Ecological Study on Arthritica reikoae (Bivalvia: Cardiida), the Dominant Microbivalve in the Inner Tidal Flats of Tokyo Bay, Japan

Arthritica reikoae (Suzuki & Kosuge 2010) is an ovoviviparous microbivalve inhabiting estuarine tidal flats in inner bays in Japan. Although A. reikoae is the dominant bivalve species in the tidal flats, its ecological characteristics remain largely unknown. We conducted ecological studies of populations in Tokyo Bay. The field research revealed that in the summer of 2010, more than 90% of adult A. reikoae bred as females. This species might be hermaphroditic similar to other congeneric species. Mature A. reikoae held 13.0 ± 5.7 (Mean ± S.D.) shelled larvae, and the shell length of the larvae just before release was about 0.3 mm. The breeding cycle was estimated to be 20 days based on laboratory culturing. A. reikoae produces large benthic larvae with limited dispersal but high survival. A. reikoae inhabits estuaries and brackish lakes where it may be exposed to a wide range of salinity. The results of a salinity tolerance experiment indicated that A. reikoae is euryhaline, which lets it inhabit such environments. The population density significantly increased through the breeding season, and the maximum density reached 1,578.4 indiv./100 cm2 in the estuary of the Tama River.