Habitat Description of Potamopyrgus antipodarum (Caenogastropoda: Hydrobiidae) in Some Areas of Japan: How Far Will It Spread?

Abstract

The habitats of Potamopyrgus antipodarum at four localities in the Kanto district (Shiobara, Chiba, Banyu Park and Yugawara) and at one location in the Kinki district (Hassaka) were investigated and the relationships between snail density and some environmental factors were analyzed. All field studies were undertaken during August and September in 2007 and 2010. The snails tended to be distributed in shallow water, which is consistent with the general features demonstrated in past studies, and in rather various velocity currents. They were distributed in high densities in a small creek entering Lake Biwa at Hassaka, but not at all in the coastal zone of Lake Biwa. At Shiobara and Yugawara, hot springs discharge into the rivers, but the mean water temperature in summer was less than 25°C, which is the upper limit for P. antipodarum reported in past studies, at all sites at both localities. Therefore, the hot spring discharges may make these areas rather suitable for P. antipodarum by increasing the water temperature from autumn to spring. At Chiba, P. antipodarum occurred in high densities in a creek in which the summer daytime water temperature reached 33.3°C. No clear relationship was determined between snail density and DO, pH, EC, NH₄-N, PO₄-P and Ca²⁺. P. antipodarum occurred at high densities in water with DO or Ca²⁺ concentration values close to the lowest limits reported in past studies. A laboratory experiment for the thermal tolerance of snails showed that all individuals from the Chiba population survived until the end of the experiment (21 days) at 31°C, but all but one snail from Kanro (near Hassaka) had died by the 14th day. The majority of the snails at Kanro were the same haplotype as those at Chiba and therefore may be the same clone. Therefore, it is likely that the high thermal tolerance of the Chiba population reflects their ability to acclimate, rather than a unique characteristic of the genotype of the Chiba population. Based on these results, the potential spread of P. antipodarum through Japan was discussed.