* Japanese name: Donko
* Scientific name: Odontobutis obscura
* Description: The enigmatically named Dark sleeper is a perch-like fish in the order Perchiformes. Members of this, the largest order of vertebrates, include 40 percent of all fish. They are typically fat-lipped, grumpy-looking fish and Dark sleepers are no exception. They vary in color, ranging from brown to gray, with black patches, mottles and swatches on the body. Even the same fish, for reasons we shall see, can be patterned differently from one minute to the next. They have bulging eyes, large, fan-shaped fins and grow up to 12-cm long.
* Where to find them: An entirely freshwater species, the Dark sleeper can be found in rivers all over Honshu. Young fish live in the shallows, but adults live on the river bed.
* Food: Dark sleepers are carnivorous and prey on other small fish, juvenile forms of fish and amphibians. The tadpoles of frogs and toads are a particular favorite; insect larvae are also taken. They do not eat detritus or plant material.
* Special features: Obscura is Latin for "dark," but the Dark sleeper can also be obscure, as in difficult to see. That's because it has a remarkable way of controlling how light is reflected from its skin. Under the skin surface are plates that reflect strongly, and cellular spaces, without plates, where light is absorbed. The fish is able to change the proportion of reflecting plates and absorbing spaces in its skin, and so change its appearance. One minute it may be light-colored, the next mottled -- then it may appear blue or very dark. The color changes are controlled by nerves and hormones. The function of the color change is unknown. Likely explanations are that it has a camouflaging effect, allowing the fish to blend in with the rocks and plants at the bottom of the river, and also a sexual selection function. Male fish might alter their coloring when competing with other males for territories, or when trying to impress a potential female mate.
PHOTO COURTESY OF BIO-IMAGE NET