Red Dwarf 'Rasbora'
Endemic to the isolated mountain lake of Inle and surrounding watershed in Shan State, eastern Myanmar. The lake lies in a karstic valley almost 900m above sea level in the Shan Plateau region and is home to many endemic animals including nine species of fish and numerous gastropods.
The water in Lake Inle is clear, shallow (2-3 metres deep in most places) and has a very fertile, loamy substrate. It’s famed for its stilted villages and local fishermen known as Intha who row their boats using only one leg. These people, thought to have migrated from the south of Myanmar in the late 1300s, use naturally-occurring floating ‘islands’ consisting of tangles of various plant species as gardens.
These islands form a wide raft around the lake margins and the Intha take them as required, removing the aerial leaves and cutting them into sections. Bamboo poles are added for support allowing fruit, vegetables, rice and flowers to be produced in commercial quantities. The gardens rise and fall with the water level and have come to form the habitats of many fish which take shelter among the tangle of roots and plant stems at their base.
Macrophytes also grow densely in the crystal-clear water and include Ceratophyllum and Elodea-like species. M. rubescens is mostly collected around the margins of the lake where grass and reed-like plants proliferate and the floating islands form thick mats composed of both live and dead vegetation. Impossible to catch with a net, fishermen use special traps placed among the plants overnight.
As the lake is situated in a karstic zone it contains neutral to slightly alkaline water with the pH value varying between 7.5 – 8.0. Its main outlet is a seasonal stream known as the Balu Chaung which floods at certain times of year allowing the transfer of fishes to pools and ponds close to Loi Kaw. During drier months these are disconnected, isolating small populations of several species. We’ve yet to obtain detailed information regarding these habitats but expect them to be characterised by similarly sluggish, clear water with dense marginal/submerged plant growth.
Maximum Standard Length
Fully-grown at just 30mm.
Aquarium SizeTop ↑
Base dimensions of 45 ∗ 30 cm or more are required.
Best kept in a densely-planted aquarium and is an excellent choice for the carefully-aquascaped set-up. The addition of some surface vegetation, floating or otherwise, 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, but it must never be introduced to a biologically immature set-up as it can be susceptible to swings in water chemistry.
Temperature: Temperatures in Lake Inle have been recorded to vary between 20 – 24 °C.
pH: Will tolerate slightly acidic conditions but a value of 7.0 – 8.0 is preferable.
Hardness: Best kept in slightly harder water of 179 – 357 ppm.
Likely to feed on small invertebrates, algae and other zooplankton in nature. In the aquarium it will accept dried foods of a suitable size but should not be fed these exclusively, and as it rarely rises to the surface there’s little point in offering floating products.
Daily meals of small live and frozen fare such as Daphnia, Artemia and suchlike will not only result in the best colouration but encourage the fish to come into breeding condition. Newly-imported specimens are often in poor condition and can be difficult to acclimatise to aquarium life. Small live foods are therefore recommended as an initial diet with dry and frozen products being introduced as the fish become settled.
Behaviour and CompatibilityTop ↑
Likely to be intimidated or outcompeted for food by larger or more boisterous tankmates, although the presence of similarly-sized, surface-dwelling species seems to help reduce its shyness. Other small cyprinids from Myanmar such as Danio choprae, D. erythromicron, D. margaritatus, or members of the genus Microdevario are suitable companions, and we suspect it might also do well alongside Dario hysginon or D. sp. ‘Myanmar’.
A community based around fishes from Lake Inle would make an interesting project with species available in the trade including Sawbwa resplendens, Danio erythromicron, Devario auropurpureus, Microrasbora rubescens, Pethia stoliczkana, Parambassis lala, and Yunnanilus brevis.
It’s a shoaling species by nature and a group of at least 8-10 specimens should be purchased. 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 and some interesting behaviour as they compete with one other for female attention.
Mature females are usually rounder-bellied and noticeably larger than the more intensely-coloured males.
