Light wavelength and intensity effects on larval settlement in the Pacific oyster Magallana gigas

Oyster larvae initially have an eyespot on settlement stage (pediveligers). This study investigated the eyespot with light wavelength-dependent absorbance, and its functional details for settlement in the Pacific oyster Magallana gigas larvae. Eyespot efficiently absorbed 500–650 nm of light wavelength. Based on the eyespot absorbance, pediveligers were subjected to different light conditions with light-emitting diodes (LEDs): near ultraviolet (NUV), blue, white, green, red, and near infrared (NIR), set at 3 intensities (5, 15, 25 W/m2, except for NIR, which was at 25, 50, 100 lux). LEDs modulated at each intensity were continuously irradiated for 2 days, and the number of settled larvae on a substrate of shell chips was counted daily. NUV at 15, 25 W/m2, and white and green light at 5, 15 W/m2 disrupted larval settlement, whereas red at 25 W/m2 and NIR at 100 lux showed strong settlement-inducing activities. Red and NIR were further tested with ceramic paper substrates; inert substrates. These lights maintained their performance, with settlement rates higher than controls in darkness. Our findings contribute to the further understanding of the function of eyespot in the settlement of oyster larvae, and to establish criteria to evaluate light pollution levels for protecting benthic ecosystem.