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Ricefish: fish for tropical and coldwater nano aquaria
29 July 2011
On a recent trip to Maidenhead Aquatics at Harlestone Heath were came across the lovely little Javanese Ricefish Oryzias javanicus, and in the last year or so several shops have started trading the spectacular Sulawesi Ricefish Oryzias woworae.

So after decades of being virtually absent from the tropical fish trade, it looks like ricefish are back in style again! That's good news, because ricefish have much to recommend them. They're small fish and generally easy to keep and breed. Several species favour hard water over soft, making them ideal choices for aquarists living in places where their local tap water doesn't suit similar small fish such as tetras. Some will even do well in coldwater tanks, making them infinitely better choices for small, unheated aquaria than goldfish.

Taxonomy

Ricefish are members of the genus Oryzias that is part of a small family of fish known as the Adrianichthyidae. Ricefish are closely related to halfbeaks and needlefishes despite their superficial similarity to killifish. Twenty-nine species were recognised as of 2010, all of them small, schooling fish with transparent, somewhat laterally-compressed bodies and usually exhibiting some sort of sexual dimorphism in terms of colours and fin shape. All Oryzias are small fish, typically reaching adult lengths of around 2-3 cm.

Distribution

Of the 29 known species in the family Adrianichthyidae, no fewer than 13 are endemic to Sulawesi while a further 12 have been described from China. The remaining species are dotted about across an area south to Indonesia and as far north as Korea and Japan. Most species are found in freshwater streams, marshes, paddy fields and other sluggish, shallow bodies of water. A few species are notably euryhaline and can be found in freshwater, brackish water, and fully marine environments; Oryzias latipes for example can be found in tide pools.

Basic care

On the whole ricefish have proven to be excellent aquarium residents. In fact Oryzias latipes is a popular lab animal because of the ease with which it can be maintained and bred. It has been widely used for all sorts of scientific studies, and is notable for being the first vertebrate to be bred in space, having been carried aboard the space shuttle Columbia in 1994.

One reason ricefish have been popular for these sorts of experiments is that they don't need much space to do well: a group can be maintained without problems in as little as 30 litres/7 gallons. Since ricefish come from slow-moving to still water habitats, avoid strong water currents. These fish appreciate plants, and for breeding purposes, floating plants like Indian fern, or fluffy plants like Java moss, are absolutely essential.

Temperature

Some ricefish are strictly tropical fish and will need to be kept in a heated aquarium, but the subtropical species are remarkably adaptable and can do well in unheated tanks. Oryzias latipes thrives at 18 C/64 F, and as such should do well in an unheated tank kept in a centrally-heated home. Given their small size and lively behaviour, such fish would make outstanding choices for small coldwater systems.

Water chemistry

Ricefish tend to be quite adaptable with regard to water chemistry. Some species are easier to maintain in hard or even slightly brackish water, but this is by no means the rule.

Diet

Ricefish feed primarily on zooplankton and very small insect larvae. Under aquarium conditions they are quite adaptable, and accept all the usual small live foods as well as wet-frozen foods such as glassworms and fortified brine shrimp. Flake and micro-pellet foods are usually taken too, and even adults enjoy finely powdered baby fish food.

Social behaviour and tankmates

All Oryzias are small, schooling fish that need to be kept in groups of at least six specimens and away from large, territorial or predatory tankmates. Ideal tankmates would include things like Cherry Shrimps, dwarf Corydoras species like Corydoras hastatus, very small tetras such as Neons, and very peaceful bottom-feeders like Kuhli Loaches and Whiptail Catfish.

Breeding

Ricefish have a very peculiar mode of reproduction. Fertilisation may be internal or external, but either way, the female carries the fertilised eggs around in a grape-like bunch attached to her vent. Clutch sizes are small, often as small as a dozen or so. The female swims through feathery-leaved plants where these eggs are rubbed off.

The eggs hatch in around 5 to 12 days depending on the water temperature, and the fry that emerge are comparatively large and well-developed, and can be easily reared on liquid fry food, finely powdered bay fish flake food, and newly-hatched brine shrimp. Growth is quite rapid under favourable conditions, Oryzias latipes reaching maturity within as little as two months.

Common Ricefish or Medaka Oryzias latipes

This species is the one usually seen in laboratories but for whatever reason isn't much seen in the aquarium trade. That's a shame, because the Medaka is a hardy, likeable fish. The standard sort is silvery-green but a golden-yellow form has been around for decades. More recently transgenic forms have been seen in labs, including fluorescent forms.

Sexual dimorphism is slight, both sexes getting to about 4 cm in length and exhibiting similar colouration. However, like many other ricefish species, males have long, ragged anal fins that set them apart from the females when the two are compared. The Medaka is essentially a subtropical fish best kept in unheated aquaria. It is hardy when maintained in hard or brackish water conditions.

Mekong Ricefish Oryzias mekongensis

The Mekong Ricefish was described in 1986 from the Mekong river system in Thailand and is characterised by its unusually small size, often little more than 1.5 cm, and the clear sexual dimorphism apparent on mature fish. Whereas females are transparent greenish-grey, males have bright orange edges on the top and bottom of the tail fin. This species is sometimes said to be a bit more delicate than other ricefish species.

Javanese Ricefish Oryzias melastigma

This ricefish is notable for its silvery-green colouration and brilliant, shiny-blue eyes. Males and females look quite similar, but the males have very much larger anal fins with distinctly ragged edges. Maximum length is about 2.5 cm.

Oryzias melastigma comes from coastal streams and mangroves, so like the Medaka it does best kept in hard or slightly brackish water. It is not fussy about water temperature and can be kept in an unheated subtropical tank or a low-end tropical aquarium without problems; 18-24 C/64-75 F is recommended.

Daisy or Sulawesi Ricefish Oryzias woworae

This species was only described in 2010 but has quickly become popular among discerning aquarists. Males are metallic blue, particularly on the upper half of the body, and have a reddish patch on the throat and chest. They also have red markings on the pectoral and tail fins. Females are rather plain. The Sulawesi Ricefish is native to the island of Sulawesi where it can be found in freshwater streams running through the rainforest. It was found living alongside Nomorhamphus species halfbeaks, but it should be borne in mind that the larger halfbeak species sometimes view very small fish as food, so mixing them under aquarium conditions is unwise.

In any case, Oryzias woworae makes an excellent aquarium fish and is now fairly regularly exported from Sulawesi. Water chemistry is not critical provided extremes are avoided, though moderately soft to slightly hard, slightly acidic to neutral water is probably best. Since they're a tropical species, this species should be kept at around 25 C/77 F. Otherwise their general care appears to match that of the other ricefish already discussed.
 
 
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Ricefish: fish for tropical and coldwater nano aquaria
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Species: Oryzias mekongensis

Ricefish: fish for tropical and coldwater nano aquaria
Picture Details:
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Species: Oryzias melastigma

 

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