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Hemimyzon formosanus (BOULENGER, 1894)

SynonymsTop ↑

Homaloptera formosana Boulenger, 1894; Hemimyzon formosanum (Boulenger, 1894); Homaloptera taiwanica Kishinouye, 1905


Hemimyzon: from the Greek hemisus, meaning ‘one half’, and Greek myzo, meaning ‘to suckle’.

formosanus: ‘native to Formosa’, i.e., Taiwan.


Order: Cypriniformes Family: Balitoridae


Endemic to Taiwan where it’s found most abundant in river systems draining the western slopes of the island’s Central Mountain Range, plus a few northern drainages.

Type locality is ‘Central Taiwan’.


Restricted to shallow, fast-flowing, highly-oxygenated headwaters and minor tributaries characterised by stretches of riffles and runs broken up by pools or cascades in some cases.

Substrates are normally composed of smaller rocks, sand and gravel with jumbles of boulders, and while riparian vegetation and patches of submerged leaf litter are common features aquatic plants aren’t usually present.

The most favourable habitats contain clear, oxygen-saturated water which, allied with the sun, facilitates the development of a rich biofilm carpeting submerged surfaces.

During periods of high rainfall some streams may be temporarily turbid due to suspended material dislodged by increased (sometimes torrential) flow rate and water depth.

Other species inhabiting such environments in Taiwan include Candidia barbata, Formosania lacustre, Sinogastromyzon puliensis and various Rhinogobius species including R. candidianus and R. gigas.

Maximum Standard Length

90 – 100 mm.

Aquarium SizeTop ↑

An aquarium with base dimensions of at least 120 ∗ 30 cm or equivalent is recommended.


Most importantly the water must be clean and well-oxygenated so we suggest the use of an over-sized filter as a minimum requirement.

Turnover should ideally be 10-15 times per hour so additional powerheads, airstones, etc. should also be employed as necessary.

Base substrate can either be of gravelsand or a mixture of both to which should be added a layer of water-worn rocks and pebbles of varying sizes.

Aged driftwood can also be used but avoid new pieces since these usually leach tannins that discolour the water and reduce the effectiveness of artificial lighting, an unwanted side-effect since the latter should be strong to promote the growth of algae and associated microorganisms.

Exposed filter sponges  will also be grazed, and some enthusiasts maintain an open filter in the tank specifically to provide an additional food source.

Although rarely a feature of the natural habitat aquatic plants can be used with adaptable genera such as MicrosorumCrinum and Anubias spp. likely to fare best. The latter are particularly useful as their leaves tend to attract algal growth and provide additional cover.

Since it needs stable water conditions and feeds on biofilm this species should never be added to a biologically immature set-up, and a tightly-fitting cover is necessary since it can literally climb glass.

While regular partial water changes are essential aufwuchs can be allowed to grow on all surfaces except perhaps the viewing pane.

Water Conditions

Temperature: For general care 16 – 22 °C is recommended but it can withstand warmer conditions provided dissolved oxygen levels are maintained.

pH6.5 – 8.0

Hardness36 – 268 ppm


Much of the natural diet is likely to be composed of benthic algae plus associated micro-organisms which are rasped from solid surfaces.

In captivity it will accept good-quality dried foods and meatier items like live or frozen bloodworm but may suffer internal problems if the diet contains excessive protein.

Home-made foods using a mixture of natural ingredients bound with gelatin are very useful since they can be tailored to contain a high proportion of fresh vegetables, Spirulina and similar ingredients.

For long-term success it’s best to provide a mature aquarium with a plentiful supply of algae-covered rocks and other surfaces.

If unable to grow sufficient algae in the main tank or you have a community containing numerous herbivorous fishes which consume what’s available quickly it may be necessary to maintain a separate tank in which to grow algae on rocks and switch them with those in the main tank on a cyclical basis.

Such a ‘nursery‘ doesn’t have to be very large, requires only strong lighting and in sunny climates can be kept outdoors. Algal type is also important with diatoms and softer, green varieties preferred to tougher types such as rhodophytic ‘black brush’ algae.

