Planktonic larvae and just settled juveniles of the sea urchin, Strongylocentrotus intermedius, were irradiated with various quantities of UV rays, and the effects 24 hrs after keeping them under a condition 10 L:14 D at 16°C (larvae) or 17°C (juveniles) were observed. The results showed that blastula and pluteus larvae were seriously damaged by irradiation of UV-B (280-320nm) and UV-A (320-400nm). A critical dose of damaging UV (DUV) which induced more than 30% morphologi-cally abnormal larvae was 0.01kJ/m2 for blastula and 4-armed larvae. It gradually increased to 0.05 and 0.08kJ/m2 as the stage advanced to the 6-armed and 8-armed stages, respectively. No effects were observed for juveniles after 24hrs of irradiation of 0.7kJ/m2 DUV, but all the juveniles which received more than 0.2kJ/m2 died within 1 month. It was suggested that the reproduction of the sea urchin might be affected by sun light in shallow water, since a maximum of the mean daily amount of DUV was reported to be 0.7kJ/m2 in July, 1994, in Sapporo.
