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Why is the neon tetra so bright? Coloration for mirror-image projection to confuse predators? “Mirror-image decoy” hypothesis

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The neon tetra (Paracheirodon innesi), which inhabits blackwater streams of the Amazon basin, has extremely bright coloration characterized by metallic blue-green stripes. To clarify the ecological function of this coloration, we examined the appearance of living neon tetra. They changed color in response to lighting and background conditions, and became less conspicuous under each condition to the human eye. Although they appeared bright in colorless clear water, their stripes appeared darker in blackwater. In addition, the visible area of their stripes was small and their brightness decreased, unless they were observed within a limited viewing angle (approximately 30° above the horizon). The results show that from the viewpoint of approaching submerged predators, a bright mirror image of the stripes is projected onto the underside of the water’s surface, providing a dramatic visual target while the real fish remains less conspicuous. Based on these results, we hypothesize that the neon tetra’s bright coloration is an effective predator evasion strategy that confuses predators using bright mirror images.

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the angle between the horizontal and the line of measurement/sight


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We express sincere thanks to Noriko Oshima of Toho University who kindly provided us with much valuable advice and a spectrometer. We show heartfelt appreciation for the kindness and information provided by Jorge Porto of INPA, Brazil. Yusuke Sekiguchi and Koji Miyamoto gave us plenty of advice and encouragement. We are obliged to Takahiro Segawa and Tetsuo Nakajima for their accompaniment in the Peruvian Amazon. We also thank all the members of our laboratories for their help with and support of our studies. This research was partly supported by COE Program (R12) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Correspondence to Takehide Ikeda.

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Ikeda, T., Kohshima, S. Why is the neon tetra so bright? Coloration for mirror-image projection to confuse predators? “Mirror-image decoy” hypothesis. Environ Biol Fish 86, 427–441 (2009).

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