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Wednesday, Nov. 23, 2005


Rice fish

* Japanese name: Medaka
* Scientific name: Oryzias latipes
* Description: This ordinary-looking little creature is one of the most important fish in biological research. Growing just 5-8 cm long, in the wild it is pale brown dorsally and silver underneath and on the flanks. The dorsal fin has 5-6 rays; the lower anal fin has 18-20 rays. There is another small fin on the lower side of the fish, used for delicate maneuvering, which also has 5-6 rays. The eye is large and black with a distinctive silver outer ring. There are some colored varieties bred for ornamental purposes, and even a fluorescent variety bred by introducing a gene from a jellyfish into the medaka genome to produce a fluorescent protein.
* Where to find them: Slow-moving streams and ponds, and even in stagnant water, from Honshu south to Kyushu. Medaka can survive in stagnant conditions because they do not need much oxygen. They are also commonly found in research labs and aquaria.
* Food: Zooplankton and larger animals such as water fleas. They will also eat aquatic worms and vegetable matter. Medaka have to be dull-looking in the wild as river birds and dragonfly larvae are major predators.
* Special features: Rice fish are resilient creatures that have a short life cycle and reproduce easily, which makes them popular aquarium and lab animals. This was the first fish found to conform to Mendel's laws of inheritance, which had already been deduced in pea plants. And it was also the first fish in which hormones were shown to induce sex change from both male to female and female to male. It is even used in cancer research. When mating, males swim quickly around the female to court her; if she is receptive, the male clasps the female with his fins, which are bigger than hers. The male bends his head toward the female, encouragingly, it appears, and both animals shudder. Eggs and sperm are then released into the water. But the eggs stay attached to the female for a time, in a cluster attached to fine threads coming from her body. Eventually the cluster of eggs falls into vegetation below. Sometimes it seems that females lack the patience to sit and wait and watch a male's courtship display. Instead, if a nearby female has mated with a male, she will copy that other female's choice, and mate with him.


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