Like many small cyprinids it’s an egg-scattering spawner that does not exhibit parental care. If the fish are in good condition they will spawn often and in a densely-planted, mature aquarium it’s possible that small numbers of fry may start to appear without 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 smaller, perhaps 10-15 litre, containers should also be set up and filled with aged water. Fill much of the available space with fine wool mops, Taxiphyllum or other fine-leaved plant. Neither lighting nor filtration is necessary although you can install a small air-powered sponge filter if you prefer.
When the adults are well-conditioned a single pair or group comprising one or two males and several females can then be introduced to each container, though it’s worth noting that the more individuals involved the greater the risk of egg predation, plus males may distract each other from females if there’s more than one in the tank. Spawning normally presents few problems with around 30 mildly adhesive eggs deposited in a typical event.
At this point the adults are best removed as they will eat any they find, plus females need a recovery period before spawning again as they’re unable to produce eggs on a daily basis. In nature it apparently breeds year-round so you could always select another pair and work a rotation system if continuous production is the aim.
Incubation is temperature-dependant to an extent but usually takes around 72 – 96 hours with the young free-swimming 1-2 days later. Initial food should be Paramecium or a proprietary dry food of sufficiently small (5-50 micron) grade, introducing Artemia nauplii and/or microworm once the fry are large enough to accept them.
This species may be seen on sale under various trade names including ‘red-line dwarf rasbora’ or ‘red dwarf rasbora’. A similar-looking, apparently undescribed congener has been collected from the type locality of Danio margaritatus near the town of Hopong, also in Shan State. It differs from M. rubescens by possessing a greenish to bluish lateral stripe plus a larger adult size, and is traded as Microrasbora cf. rubescens, M. ‘thuzari’, ‘Asian Cardinal Rasbora’, or ‘flame red rasbora’.
Devario sondhii can also appear comparable at first glance (it also occurs sympatrically with M. cf. rubescens in nature) but on close inspection the orange colouration is restricted to the rear, ventral portion of the body and it possesses a dark cleithral spot just behind the gill plate. Devario acuticephala is also broadly similar in patterning but of Indian origin and it possesses a rough-edged, dark stripe extending laterally from the caudal peduncle along the majority of the body.
The genus has undergone a number of systematic changes in recent years but currently contains only M. rubescens and M. microphthalma. A phylogenetic study by Fang et al. published in 2009 revealed that other species formerly included in the grouping are more closely allied with Devario and reclassified them as members of new genus Microdevario, further separated from Microrasbora by differences in some aspects of internal morphology.
M. rubescens was also found to be closely related to Devario and the authors conclude that ‘its status as a valid genus is contentious and the molecular data suggest inclusion or close relationship with Devario‘. The two were not synonymised because, unlike those species moved into Microdevario, M. rubescens does not share any derived morphological characters with Devario and in the tests performed the precise level of relatedness between the two could not be settled upon.
The results for Microdevario species were more consistent, distinguishing them markedly from M. rubescens and demonstrating that they possess enough unique characters to be classified separately from both Microrasbora and Devario. M. microphthalma was not included in the study.
The small adult size 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, Horadandia, Boraras, Sawbwa, Sundadanio, Danio, Laubuca and Rasbora.
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 such as the tooth-like projections in male Danionella dracula. 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’.
- Annandale, N., 1918 - Records of the Indian Museum (Calcutta) 14: 33-64
Fish and fisheries of the Inlé Lake.
- Fang, F., M. Norén, T. Y. Liao, M. Källersjö and S. O. Kullander, 2009 - Zoologica Scripta 38(1): 1-20
Molecular phylogenetic interrelationships of the south Asian cyprinid genera Danio, Devario and Microrasbora (Teleostei, Cyprinidae, Danioninae).
- Liao, T. Y., Kullander, S. O. and F. Fang, 2009 - Zoologica Scripta 39(2): 155-176
Phylogenetic analysis of the genus Rasbora (Teleostei: Cyprinidae).
- Rüber, L. , M. Kottelat, H. H. Tan, P. K. L. Ng and R. Britz, 2007 - BMC Evolutionary Biology London 7: 1-10
Evolution of miniaturization and the phylogenetic position of Paedocypris, comprising the world's smallest vertebrate.