Wavelength-specific thresholds of artificially reared Japanese eel Anguilla japonica larvae determined from negative-phototactic behaviours.

We report wavelength-specific thresholds of leptocephali of Japanese eels Anguilla japonica determined from their negative-phototactic behaviour. Leptocephali are most sensitive to wavelengths 400-500 nm and at very short wavelengths. Their visual sensitivity decreases more sharply at wavelengths >500 nm than it does at wavelengths <400 nm. The spectral sensitivity of leptocephali adapts to the optical conditions of their habitat. The mean visual sensitivity threshold of leptocephali is 7.22 x 10(-4) mu mol m(-2) s(-1) between 400 and 500 nm. Based on visual sensitivity thresholds of 475 nm, the most transparent wavelength in waters where these leptocephali occur, the daytime depth of occurrence of these larvae may exceed 250 m. LEDs emitting light of wavelength 625 nm in culture environments would minimise disturbance to leptocephali during facility maintenance.