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Microrasbora rubescens ANNANDALE, 1918

Red Dwarf 'Rasbora'

Etymology

MicrorasboraMicrodevario: from the Ancient Greek μικρός (mikrós), meaning ‘small’, and the generic name Rasbora, in allusion to the small size of this genus and its superficial similarity to the Rasbora grouping.

rubescens: from the Latin rubescens, meaning ‘reddish’, in allusion to the orange-scarlet flanks, ventral surface of head, caudal, anal and sometimes dorsal fins.

Classification

Order: Cypriniformes Family: Cyprinidae

Distribution

Endemic to the isolated mountain lake of Inle and surrounding watershed in Shan State, eastern Myanmar.

Type locality is ‘Shan States, Lake Inle and Heca plains, Myanmar’.

Habitat

Lake Inle 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 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 are 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

25  – 30 mm.

Aquarium SizeTop ↑

Base dimensions of 60 ∗ 30 cm or equivalent are recommended.

Maintenance

Best maintained in a densely-planted tank and is an excellent choice for the carefully-aquascaped set-up.

Filtration should be relatively gentle, while the addition of some floating plants and driftwood roots or branches to diffuse the light entering the tank also seems to be appreciated.

Water Conditions

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.

Diet

Likely to feed on small invertebrates, algae and other zooplankton in nature.

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.

A community based around fishes from Lake Inle would make an interesting project with species available in the trade including Sawbwa resplendens, Celestichthys erythromicron, Inlecypris auropurpureus, Pethia stoliczkana, Parambassis lala, and Petruichthys brevis.

The closely-related Microrasbora cf. rubescens is not recommended as these two may be capable of hybridisation.

It is 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 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.

Sexual Dimorphism

Mature females are usually deeper-bodied and noticeably larger than the more compact, intensely-coloured males.

Reproduction

Like many small cyprinids this species scatters eggs randomly, typically among aquatic vegetation, and does not exhibit parental care. If the fish are in good condition they will spawn often and in a mature planted aquarium it is possible that small numbers of fry may start to appear without intervention.

However if you want to increase the yield a slightly more controlled approach is required. The adult group can still be conditioned together but one or more smaller 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 a small air-powered sponge filter can be installed if you prefer.

When the adults are suitably-conditioned a single pair or group comprising several males and females is then introduced to each container; the more individuals involved the greater probability of egg predation. Males may also distract one another from females if several are present

Spawning normally presents few problems with around 30 mildly adhesive eggs deposited in a typical event. The adults are best removed post-spawning as they will eat any eggs or fry they find. Incubation is temperature-dependent to an extent but normally around 72-96 hours with the young free-swimming 1-2 days later. Initial food should be 5-50 micron grade, introducing Artemia nauplii, microworm, etc., once the fry are large enough to accept them.

NotesTop ↑

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 Celestichthys 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’, M. sp. ‘rose blue line’, ‘Asian Cardinal Rasbora’, or ‘flame red rasbora’.

Devario sondhii can also appear comparable at first glance but orange pigmentation 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. 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 evolutionary pathway leading to small adult size in Microrasbora has been referred to 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 across numerous genera. Most show a preference for still or slow-moving, often nutrient-poor, habitats such as forest peat swamps.

References

  1. Annandale, N., 1918 - Records of the Indian Museum (Calcutta) 14: 33-64
    Fish and fisheries of the Inlé Lake.
  2. 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).
  3. Kottelat, M., 2013 - Raffles Bulletin of Zoology Supplement 27: 1-663
    The fishes of the inland waters of southeast Asia: a catalogue and core bibiography of the fishes known to occur in freshwaters, mangroves and estuaries.
  4. Liao, T. Y., Kullander, S. O. and F. Fang, 2009 - Zoologica Scripta 39(2): 155-176
    Phylogenetic analysis of the genus Rasbora (Teleostei: Cyprinidae).
  5. 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.

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