Balitorids are often seen on sale in an emaciated state which can be difficult to correct. A good dealer will have done something about this prior to sale but if you decide to take a chance with severely weakened specimens they’ll initially require a constant source of suitable foods in the absence of competitors if they’re to recover.

Behaviour and CompatibilityTop ↑

Largely peaceful although its environmental requirements limit the choice of suitable tankmates somewhat, so research your choices before purchase in order to be sure.

Species inhabiting similar environments include Barilius, Discherodontus, GarraDevario, some Rasbora, Rhinogobius, Sicyopterus and Stiphodon gobies plus catfishes like Glyptothorax, Akysis and Oreoglanis.

Many loaches from the family Nemacheilidae, Balitoridae and Gastromyzontidae are also suitable although harmless squabbles may occur with the latter group in particular.

Like many loaches it’s loosely territorial towards conspecifics but seems to require their presence to truly thrive. A group of four or more specimens should be the smallest considered.

Sexual Dimorphism

Unconfirmed but as in related genera adult females should be heavier-bodied and a little larger than males.


Presumably a seasonal spawner in nature but nothing has been recorded in aquaria.

NotesTop ↑

Hemimyzon spp. are rare in the aquarium trade and only turn up occasionally, most often among shipments of other wild-caught fishes from their native waters.

There currently (February 2016) exist 14 officially-recognised species, 3 of which are endemic to Taiwan.

Current knowledge suggests that H. formosanus is the most widely-distributed and following Chen and Fang (2009) it can be distinguished by the following suite of characters: 3+7 dorsal-fin rays; 10-12 pectoral-fin rays (modally 11) +9-11 (modally 10) (total 20-22; modally 21); inner, posterior barbels present only as low, rounded projections; distance from posterior tip of pelvic-fin to anus about equal to that from anus to anal-fin origin.

The authors also proposed that among the Taiwanese species it’s likely to be the closest related to the ancestral form because its distribution lies closest to mainland China, and that speciation in Taiwan may have occurred via river piracy allowing colonisation from west to east.

Hemimyzon can be told apart from most related genera since the pelvic fins do not form a sucking disc, although they may be fused at the posterior base. It’s most similar to Jinshaia but has an emarginate to slightly-forked caudal-fin vs. deeply-forked in Jinshaia.

The family Balitoridae as recognised by Kottelat (2012) is widely-distributed across much of the Indian subcontinent, Southeast Asia and China.

Kottelat (2012) also considers the family Gastromyzontidae valid and many former balitorids are now contained within that group, but Hemimyzon is retained in Balitoridae.


  1. Chen, I-S. and L.-S. Fang, 2009 - Environmental Biology of Fishes 86(1): 185-192
    Hemimyzon sheni, a new species of balitorid fish (Teleostei: Balitoridae) from Taiwan.
  2. Doi, A. and M. Kottelat, 1998 - Ichthyological Research 45(1): 7-11
    Hemimyzon nanensis, a new balitorid fish from the Chao Phraya basin, Thailand.
  3. Kottelat, M., 2012 - Raffles Bulletin of Zoology Supplement 26: 1-199
    Conspectus cobitidum: an inventory of the loaches of the world (Teleostei: Cypriniformes: Cobitoidei).
  4. Tang, Q., H. Liu, R. Mayden and B. Xiong, 2006 - Molecular Phylogenetics and Evolution 39(2): 347-357
    Comparison of evolutionary rates in the mitochondrial DNA cytochrome b gene and control region and their implications for phylogeny of the Cobitoidea (Teleostei: Cypriniformes).
  5. Wang, T-Y., T-Y. Liao, and C-S. Tzeng, 2007 - Zoological Studies 46(5): 547-560
    Phylogeography of the Taiwanese Endemic Hillstream Loaches, Hemimyzon formosanus and H. taitungensis (Cypriniformes: Balitoridae).
  6. Šlechtová, V., J. Bohlen and H. H. Tan, 2007 - Molecular Phylogenetics and Evolution 44(3): 1358-1365
    Families of Cobitoidea (Teleostei; Cypriniformes) as revealed from nuclear genetic data and the position of the mysterious genera Barbucca, Psilorhynchus, Serpenticobitis and Vaillantella.

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