Continuing reproductive abnormalities observed in a rock-shell population near the Fukushima Daiichi Nuclear Power Plant, Japan

In 2012, following the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) that followed the Tohoku earthquake and tsunami, no rock shells (Reishia clavigera; Gastropoda) were observed in an area near the plant stretching from Hirono to Futaba Beach (a span of approximately 30 km). In July 2016, however, rock shells were again observed in this area. From April 2017 to June 2023, we collected rock-shell specimens monthly at a site near the FDNPP (Ottozawa) and at a reference site approximately 120 km south (Hiraiso). We examined their gonads for histological evaluation of the reproductive cycle and sexual maturation. The specimens collected from Ottozawa (approximately 1 km south of the FDNPP) exhibited consecutive sexual maturation (CSM), a reproductive abnormality, throughout the collection period, whereas the specimens from Hiraiso were sexually mature only in summer. The CSM observed in the specimens collected at Ottozawa might represent a site-specific phenotype, possibly caused by environmental factors near the FDNPP. In fact, CSM was observed only at sites within 3 km south of the FDNPP, and specimens from Ottozawa were more severely affected than those at Koirino (3 km south of the FDNPP). This suggests that unknown factors derived from the FDNPP altered the rock-shell physiology at these sites and triggered CSM as an irreversible syndrome. Further studies are needed to investigate the causal factors of CSM and to elucidate the mechanisms that induce and promote CSM in rock-shell populations near the FDNPP.