A discovery of the northernmost population of the giant mangrove whelk Terebralia palustris (Mollusca: Gastropoda: Potamididae) on Zamami Island and its genetic variability

The giant mangrove whelk Terebralia palustris is known to have wide distribution ranging from east coast of African Continent to Western Pacific region. Up to date, the population reported from Miyako Island, Japan has been recognized as northernmost population of this species. However, we have found a new population from Zamami Island, Kerama Islands, Japan, and thus report here. The population was found in a muddy wetland with vegetation of sparse Rhizophora stylosa and abundant Phragmites sp. throughout the habitat. The population consists of both fully-developed (with thick edge of shell-opening) and still growing (without thick edge) individuals sympatrically, thus assumed to comprise at least two generations. Whereas, mean population density was 5.3 inds/m2 and the total number of individuals found in the entire-habitat survey was 102, those are both substantially smaller number when compared to other habitats in Ryukyu Islands. In population genetic analysis, both haplotype and nucleotide diversity indices were low. Also, haplotype network and maximum likelihood phylogenetic analysis showed that there is no distinct genetic isolation between Zamami and other areas in southern Ryukyu Islands. The dissimilar habitat from previous reports, low population density and small body size may imply that this population is newly recruited (naturally or artificially), or under strong environmental stress at an edge of its distribution, however, it is still unclear about the origin of this population form our data. To monitor the demography of this population, following observation to reveal growth and reproductive cycle is needed in the future.