in pools and backwaters of large rivers" whereas the second has a deeper body shape, dark outlining to the body scales and occurs in clear forest streams.

Rainboth's 'Fishes of the Cambodian Mekong' characterised members of Rasbora by possession of an unbranched, non-spiny first dorsal fin ray and 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 and a lack of barbels. It's long been recognised as a polyphyletic lineage as noted by Kottelat (1999) amongst others, and in 2009 the results of a phylogenetic analysis by T. Y. Liao et al. suggested a number of changes in order to improve the taxonomy. The authors found species of rasborin genera to actually represent a monophyletic grouping existing in six clades and erected four new genera (all containing former members of Rasbora) in order to preserve monophyly of the existing groups i.e. Boraras, Horadandia, Rasbora, Rasboroides and Trigonostigma.

According to the authors the first two clades are monotypic; R. brittani should now be referred to as Kottelatia brittani  and R. dorsiocellata as Brevibora dorsiocellata . The third clade comprises Boraras brigittae , Horadandia atukorali , Rasboroides vaterifloris , Trigonostigma heteromorpha  and three species previously included in Rasbora but also moved into new genera; Trigonopoma gracile , T. pauciperforatum  and Rasbosoma spilocerca . 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.

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 , R. kobonensis, R. ornata and R. cf. daniconius. Clade six, meanwhile, 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, similar fish), R. caudimaculata  and R. trilineata . As this final clade contains the type species (see below) its members retain the generic name Rasbora as do clade five species because they don't differ sufficiently to warrant a the erection of a new genus/genera.

Unfortunately many species weren't included in the analysis, meaning inevitable questions are raised regarding the correct placement of the 40 or so other Rasboras, in particular. As the genus had previously been split into various 'species groups' (groups of closely-related species) 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 with further phylogenetic studies:

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, Rasbora 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 not Rasboras) and R. zanzibarensis (identity in question).

The identity of the type species, often given as R. rasbora  in the past, is no longer in question; when Bleeker first referred to the name Rasbora in 1859 only four nominal members were included of which R. cephalotaenia  (known as Leuciscus cephalotaenia at the time) should be considered the type. Howes (1980) suggested the separation of a number of species into the new genus Parluciosoma with type species P. (Rasbora) argyrotaenia but the monophyly of that grouping was not recovered by Liao et al.

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. Kottelat, M. 1999 - Raffles Bull. Zool. 47(2): 591-600.
   Nomenclature of the genera Barbodes, Cyclocheilichthys, Rasbora and Chonerhinos (Teleostei: Cyprinidae and Tetraodontidae), with comments on the definition of the first reviser.
3. www.fishbase.org
4. Rainboth, W.J. 1996 - FAO, Rome, 265 p.
   Fishes of the Cambodian Mekong. FAO Species Identification Field Guide for Fishery Purposes.
5. Saowakoon, H., Saowakoon, S., Padoongpoj, A. and Jindapol, K. 2005 - Proceedings of 7th Technical Symposium on Mekong Fisheries: 225-235
   Surveys of native freshwater fishes in Surin province, Thailand.