Oryzias minutillus SMITH, 1945

Dwarf Medaka

Etymology

Oryzias: from the Greek ὄρυζα (oryza), meaning ‘rice’, in reference to the tendency of some members of the genus to inhabit rice paddy fields.

minutillus: from the Latin minutus, meaning ‘small’.

Classification

Order: Beloniformes Family: Adrianichthyidae

Distribution

The type series was collected from ‘a small canal in Bangkok, Central Thailand’, and it’s subsequently been recorded throughout the country in the Chao Phraya and Mekong river drainages, on the island of Phuket, plus the lower Mekong in Cambodia and parts of the Salween river system in northern Thailand, Myanmar and Yunnan province, southern China.

Some of these records may pertain to other species, however.

Roberts (1998) speculated it should also occur in the middle section of the Mekong in Laos and also proposed the Yunnan collections to represent a distinct, undescribed species but we’ve been unable to obtain confirmation of the former while Parenti (2008) considers the latter to be conspecific with Thai populations.

However, in the description of O. songkhramensis (Magtoon 2010) the distribution map depicts it to occur only in the middle-to-lower Chao Phraya basin, plus Peninsular Thailand, suggesting it may be restricted to eastern and southern parts of Thailand.

Habitat

An exclusive inhabitant of fresh, normally standing, waters including shallow pools, ditches, rice paddies and clear water swamps.

Aquatic vegetation often proliferates such habitats with other fishes typically including Dermogenys, Rasbora, Trichopodus, Trichopsis and Channa spp..

Maximum Standard Length

10 – 17 mm.

Aquarium SizeTop ↑

An aquarium with base dimensions of 40 ∗ 20 cm or more is recommended for a group.

Maintenance

Best maintained in a heavily planted set-up, ideally with a dark substrate, patches of dense vegetation, and some open areas.

Other décor can consist of twisted roots and pieces of bogwood, while surface vegetation is also appreciated by the fish.

When maintained under such conditions they’re more likely to display their best colours, and planted aquaria also offer fry a more favourable chance of survival alongside the adults.

Water Conditions

Temperature: 23 – 32 °C

pH: 6.0 – 7.5

Hardness: 36 – 268 ppm

Diet

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 be offered daily meals of small live and frozen fare such as Daphnia, Artemia, chopped bloodworm, etc., along with good quality, suitably-sized flakes and granules.

Behaviour and CompatibilityTop ↑

Generally peaceful but does not make an ideal addition to many communities due to its small size.

Should you wish to maintain it alongside other fishes diminutive species enjoying similar conditions such as Boraras, Trigonostigma, Danionella and some Danio spp. constitute the best options, while freshwater shrimp of the genera Caridina and Neocaridina are also suitable.

It may also do well alongside smaller anabantoids such as Sphaerichthys, Parosphromenus or the more diminutive Betta species.

If the intention is breeding then obviously it should ideally be maintained alone, and we don’t recommend keeping it with other Oryzias spp. due to the potential of hybridisation, already proven in laboratory experiments.

It’s mostly non-aggressive towards conspecifics, and tends to look most effective and behave more confidently in a group of 8 or more.

Sexual Dimorphism

In males the distal margin of the anal-fin is convex, the genital papilla forms a short tube, and the anterior dorsal and anal-fin rays are extended.

In females the anal-fin margin is slightly concave,  genital papilla bilobed, and pelvic-fin rays extended.

Reproduction

Quite easy to breed and fairly prolific, with females capable of producing batches of eggs every few days or even on a daily basis when in good condition.

Spawning normally occurs in the early morning, with males darkening in colouration and defending small, temporary territories against one another while attempting to entice females.

The adhesive eggs are typically expelled as a single mass and fertilised simultaneously, after which they continue to hang from the genital pore of the female for a period before eventually being deposited singly or in small clumps among vegetation or other suitable media.

Fine-leaved plants such as Cabomba, Ceratophyllum or Taxiphylum spp. are ideal, but synthetic spawning mops or other artificial alternatives also work.

The incubation period is temperature dependant to an extent but typically 1-3 weeks, and while the adults tend to ignore the eggs they do predate free-swimming fry, though if the tank is densely-planted some will usually survive.

Alternatively the eggs or fry can be removed to a separate rearing container filled with water from the adults tank. Once free-swimming the fry are able to accept microworm, Artemia nauplii, etc.

Caution is recommended when housing juveniles of different ages together as the older will predate on the younger if there is a large enough discrepancy in size.

