The influence of temperature and light on larval pre-settlement metamorphosis: a study of the effects of environmental factors on pre-settlement metamorphosis of the solitary ascidian Styela canopus

This study investigated the effects of two environmental factors, temperature and light, on larval settlement and metamorphosis in the solitary ascidian Styela canopus. The results revealed that larval settlement rates decreased with increasing temperature in the range 12–30°C. We also demonstrated for the first time that pre-settlement metamorphosis of ascidian larvae can occur as a function of temperature. We suggest this could be an adaptation to avoid the greater energetic cost of active larval swimming, presumably resulting from the increasing temperature. They are able to metamorphose into passive drifting post-larvae and to continue planktonic life. This finding has implications for larval dispersal, especially under conditions of ocean warming. In addition, the effect of light intensity on larval settlement and metamorphosis was significantly different between photoperiods of 24 L : 0 D and 12 L : 12 D. These results provide some insight into the complex cues affecting settlement and metamorphosis of ascidian larvae and ascidian distribution in nature.