Consecutive sexual maturation observed in a rock shell population in the vicinity of the Fukushima Daiichi Nuclear Power Plant, Japan

Abstract

In 2012, after the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) that followed the Tohoku earthquake and tsunami in March 2011, no rock shell (Thais clavigera; currently recognized as Reishia clavigera; Gastropoda, Neogastropoda, Muricidae) specimens were found near the plant from Hirono to Futaba Beach (a distance of approximately 30 km). In July 2016, however, rock shells were again found to inhabit the area. From April 2017 to May 2019, we collected rock shell specimens monthly at two sites near the FDNPP (Okuma and Tomioka) and at a reference site ~ 120 km south of the FDNPP (Hiraiso). We examined the gonads of the specimens histologically to evaluate their reproductive cycle and sexual maturation. The gonads of the rock shells collected at Okuma, ~ 1 km south of the FDNPP, exhibited consecutive sexual maturation during the 2 years from April 2017 to May 2019, whereas sexual maturation of the gonads of specimens collected at Hiraiso was observed only in summer. The consecutive sexual maturation of the gonads of the specimens collected at Okuma might not represent a temporary phenomenon but rather a site-specific phenotype, possibly caused by specific environmental factors near the FDNPP.

What Caused Declines in Intertidal Invertebrate Populations around Fukushima Daiichi Nuclear Power Plant after the 2011 Great East Japan Earthquake, Tsunami, and Nuclear Disaster?

“…Although no R. clavigera specimens were found near the plant from Hirono to Futaba Beach (a distance of approximately 30 km) in April 2012 [ 3 ], rock shells were again found to inhabit the area, such as at Okuma, located at approximately 1 km south of FDNPP in July 2016. Horiguchi et al [ 5 ] then collected rock shell specimens monthly at two sites near FDNPP (Okuma and Tomioka, Japan) and at a reference site approximately 120 km south of FDNPP (Hiraiso, Japan) from April 2017 to May 2019. They examined the gonads of the specimens histologically to evaluate their reproductive cycle and sexual maturity.…”

“…In reality, there was a gradual redistribution of the rock shell from northern and southern sites to the sites near FDNPP in the central area of Fukushima Prefecture from 2012 to 2016 [ 4 ]. Despite the recovery of the rock shell populations near FDNPP, however, continuous sexual maturation, a strange, abnormal phenomenon, has been observed in rock shell collected at Okuma, ~1 km south of FDNPP, since April 2017 [ 5 ].…”

“… Continuous sexual maturation in the rock shell population. It is unlikely that radionuclides that leaked from FDNPP or high seawater temperatures are causal factors for these observed effects, as discussed in Horiguchi et al [ 5 ]. However, it is possible that some unknown harmful substance(s) that may have leaked from FDNPP may be related to the induction/promotion and continued sexual maturation, or the failure to terminate maturation, in the rock shell population near FDNPP throughout the year.…”

Concomitant downregulation of neuropeptide genes in a marine snail with consecutive sexual maturation after a nuclear disaster in Japan

“…It became rare to find Reishia clavigera from late 2011 to early 2013, probably because most died due to exposure to radioactive substances and subsequent inhibition of larval recruitment by exposure to harmful substances, such as radionuclides and heavy metals. Horiguchi et al found that, within the area 3-km from FDNPP, the testis and ovary of R. clavigera are sexually matured year-round, whereas this marine snail, collected at a reference site, approximately 120 km south of the FDNPP (Hiraiso), normally exhibits gonadal maturation in early summer and regression in winter (15). This unusual phenomenon was named "consecutive sexual maturation (CSM)" (15).…”

“…Horiguchi et al found that, within the area 3-km from FDNPP, the testis and ovary of R. clavigera are sexually matured year-round, whereas this marine snail, collected at a reference site, approximately 120 km south of the FDNPP (Hiraiso), normally exhibits gonadal maturation in early summer and regression in winter (15). This unusual phenomenon was named "consecutive sexual maturation (CSM)" (15). Because of the high tidal level and rough waves in winter in that area, it is hard to determine whether the CSM snail lays eggs in mid-winter in that area.…”