NotesTop ↑

O. minutillus is traded occasionally and is also known by the vernacular name ‘dwarf ricefish’.

Within the genus it appears to be most closely-related to the other miniature species O. pectoralis, O. setnai and O. uwai, sharing with the latter pair elongated anterior anal-fin rays which are set off from the rest of the fin, a medial extension to the ethmoid cartilage and only 7 principle caudal-fin rays.

All four species possess a pigmented anal or urogenital region and a rounded caudal-fin.

It can be distinguished from all congeners by the following combination of characters: small adult size; caudal-fin rounded; principal caudal-fin rays i,3/4,i; 5-7 dorsal-fin rays; 17-21 anal-fin rays with the last unbranched, and no bony contact organs; 4-5 pectoral-fin rays; large teeth on premaxillary and dentary absent; pigmented anal or urogenital region.

Members of the family Adrianichthyidae are often referred to collectively as ‘ricefishes’ and were traditionally considered to be members of the family Cyprinodontiformes and thus closely-related to toothcarps.

This misconception is sometimes still upheld despite the fact that Rosen and Parenti reclassified them within the cyprinodontiform sister group Beloniformes as long ago as 1981.

The best-known member of the family is the medaka or Japanese ricefish, Oryzias latipes, which has been widely used as a model organism in genomic and experimental biology for well over a century and was the first vertebrate animal to mate in space during the mid-1990s.

There are currently just two genera included in the family, Oryzias and Adrianichthys, with the historically-recognised groupings Xenopoecilus and Horaichthys having been synonymised with Oryzias by Parenti (2008).

Of the three species previously included in the paraphyletic Xenopoecilius, X. oophorus and X. poptae were moved into Adrianichthys with the third, X. sarasinorum currently recognised as Oryzias sarasinorum.

In addition the formerly monotypic Indian species Horaichthys setnai is currently classified as O. setnai.

References

1. Smith, H. M., 1945 - Bulletin of the United States National Museum 188: 1-622
   The fresh-water fishes of Siam, or Thailand.
2. Herder, F. and S. Chapuis, 2010 - The Raffles Bulletin of Zoology 58(2): 269-280
   Oryzias hadiatyae, a new species of ricefish (Atherinomorpha: Belonifornes: Adrianichthyidae) endemic to Lake Masapi, Central Sulawesi, Indonesia.
3. Magtoon, W., 2010 - Tropical Natural History 10(1): 107-129
   Oryzias songkhramensis, a new species of ricefish (Beloniformes; Adrianichthyidae) from northeast Thailand and central Laos.
4. Magtoon, W. and A. Termvidchakorn, 2009 - The Natural History Journal of Chulalongkorn University 9(1): 35-68
   A Revised Taxonomic Account of Ricefish Oryzias (Beloniformes; Adrianichthyidae), in Thailand, Indonesia and Japan.
5. Parenti, L. R., 2008 - Zoological Journal of the Linnean Society 154(3): 494-610
   A phylogenetic analysis and taxonomic revision of ricefishes, Oryzias and relatives (Beloniformes, Adrianichthyidae).
6. Parenti, L. R. and B. Soeroto, 2004 - Ichthyological Research 51(1): 10-19
   Adrianichthys roseni and Oryzias nebulosus, two new ricefishes (Atherinomorpha: Beloniformes: Adrianichthyidae) from Lake Poso, Sulawesi, Indonesia.
7. Parenti, L. R. and R. K. Hadiaty, 2010 - Copeia 2010 (2): 268-273
   A new, remarkably colorful, small ricefish of the genus Oryzias (Beloniformes, Adrianichthyidae) from Sulawesi, Indonesia.
8. Roberts, T. R., 1998 - Ichthyological Research 45(3): 213-224
   Systematic observations on tropical Asian medakas or ricefishes of the genus Oryzias, with descriptions of four new species.
9. Rosen, D. E. and L. R. Parenti, 1981 - American Museum Novitates 2719: 1–25
   Relationships of Oryzias, and the groups of atherinomorph fishes.
10. Takehana, Y., K. Naruse K and M. Sakaizumi, 2005 - Molecular Phylogenetics and Evolution 36(2): 417–428
    Molecular phylogeny of the medaka fishes genus Oryzias (Beloniformes: Adrianichthyidae) based on nuclear and mitochondrial DNA sequences.
11. Uwa, H. and L. Parenti, 1988 - Japanese Journal of Ichthyology 35(2): 159-166
    Morphometric and meristic variation in ricefishes, genus Oryzias: a comparison with cytogenetic data.
12. Uwa, H. and W. Magtoon, 1986 - Copeia 1986 (2): 473-478
    Description and karyotype of a new ricefish, Oryzias mekongensis from Thailand.