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What is a Medaka?

By J.L. Drede
Updated Jun 04, 2024
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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.

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.

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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