Interannual changes in population structure and tidal zonation of sympatric batillariid snails Batillaria multiformis and B. attramentaria: A case study in the Ashizaki tidal flat, Mutsu Bay

: The Ashizaki tidal flat in inner Mutsu Bay, northern Japan, represents the northern limit of the range of the endan gered mud snail Batillaria multiformis in Japan. In this study, the population structure and tidal zonation of sympatric B. multiformis and B. attramentaria were examined at stations on the Ashizaki tidal flat from 2010 to 2015. A cohort of smaller in dividuals （ shell length ［ SL ］ : 10.0–17.1 mm ） of B. multiformis was formed in 2010; however, few cohorts formed during the following years, leading to a decrease in the population density. Batillaria multiformis recruited in the summer and fall, with SL growth of 5–6 and 10–17 mm in June–July and September, respectively, of the second year, and of 20 mm in May– July of the third year. By contrast, B. attramentaria recruited every year （ SL growth: 5–6 and 10–17 mm in May and sum -mer, respectively, of the second year, and 15–20 mm in the third summer ） and maintained a high population density throughout the study period. The two species recruited in low- to mid-intertidal zones in most cases. Subsequently, a propor tion of B. attramentaria moved to the high-intertidal zone, while B. multiformis remained in the low-intertidal zone. Our data suggest that the populations of the two batillariid species studied herein exhibited slower growth rates and attained larger adult sizes compared with other populations in Japan. Less frequent recruitment of B. multiformis in our study site implies that this population of B. multiformis may be vulnerable.