2016 Volume 70 Issue 2 Pages 43-49
The Hitotsuba Lagoon is a small estuary environment on the east coast of Kyushu island with about 12 ha of water area, half of which may appear as a continuous tidal flat. After a series of line transect and quadrate surveys in the lagoon from 2006 to 2014, we estimated the total number of three potamidid gastropods and discussed on their life history and population size. The Hitotsuba population of Cerithidea rhizophorarum A. Adams, 1855 was rather stable and not exceeded over 20,000 individuals in total within the lagoon. Two other species, Cerithideopsilla djadjariensis (K. Martin, 1899) and Cerithidea cingulata (Gmelin, 1791), fluctuated in the population size. The last two species are thought to increase in number year by year within the lagoon. The seasonal changes in size distribution of these two potamidid species in the Hitotsuba Lagoon were studied based on the periodical samplings in the intertidal zone from April to December, 2006 and another series in the subtidal from April, 2007 to February, 2008. As a conclusion of these surveys, small size gastropods with less than 5 mm in shell length of these species may appear on the tidal flat until October, then grow throughout the next year and finally reach to the full grown size until the second spring after entering to the tidal flat. Stopping the growth in the shell length after the development of the retroflexed lip of shell mouth, the adult gastropods may start to spawn in early summer. In the lagoon, Cerithidea cingulata may have a longer spawning period than Cerithideopsilla djadjariensis and recorded their small subtidal specimens throughout the sampling year.
