Oryzias javanicus (BLEEKER, 1854)

Javanese Ricefish

SynonymsTop ↑

Aplocheilus javanicus Bleeker, 1854

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.

javanicus: ‘of Java’, in reference to the type locality of this species.

Classification

Order: Beloniformes Family: Adrianichthyidae

Distribution

This species is widely-distributed throughout Peninsular Thailand, Malaysia (Malay Peninsula plus the states of Sabah and Sarawak, Borneo), Singapore and Indonesia, with records from the Riau Archipelago, Sumatra, Java, Borneo, Bali, Lombok, and Sulawesi existing in the latter.

Type locality is ‘Panimbang River, Perdana, Indonesia’.

Habitat

Typically found in coastal, normally brackish, streams and pools including mangrove swamps and forests, but has also been recorded in freshwater habitats and is known to be tolerant of a wide salinity range.

Maximum Standard Length

25 – 35 mm.

Aquarium SizeTop ↑

An aquarium with base dimensions of 45 ∗ 30 cm or more is recommended for a group.

Maintenance

This species has no special requirements in terms of décor though it tends to be less colurful when maintained in a sparsely-decorated set-up.

It arguably looks best in a display inspired by its natural habitat which could consist of a sandy substrate, some water-worn tree roots and branches plus a few handfuls of leaf litter.

Planted aquaria are also suitable and may offer fry a more favourable chance of survival alongside the adults, though the majority of aquarium plant species will fail to thrive under brackish conditions.

Try to find out the origin of your fish as some populations will do best in freshwater but if in doubt the addition of marine salt in the ratio of 1-3 g/L is recommended.

Water Conditions

Temperature: 24 – 29 °C

pH: 7.0 – 9.0 will probably fail to thrive if maintained long term in acidic water.

Hardness: 90 – 447 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 some Dermogenys, Pseudomugi and Brachygobius spp. constitute the best options, while many neritid snails are also suitable.

If the intention is breeding then 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 congeners although rival courting males can be aggressive towards one another, and tends to look most effective and behave more confidently in a group of 8 or more.

Sexual Dimorphism

Adult males have yellow submarginal bands in both caudal-fin lobes, possess elongate, filamentous rays in the dorsal and anal fins, and have a slimmer body shape than females.

The anal-fin in males has a series of bony contact organs plus a convex convex distal margin, and the genital papilla forms a short tube, while in females the anal-fin margin is slightly concave and the genital papilla bilobed.

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.

NotesTop ↑

This species is infrequently seen in the hobby but is exported on occasion and sometimes referred to as the ‘blue-eyed ricefish’.

It has a truncate, rather than lunate or emarginate, caudal-fin which distinguishes it from the congeners O. bonneorum, O. nebulosus, O. nigrimas, O. orthognathus, and O. sarasinorum, and in this respect appears to be a member of a large, unnamed clade or ‘species group’ containing all other members of the genus.

It’s further distinguishable from congeners by the following combination of characters: 10-13 pectoral-fin rays; 18-25 (usually 23) anal-fin rays; anal-fin rays in males with bony contact organs; 6-8 (usually 7) dorsal-fin rays; caudal-fin with yellow submarginal bands in both lobes; premaxillary with enlarged teeth on posterior side; relatively deep body.

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. 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.
2. 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.
3. 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.
4. 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).
5. 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.
6. 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.
7. 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.
8. Rosen, D. E. and L. R. Parenti, 1981 - American Museum Novitates 2719: 1–25
   Relationships of Oryzias, and the groups of atherinomorph fishes.
9. 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.
10. Uwa, H. and L. Parenti, 1998 - Japanese Journal of Ichthyology 35(2): 159-166
    Morphometric and meristic variation in ricefishes, genus Oryzias: a comparison with cytogenetic data.