The Incredible story of Longfin Medaka Youhiki Ricefish

Oryzias latipes is the scientific name for a fish that is more well known as the Japanese Ricefish; a fish with a rich, biologically significant history. It's also sometimes called a Japanese Killifish, based on its looks, but there isn't much of a genetic connection between the two. There are now many fish that share this scientific name, a result of countless generations of selective breeding, and even some genetic engineering.

These fish are famous for a number of reasons, though perhaps mostly because they were the first vertebrate animal that was bred in space (at least that humans know about). In 1994, over a 15 day mission, a group of four fish successfully mated, then laid eggs in high orbit. Eight of these fry hatched in space, and another 30 hatched after returning to earth. The fish that were born in space later were able to reproduce as well. The scientific bent of Oryzias latipes doesn't stop there; later, a group of Taiwanese scientists inserted a glow-in-the-dark gene from jellyfish and corals into some of these fish in order to make certain genes stand out in their genetic studies. As a result of this, there are now glow in the dark green, yellow and red variations of them!

It is their incredible hardiness and ease of reproduction that has made them a stalwart in a wide range biological study; these same attributes are also responsible for them becoming popular aquarium fish, with now over 30 different varieties to choose from! Longfin Medaka Youhiki Ricefish are one of these. They are small fish that never get much more than an inch and half long, perhaps two inches with the longer than usual fins. They are a burnished orange and more deeply colored (and rarer) than the “Orange Japanese Ricefish.”

In the wild, Ricefish were originally found in slow streams, estuaries, rice paddies ponds and even tidepools all over Japan, China, Korea and Vietnam. In their wild form, they can still be found throughout this range. They can live their lives out in the full range of salinity from full saltwater to full freshwater. Further demonstrating their adaptability, they've been documented in water everywhere from under 40 degrees to over 105 degrees Fahrenheit! This isn't to say that they should be kept in these temperatures, or that they would survive for long in them. A more reasonable temperature range is from the high 60s to the high 70s or even low 80s.

This adaptability leaves the home aquarist with lots of options for how to display the Longfin Medaka Youhiki Ricefish. Their small size means a group of eight of them (which is our minimum suggested group size) could be kept in a tank as small as 20 gallons. This display could be anything from a beautifully detailed planted aquarium to a marine tank (with enough acclimation) to a mini-pond or water feature outside in many environments that were temperate enough. Many hobbyists who have kept Ricefish long term say they do best when they actually have some seasonal temperature fluctuation, and so inside tanks might not even need a heater! They are not picky about much in regard to water quality parameters as you've probably guessed from their wild forebears being found just about everywhere, though they seem to do best in water that has a very slightly elevated pH, between 7 and 8 would be fine.

Medaka Youhiki Ricefish are very peaceful and could be kept with a variety of other small peaceful fish, some aquarists even keep them with decorative shrimp, but in our experience, this should only be done in very densely planted tanks where there is a lot of cover for the shrimp to hide in. Expect some shrimp losses, especially among the babies. Always keep your Ricefish in groups of at least eight with no ceiling as to how many to keep together as long as there is enough space. In most saltwater setups, these are so small that they would likely be food for many fish common to those displays, so unless this is your intent (to rear them as live food), choose tank mates carefully.

Like everything else about Oryzias latipes and their not-so-challenging husbandry, they are also incredibly easy to feed. Here we are feeding them Nutramar Complete Freshwater Pellets which are accepted with gusto. They are also being a fed a variety of appropriately sized thawed foods. The meaty mix we provide them with changes but is often made up of a variety of all the different enhanced Gamma Brine Shrimp, plus small meaty stuff like Tubifex and Mysis, etc. They will also readily take live food of these types as well.

One of the primary reasons that Ricefish have been used for so many biological studies is that they don't have a breeding season, and don't need any spawning triggers. Happy Oryzias latipes might spawn as often as every three days. We've seen reports of this being possible every day but have not documented this ourselves. Some aquarists have reported that increasing the number of feedings daily will shorten the time between spawns, which seems logical. We know that keeping the water temperature in the low 70s does make a huge difference in getting them to spawn. Males will set up and defend small, temporary territories, and females will visit. The females will hold eggs externally for a few days before she attaches them to plants or mops. The eggs will hatch 10 to 12 days later, longer in colder temps, faster in slightly warmer water. Baby Ricefish can be fed infusoria (see our homegrown food article) and later things like microworms and artemia nauplii. They grow quite quickly, but never get very big. Adults will generally not consume the eggs, but free-swimming fry will be eaten, even by other juveniles from earlier spawns. If you want to get a good survival rate, you'll need to remove the eggs to a rearing tank before they hatch. In aquariums with enough cover, some will inevitably survive.

Longfin Medaka Youhiki Ricefish are the gorgeous products of decades of selective breeding culminating in a fish perfect for the home aquarium or pond. They are the picture of hardiness and ease of care and are very likely to breed in captivity for you! For those of you who are getting into the “mini-pond” craze, this is the perfect fish for your new experiment! If you're interested in keeping the first earthling vertebrate spawned in space, in your own aquarium then head to your LFS and ask them about getting you some Ricefish from Aquatropic today!