Microdevario nanus (KOTTELAT & WITTE, 1999)

SynonymsTop ↑

Microrasbora nana Kottelat & Witte, 1999

Etymology

Microdevario: from the Ancient Greek μικρός (mikrós), meaning ‘small’, and the generic name Devario, in allusion to the small size of genus members and their evolutionary relationship with the genus Devario.

nanus: from the Latin nanus, meaning ‘dwarf’, presumably in reference to this species’ small adult size.

Classification

Order: Cypriniformes Family: Cyprinidae

Distribution

Known only from the lower Sittaung and Bago river drainages, both of which are tributaries within the lower Irrawaddy system, and may be endemic to the Irrawaddy Delta region.

Type locality is ‘Sittang river basin: Kyauk Tan Chaung stream, 2.5 km from Daik U (4th and last bridge), 65.5 km on road from Bago’.

Habitat

The Irrawaddy Delta is a very flat, low (mostly 1 – 15 m AMSL), alluvial plain transected by a number of rivers and streams, with much of the central and upper portions converted to rice agriculture. Most waterways are natural, and there exists no extensive system of dredged canals, with the only the manmade Twante canal linking Rangoon city with the western part of the delta.

Water levels vary depending on season with the lowest flow between Februray and March followed by a sharp increase in April and May die to snow melt in the upper part of the basin. There is another steep rise in May-June corresponding with the onset of the annual monsoon. Embankments have been constructed in some areas in order to protect agriculatural land, but in others a complex of temporal and permanent oxbow lakes and marshes remains.

The larger river channels tend to be quite turbid due to suspended silt and M. nanus is likely to inhabit sluggish and standing freshwater environments such as rice paddies, swamps, weedy ponds and ditches with dense submerged vegetation.

Towards the Andaman Sea the lower part of the delta gives way to brackish mangrove swamps. The climate is wet, with annual precipitati0n of 1500 – 2500 mm, up to 90% of which falls between mid May and mid November, and warm with average air temperatures of 22 – 37 °C over the course of the year.

At one locality M. nanus was collected in less then 30cm of water from an oxbow lake among floating Pistia-like vegetation and grasses alongside Badis ruber, Colisa labiosa, Brachygobius nunus, Aplocheilus panchax, unidentified species of Esomus, Oryzias, and Parambassis plus a member of the shrimp genus Caridina.

Maximum Standard Length

14 – 16 mm.

Aquarium SizeTop ↑

An aquarium with base dimensions measuring 45 ∗ 30 cm or equivalent.

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: 20 – 27 °C

pH: 6.0 – 7.5

Hardness: 18 – 215 ppm

Diet

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 mostly be offered small live and frozen fare such as Daphnia, Artemia, etc.

Behaviour and CompatibilityTop ↑

Very peaceful but best maintained alone or with comparably-sized species that enjoy similar conditions.

Other small fishes from Myanmar such as Celestichthys choprae, Brachydanio aesculapii, B. nigrofasciatus, Pethia phutunio, Acanthocobitis zonalternans, Petruichthys, and Akysis spp. make excellent companions, and we suspect it might also do well alongside some species not normally recommended for community aquaria such as Parambassis lala, Dario hysginon, or D. sp. ‘Myanmar’.

It is a schooling species by nature, therefore 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 as they compete with one other for female attention.

Sexual Dimorphism

Mature females tend to be deeper-bodied and noticeably larger than the more compact, intensely-coloured males.

Reproduction

It is unclear whether M. nanus has been bred in aquaria but success has been achieved with its congener M. kubotai. Like many small cyprinids Microdevario spp. scatter their eggs randomly, typically among aquatic vegetation, and do 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.

The adults are best removed after 2-3 days as they will eat any eggs or fry they find. Incubation is temperature-dependent to an extent but normally around 72 hours with the young free-swimming 3-4 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 ↑

M. nanus is sometimes referred to as ‘green dwarf rasbora’ or ‘nana rasbora’ in the ornamental trade.

It is immediately distinguished from congeners by presence of a black blotch at the tip of the dorsal-fin and a greyish marking at the tip of the anal-fin. In addition, it possesses 25-26 + 1-2 scales in the longitudinal series, 11-13 predorsal scales, and 10-11 branched anal-fin rays.

The genus Microdevario was raised by Fang et al. (2009) after phylogenetic analyses revealed that some species formerly included in Microrasbora to be more closely allied with Devario and in need of reclassification. Microdevario is largely separated from Devario and other closely related genera such as Chela and Laubuca by virtue of internal characters but also the following: small adult size; anal and dorsal fins with concave-shaped distal margin; absence of barbels; absence of perforated scales; 9-10 branched anal-fin rays; 7 branched dorsal-fin rays; less abdominal than caudal vertebrae.

The evolutionary pathway leading to small adult size in Microdevario 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. 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. Conway, K. W., W.-J. Chen and R. L. Mayden, 2008 - Zootaxa 1686: 1-28
   The 'Celestial Pearl danio' is a miniature Danio (s.s) (Ostariophysi: Cyprinidae): evidence from morphology and molecules.
3. 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).
4. Jiang, Y.-E., X.-Y. Chen and J.-X. Yang, 2008 - Environmental Biology of Fishes 83(3): 299-304
   Microrasbora Annandale, a new genus record in China, with description of a new species (Teleostei: Cyprinidae).
5. 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.
6. 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, S. He, 2007 - Journal of Experimental Zoology, Molecular Development and Evolution 308B: 642-654
   Phylogenetic relationships of Danio within the order Cypriniformes: a framework for comparative and evolutionary studies of a model species.
7. 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.