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What is a Medaka?

A Medaka, or Japanese rice fish, is a tiny, captivating creature often found darting in freshwaters of East Asia. Renowned for its genetic research contributions and ease of care, this fish is a favorite among aquarists and scientists alike. Its simplicity belies a complex biology that has much to teach us. What secrets does the Medaka hold? Join us to uncover more.
J.L. Drede
J.L. Drede

A medaka is a fish of the Adrianichthyidae family and Oryzias genus that is usually referred to as ricefish. It is sometimes called the Japanese killifish. In addition to Japan, it is found in most of Asia, including Korea, China, and Vietnam, usually in ponds, marshes, and rice paddies.

The medaka is a very small fish, usually reaching about 1.6 inches (4 cm) in length. Its body is very slender and elongated, with an arched back. Its very small size does not make the medaka a popular fish for fishermen, but its bright coloration and appearance has long made it a favorite of aquarium enthusiasts. In nature, the fish is usually brown or yellowish gold. Captive breeding has increased their color range, with white, yellow, or pure orange variations not uncommon.

Medaka fish can be found in rice paddies.
Medaka fish can be found in rice paddies.

As aquarium fish go, the medaka is a fairly hardy fish that can survive in a wide range of water conditions. Water temperatures can vary from 65 to 75 degrees Fahrenheit (18 to 24 Celsius), and pH levels can range from 7 to 8. As long as it is mixed with non-aggressive fish of a similar size, it makes a very good fish for community tanks.

Caring for the fish is also easy. The medaka will eat a variety of foods, from flake and freeze-dried foods to a wide range of frozen foods. Like many smaller fish, it prefers to have plenty of cover in the tank to hide when stressed. It also requires some degree of water movement, which can be provided by an air stone or filter.

In 1994, four medaka fish were taken aboard the USS Columbia space shuttle for a variety of tests.
In 1994, four medaka fish were taken aboard the USS Columbia space shuttle for a variety of tests.

The medaka is also a popular fish for researchers thanks to its hardiness and relatively quick breeding cycle. Medaka have been extensively used in environmental research. Tests involving carcinogenesis and related fields of study have been conducted on the fish in the past. Genetic manipulations on the fish have created captive-only variants of the fish, including a type that glows fluorescent green.

The use of the Japanese killifish as a research tool has even extended to outer space. In 1994, four medaka were taken aboard the USS Columbia space shuttle for a variety of tests, as they were the only fish species known to withstand micro-gravity. Not only did the fish thrive in the shuttle, but they bred, becoming the first vertebrates to produce offspring in space. The medaka kept aboard the Columbia laid 43 eggs, some of which hatched while still in space. All the fish that hatched both in space and back on Earth exhibited no noticeable side effects from their travels.

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    • Medaka fish can be found in rice paddies.
      By: kamonrat
      Medaka fish can be found in rice paddies.
    • In 1994, four medaka fish were taken aboard the USS Columbia space shuttle for a variety of tests.
      By: Kim Warden
      In 1994, four medaka fish were taken aboard the USS Columbia space shuttle for a variety of tests.