Effect of thermal stress on survival and delay of metamorphosis in larvae of the purple sea urchin Strongylocentrotus purpuratus

The effect of thermal stress (heat shock) on survival was evaluated for advanced larval stages and the possible effect on delay of metamorphosis in competent larvae was also evaluated in the purple sea urchin Strongylocentrotus purpuratus under laboratory conditions. Eight-arm precompetent and competent larvae were kept at 15, 19, 23, 28, and 31 degrees C during 24 h to estimate their thermal tolerance and measure their survival in each treatment. Delay of metamorphosis was evaluated in competent larvae exposed to heat shock treatments of 15, 20, 24, 28, and 32 degrees C for 30 min. After this, larvae were induced to metamorphose in the presence of biofilms and KCl. Our results indicated that the thermal tolerance limit for precompetent and competent larvae was 27 degrees C; temperatures equal or above this value are lethal for both stages of development, where we observed 100% mortality in a period of 24 h for the treatment at 27 degrees C and of 2 h at 31 degrees C. Competent larvae subjected to a heat shock treatment of 28 degrees C for 30 min delayed the onset of metamorphosis (<40%) and showed high percentage of larvae with incomplete metamorphosis (55%). These results indicate the sensitivity of late larval stages to temperatures above 27 degrees C for short periods of time, and that this sensitivity may compromise the onset of metamorphosis. This could have some effect on the settlement and recruitment patterns of the purple sea urchin, particularly in intertidal sites exposed to high temperatures during the summer.