Antioxidant enzymes were analysed in spawn, larvae and in adult individuals of the polychaete Phyllodoce mucosa from an intertidal sandflat of the German Wadden coast. The analyses were carried out during two successive years (1992 and 1993) which were markedly different with respect to the weather conditions during the spawning period of P. mucosa. Extremely sunny weather conditions in 1993 led to hyperoxia and maximum peroxide concentrations of 4000 nmol I-1 in intertidal pools during daytime. Egg masses deposited into these intertidal pools were directly exposed to solar radiation and to hyperoxia stress. They had higher levels of the oxygen radical scavenger superoxide-dismutase (SOD) and of glutathione-reductase (GR, analysed only in 1993), a constituent of the glutathione antioxidant cycle, than the adults. Moreover, SOD and GR activities in the spawn were higher in 1993 than in 1992. By contrast, catalase activities were higher in adult individuals than in eggs and larvae. The haem derivative biliverdin, to which antioxidative properties have been ascribed, was present in high amounts in adult worms and at lower concentrations in the eggs. We postulate that high UV irradiation during early spring, which causes elevated H2O2 concentrations in tidal pool waters, may induce radical stress in polychaete spawn on the sediment surface. © 1995 Taylor & Francis Group, LLC.