Behavior of newly-hatched Octopus tetricus paralarvae in response to different light sources

Research aimed at resolving commonly observed high mortalities in the culture of octopus paralarvae has largely focused on their nutritional requirements, while zootechnical aspects have been somewhat disregarded. Light conditions are a key environmental factor often involved in the successful larval culture of many marine organisms, with little known about the optimal light conditions for the growth and survival of cultured octopus paralarvae. This study evaluates the behavior (positive or negative phototaxis) of newly-hatched O. tetricus paralarvae aged 0 days when held under four different light sources (blue - 460 nm, green - 517 nm, pale blue - 460-517 nm, white - 440-530 nm). To test phototaxis behavior, 200 newly-hatched paralarvae were placed in a kreisel tank (7 l volume), held in darkness for 30 min before exposed to one of the light sources above and the distribution of the paralarvae within the tank recorded with a video camera at 0, 15 and 25 min among the bottom, middle and top layers of the kreisel tank. Light intensity was highest in the top layer for all four light sources and decreased by up to 49% and 54% in the middle and bottom layer of the kreisel tank, depending on the wavelength of each light source. Newly-hatched paralarvae were evenly distributed among the three tank layers for the four light sources at 0 min. In contrast, the majority (52.1-57.8%) of paralarvae were found in the top layer at 15 min and 25 min for all four light sources. The results demonstrate that newly-hatched O. tetricus paralarvae show positive phototaxis towards blue, green and white light, which is important knowledge required to provide O. tetricus paralarvae with suitable culture conditions.