Light wavelength and intensity effects on larval phototaxis in the Pacific oyster Crassostrea gigas (Thunberg, 1793)

Oyster larvae in the settlement stage, referred to as pediveligers, initially have a light sensor eyespot to perceive light wavelength and intensity. Based on this ontogenetic characteristic of oyster larvae, this study investigated the effects of light wavelength and intensity on larval phototaxis in the Pacific oyster Crassostrea gigas. The eyespots of pediveligers were characterized with wavelength-dependent light absorbance, measured by a microspectrophotometer system, and phototaxis was analyzed with the following processes. Horizontal larval phototaxis was observed under lighting conditions with six light wavelengths and three intensities using light-emitting diodes: 5, 15, and 25 W/m² for near-ultraviolet light and four visible light wavelengths or 25, 50, and 100 lx for near-infrared light. Pediveligers were subjected to each lighting condition for 15 min, and then their distribution inside the experimental container was estimated. The eyespot exhibited maximum absorbance at a visible red light wavelength of approximately 638 nm. The larvae had a propensity for positive phototaxis under red light and negative phototaxis under blue light. The phototactic response varied with the light intensities. The obtained results indicate a possible mechanism for maintaining optimal water depth for the settlement of C. gigas larvae using light wavelength and intensity.