Ask the WWM Crew
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After decades of being virtually absent from the
tropical fish trade, it looks like ricefish--Oryzias spp.--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.
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.
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.
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.
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
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.
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.
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.
Above: Oryzias mekongensis, also known as the Mekong Ricefish, a new import from Burma. Below, a male O. melastigma clearly showing the long fins that make them easily distinguishable from the females. Photos by Neale Monks
Oryzias woworae tankmate options