The Japanese Ricefish, Oryzias latipes, is an excellent contender for the small unheated aquarium. But sadly this little fish is nowadays extremely difficult to track down within the UK hobby.

A few years ago, Japanese Ricefish could be found in modest numbers in the trade, usually appearing in the shops during the spring and summer months along with various North American natives such as Sunfishes, Rosy Red Minnows, Rainbow Shiners and Redbelly Dace. I suspect that the UK Government's restrictions on imports of exotic coldwater fishes has inhibited exporters from shipping Japanese Ricefishes to the UK, despite the fact that you do not need a licence to keep them.

The Ricefish returns?

Elsewhere in the world the Japanese Ricefish is making a comeback, but for all the wrong reasons (in my view). Along with Zebra Danios, the Ricefish has been the subject of genetic modification experiments. This has culminated in the commercial production of "'transgenic"* fish that glow in the dark. In certain countries it is possible to buy ornamental Zebra Danios and Ricefish that emit a green, red, or yellow glow. The glowing colour is produced by fluorescent protein molecules derived from jellyfish or coral.

The genes that encode for these proteins have been inserted into the fish's own genetic material (its DNA). These "'man-made" glowing fish have caused quite a stir, and generated lots of controversv as to whether we should trade in geneticallv-modified (GM) pets. As a result, some countries have banned their sale whereas others are openly trading them as ornamental novelties.
As far as I am aware, glowing Ricefish (and Zebra Danios) cannot be imported into the UK for the aquarium hobby but some are legitimately brought in for laboratory research into molecular genetics.

*Transgenic: A transgenic animal (or plant) is one that possesses genetic material from another species.

Everyone should keep them!

The reason for this article is to try and foster a renewed interest in keeping Japanese Ricefish (the natural genetically unmolested fish!).
Along with the White Cloud Mountain Minnow, the peaceful little Ricefish is far better suited to small unheated aquariums than is the considerably larger (and messier) Goldfish.
As such, the Ricefish is an ideal beginner's coldwater fish. What's more, it will readily breed in a small aquarium, revealing its fascinating reproductive habit discussed below. (Incidentally, I am not the first to promote Oryzias latipes in this magazine: Dave Page of Corbv AS wrote about this species a few years ago.)

Ricefish reproduction

In their native Japan, the Ricefish reproductive season is between April and October. Under aquarium conditions they will spawn when the water temperature reaches the upper 60's F, provided there are sufficient hours of light per day (short day lengths inhibit spawning).

Sexual differences are fairly obvious in the case of adult fish; the males have slightly elongated rays on the dorsal and anal fins and much slimmer bodies than the females. Fertilisation occurs externally but these fish are apparently also capable of internal fertilisation: this ability to switch between reproductive modes is extremely uncommon among fishes. Females may become heavily swollen with eggs reaching almost bursting proportions similar to that of gravid livebearers. In my experience, females may continue to extrude egg clusters even ifn o males are present.

The female Ricefish does not scatter her eggs but instead carries them as a small cluster which hangs from her genital opening. Each cluster, resembling a tiny bunch of grapes, typically contain between 10 and 20 eggs, some of which may be seen hanging from fine opaque threads.


Egg clusters are more likely be observed in the early morning which is when external fertilisation by the males takes place. A female Ricefish can produce an egg cluster every day over a number of consecutive days, even weeks. Within a few hours after fertilisation, the female will begin to attach her eggs to aquatic vegetation. This she does by slowly brushing her vent over a stem or leaf.
Often several repeated brush strokes are required in order to dislodge one or more eggs.

The eggs are not too difficult to spot within the aquatic vegetation, especially if observed in good light. Floating strands of fine leaved plants are ideal for egg deposition, but other forms of vegetation will also be used, especially if the plant material lies close to the water surface.

The eggs, which are relativelv large and tough, should be removed to rearing facilities. They call either be transferred along with the plant or gently picked off, one by one, using the fingers. Hatching takes about 10 to 14 days, depending on water temperature.
Good water quality is important during the long egg incubation period, but even under hygienic conditions a small proportion of eggs succumb to fungus (perhaps some were infertile to begin with?).
Next Spring (when my Ricefish should commence spawning), I plan to add API's new Pimafix anti-fungus remedy to the incubating eggs in the hope of reducing the incidence of egg fungus. The fry are easily reared on finely powdered or liquid foods and brine shrimp nauplii.

Japanese Ricefish Fact File:

Other common names: Medaka, Geisha-girl.
                               The common name "Ricefish" refers to the rice paddies
                                in which these fish are found within their native Japan.
Taxonomy: Oryzias latipes first described by Temminck and Schlegel in 1846.
The genus Oryzias is now placed within the Family Adrianichthyidae.
Natural distribution: Japan, China, Korea and Formosa
Size: to about 3.5 cm
Colour: Wild forms are silverv grev. A yellow (xanthic) form is most commonly
             seen in the hobby: individuals vary from pale yellow to orange.

Feeding: I feed Aquarian Goldfish Flakes plus frozen/live brine shrimp nauplii,
             small Daphnia and mosquito Iarvae. Ricefish generally feed at the
             surface but will descend to lower leveIs in order to take a meal.

Housing: These peaceful fish fare best by themselves, in groups of six or more
             individuals. Provide lots of bushy surface plants for egg deposition.

Water conditions: Not fussy, provided good hygiene is maintained.
                           Ideally neutral to slightly alkaline pH.
                           Their wide temperature tolerance enables these fish to he kept
                           in outdoor unheated aquariums or in outdoor ponds and water
                           features. I have kept populations of these fish outdoors year
                           round in the south of England, although I prefer to move them
                           to a cool outhouse during the depths of winter. But, as Dave
                           Page has observed, they will survive under ice with seemingly
                           no ill effects.

Other Oryzias species

There are 22 described species of Oryzias Ricefishes (according to the www.fishbase.org website. Unlike 0.latipes, these are strictly tropical species and require a heated aquarium. Those listed below were unknown or undescribed prior to the 1990s and I suspect more await discovery:

Oryzias species                Description

O. haugiangensis               Roberts 1998
O. hubbsi                        Roberts 1998
O. nigrimas                      Kottelat 1990
O. orthognathus                Kottelat 1990
O. pectoralis                    Roberts 1998
O. profundicola                Kottelat 1990
O.uwai                             Roberts 1998

Acquiring specimens:

Keeping and breeding O. latipes is relatively straightforward, but acquiring specimens is the difficult bit! Last year I added some "new blood" to my stock, courtesy of Phil Austen who often has a few Ricefish on display at the FBAS Festival of Fishkeeping. If you visit the Festival you may be able to acquire some Ricefish for yourself (watch this website for Festival updates).
I will endeavour to bring a few specimens along too: so come and see me on the Aquarian Stand!

You never know, Oryzias latipes might eventually make it into the top ten most popular coldwater fish – it certainly deserves to be there. As for the transgenic fluorescent Ricefish? Well, no thanks, I'll stick to the ones that natures created…..

You can contact Peter via email: AquaticsDoctor@aol.com

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