Recently, the golden mussel Limnoperna fortunei, an alien species, has invaded the Tenryu, Yahagi and Toyo rivers in Central Japan. This has raised concerns about the impact it may have on hydraulic power generation facilities. We investigated the period of occurrence and attachment of its planktonic larvae, and their growth. The purpose was to understand the current status of their distribution and consequently develop control methods. The results indicate a clear correlation between water temperature and the period of occurrence of planktonic larvae and the growth of this species.

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Limnoperna fortunei (the golden mussel) is an invasive freshwater bivalve species that is native to China and southeastern Asia. Since L. fortunei has both economic and environmental effects, Japan's Ministry of the Environment has officially designated it as an “Invasive Alien Species”. The occurrence of the mussel was initially reported in 2005 in central Japan, but its spatial distribution in this area was unknown. We examined the spatial distribution of the mussel in the Kanto region from 2006 to 2009.
In the lower Tone River, we found the mussel up to about 120 km from the river mouth. The mussel was also found in several other rivers, including the Kokai, Hitachitone, and Edo Rivers; in ponds and lakes (Lake Kasumigaura, Lake Teganuma, Lake Ushikunuma, etc.); and in canals. Size distribution data suggest that the mussel invaded these areas no later than 2004. Previous studies showed L. fortunei to be already present around the Oshio reservoir (Gunma Pref), which is a part of the Tone River system. Spatial distribution and genetic analysis revealed that the samples from the lowlands of the Kanto district (the lower Tone River, Lake Kasumigaura, etc.) show different genetic structure from those of the Oshio reservoir. From these results, we considered that the mussel invasion in this region occurred in at least two separate occasions. The spatial distribution and genetic analysis of the mussel in the lowlands of the Kanto district suggest that expansion of the mussel distribution was brought about by the water current via river, water-way and pipeline. Early detection and monitoring of the mussel invasion will be important, especially in the downstream of the invaded areas.

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In this study, we investigated the density of the golden mussel (Limnoperna fortunei) that were attached to the channel walls and the population dynamics of its larvae in lakes Ohshio and Takenuma. We also examined the effect of temperature on the survival of L. fortunei. The density of attached mussels at a given site in the channel decreased as the distance between the site and the lake increased. In Lake Ohshio, larvae were detected in June 2008 when the water temperature increased to 17°C. L. fortunei larval densities in Lake Ohshio were more than 10,000 individuals m^-3 for 1 month from mid August to mid September 2007. By contrast, larval densities were very low in Lake Takenuma (max: ca. 200 individuals m^-3) during the same period. A comparison of the two lakes showed that water temperature and dissolved oxygen concentration greatly influenced the survival and breeding of L. fortunei. The survival rate of the mussels was independent of the temperature from 5 to 30°C. On the other hand, the survival rate was much lower at 35°C during the 114-day rearing experiment, although it was initially unaffected for 20 days. Half of the mussels died after 309 days of rearing at 5°C. These results suggest that L. fortunei can survive under a wide range of temperatures, and is able to inhabit most lakes and ponds in Japan.

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The golden mussel Limnoperna fortunei is an invasive freshwater bivalve species. Recently, gregarious settlement of these mussels in the dam and aqueducts of hydroelectric power stations in Japan has been the cause of various problems. We developed a molecular based method for the quantitative detection of larvae of L. fortunei using real-time polymerase chain reaction (real-time PCR). Designed primers were able to specifically detect larvae of L. fortunei in freshwater planktonic samples. Real-time PCR was performed using fluorescent dye (SYBR^® Green) detection. Threshold cycles correlated well with the number of larvae in logarithmic values, and the R-square value of the standard curve was 0.99. This molecular based method can be utilized not only for the identification of L. fortunei in a particular habitat, but also for the quantification of larvae of this species in natural environments.

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The green mussel, Perna viridis, is an invasive species, that has been expanding its geographic range in warmer Japanese waters recently. To elucidate its habitat use pattern, its distribution was investigated in Uranouchi Inlet, Kochi Prefecture, Japan (33º 26' N, 133º 25' E), from March to May, 2008. Intertidal surveys revealed that P. viridis uses natural rocky shores and cobble shores as well as artificial vertical seawalls and block seawalls. No difference in its abundance was detected among the four habitat types. Green mussels were more abundant in the inner bay and middle bay than in the outer bay. They were also found on floats of fish-farming rafts. The total abundance of P. viridis in Uranouchi Inlet was estimated 240,000 individuals on floats and 50,000 on intertidal shores. The distribution of another invasive mussel, Xenostrobus securis, was also surveyed in Uranouchi Inlet, along with the Japanese native mussels X. atratus, Hormomya mutabilis, Septifer bilocularis, and S. virgatus. Perna viridis overlapped with H. mutabilis and S. bilocularis. The high abundance of P. viridis and its invasion of natural shores possibly represents a threat to the indigenous mussel species community, but further ecological research is needed to evaluate the scale of this threat.

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Anti-fouling properties of various coating materials for steel wires, i.e. normal zinc-galvanized coating, polyethylene (ionomer resin) coating, and magnesium oxide (HUS)-added (5%) polyethylene coating, were evaluated by suspending coated wires for 17 months under an oyster raft in Hiroshima Bay. The dominant macro-fouling animals were the anascan bryozoan Bugula neritina after four months (February), the bivalve Mytilus galloprovincialis after eight months (June), the solitary ascidian Styela plicata after 12 months (October), and the colonial ascidian Didemnum moseleyi after 17 months (March). Normal zincgalvanized coating was generally superior in its anti-fouling properties to the other two coating materials.

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