Boraras micros

Cyprinidae

Mostly restricted to parts of the Mekong basin in northeastern Thailand where it has been collected in the provinces of Nong Khai, Udon Thani and Sakon Nakhon. It is also said to occur in Laos but we have been unable to obtain any definitive information. One interesting collection locality is an area of wetland called Kud Ting in Nong Khai where it lives sympatrically with Rasbora rubrodorsalis  and R. spilocerca . In 2008 the Thai government proposed that Kud Ting be designated a protected Ramsar site due to the diversity of species found there.

Inhabits shallow bodies of water such as swamps, marshes, floodplains and rice paddies. Submerged vegetation tends to grow thickly in these predominantly clear waters and it is also known to move into temporarily inundated areas during the wet season.

Taking Bung Khong Long, a lake in Nong Khai, as an example for those wishing to attempt a biotope set-up submerged aquatic plant species in the lake and surrounding floodplain include Hydrilla verticillata, Cerotophyllum dermersum, Chara zeylanica and Blyxa echinosperma, Salvinia cucullata is found floating on the surface and emergent plants comprise Eleocharis dulcis, Nelumbo nucifera, Lepironia articulata and Nymphaea lotus. Other fish species are Brachygobius sp., Betta splendens , Betta smaragdina , Clupeichthys aesamensis, Odontobutis aurarmus, Krytopeterus bicirrhis, Clarias batrachus , Rasbora spilocerca and Tetraodon leiurus although it should be noted that not all of these can be kept together in an aquarium and some are likely to be unavailable in the trade.

Tiny at around 0.53"/1.3cm.

Though small it still needs space to swim and dominant males will form temporary territories when spawning so a group is best kept in a tank measuring at least 18" x 10" x 10"/45cm x 25cm x 25cm/29.5 litres.

Best kept in a densely-planted tank and is an excellent choice for the carefully-aquascaped set-up. The addition of some floating plants and driftwood roots or branches to diffuse the light entering the tank 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 and may struggle if there is a fast current. Do not add this fish to a biologically immature tank as it can be susceptible to swings in water chemistry.

75 - 79°F/24 - 26°C

6.0 - 7.0

2 - 10°H

As with other Boraras species it is likely to be a micropredator feeding on small insects, worms, crustaceans and other zooplankton in nature. 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 along with good quality flakes and granules will result in the best colouration and encourage the fish to come into breeding condition.

This species is very peaceful but does not make an ideal community fish due to its small size and rather timid nature. It will do best when maintained alone or with other diminutive species such as Microdevario, Sundadanio, Danionella, Eirmotus, Trigonostigma, pygmy Corydoras and small Loricariids such as Otocinclus. It also makes an ideal companion for shy anabantoids such as Sphaerichthys, Parosphromenus or the more diminutive Betta species and in a planted set-up can be housed alongside freshwater shrimp of the genera Caridina and Neocaridina. We don't recommend keeping it with other Boraras as hybridisation might occur.

It's a shoaling species by nature and really should be kept in a group of at least 8-10 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 many small cyprinids this species is an egg-scattering, continuous spawner that exhibits zero parental care. That is to say when in good condition and in the presence of both males and females relatively small numbers of eggs will be laid daily. In a well-furnished, mature aquarium it is therefore 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 trying a method that has proved successful for other species in the genus. The adult group can still be conditioned together but one or more separate, say 12" x 8" x 8"/30cm x 20cm x 20cm/12.5 litre containers should also be set up. These 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 be of pH 6.0-7.0, 5-10°H with a temperature towards the upper end of the range suggested above. A decent-sized clump of Java moss or other fine-leaved plant should also be added filling perhaps half the available space. Filtration is not really necessary but you can use a small, air-powered sponge filter if you prefer.

Two or three pairs of well-conditioned adult fish should then be introduced to each container. It is wise to make the transfer slowly in order to avoid excessive levels of stress but if conditions are to their liking they should begin to spawn the following morning. While this species certainly will eat its eggs it appears not to actively hunt for them as is the case with many small Puntius and other related species. Once spawning has commenced it should continue on a daily basis.

The pair(s) should be left in situ for no more than a couple of days before being removed as the first eggs should hatch by the second day after the initial spawning. The tiny young will survive on their yolk sacs for another 24 hours or so after which they will require Paramecium or other microscopic food. After a week to ten days they should be large enough to accept Artemia nauplii/microworm etc. As the days pass additional fry should start to appear from later spawning events. It's best to wait a week or two before starting to perform small water changes in order to avoid unduly shocking the young fish.

This species is poorly-documented and rarely seen in the trade with many of the available images depicting one of its more colourful congeners. It is actually difficult to confuse with any of its close relatives as it is a rather plainly-patterned fish and this may be partly the reason why it is not more widely-available to hobbyists. Fish seen on sale as Boraras micros var. 'red' or 'strawberry rasbora' are always B. maculatus , B. brigittae  or the undescribed B. sp. 'Southern Thailand'.

Boraras was erected in 1993 in order to separate a small band of species from the larger Rasbora grouping on the basis of differences in morphology and reproductive strategy. In older literature they are therefore referred to as members of Rasbora and following Liao et al. (2009) the genus is a member of the rasborin sub-group within the subfamily Danioninae (the other sub-group contains the danionins). This group is further subdivided into six clades (clusters of closely-related species) of which Boraras (or at least B. brigittae ; the only species involved in the study) is included in "clade three" alongside Horadandia atukorali , Rasboroides vaterifloris , Trigonostigma heteromorpha  and three species previously included in Rasbora but moved into new genera by the authors; Trigonopoma gracile , T. pauciperforatum  and Rasbosoma spilocerca  (note: since the new names have yet to find general acceptance the latter are all still listed within Rasbora here on Seriously Fish). Of these it was found to form a monophyletic group with Rasboroides meaning these two derived from the same common ancestor. 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 small adult size in Boraras species evolved via a process known as miniaturisation, characterised by sexually mature adults with a significantly reduced size of less than 20mm 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; 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".

Boraras species from the Malay Archipelago (B. brigittae , B. maculatus  and B. merah ) tend to have a slimmer, more elongate body profile than those from Indochina (Boraras micros, B. urophthalmoides, B. sp. 'South Thailand') which may assist when attempting to identify them.

Literature cited

1. Conway, K. W. 2005 - Ichthyol. Explor. Freshwaters, Vol. 16, No. 3, pp. 249-264.
   Monophyly of the genus Boraras (Teleostei: Cyprinidae).
2. 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.
3. www.siamensis.org
4. www.petfrd.com
5. www.fishbase.org
6. Liao, T. Y., Kullander, S. O. and F. Fang. 2009 - Zoologica Scripta Early View (Articles online in advance of print).
   Phylogenetic analysis of the genus Rasbora (Teleostei: Cyprinidae).