What prevents Musculista senhousia from constructing byssal thread mats in estuarine environments? A case study focusing on Lake Shinji and nearby estuarine waters

Musculista senhousia is a marine mytilid mussel native to Japan that constructs byssal thread mats (hereafter “mats”) that destroy local fisheries, and so it is regarded as a biofouling species. To elucidate the factors that prevent the formation of mats of M. senhousia, we first observed the seasonal changes in the populations of M. senhousia in two rivers with similar salinities. Then we compared the condition of Lake Nakaumi, which is covered with dense mats at present, with the condition of the same lake before the decline of the eelgrass beds. The results showed that a particular set of conditions (in terms of salinity, dissolved oxygen concentration and sediment type) did not lead to the dense habitation of M. senhousia in one river, while mat construction was observed in another river with a similar set of conditions. Together with the past description of Lake Nakaumi, we conclude that larvae of M. senhousia head upstream all year round, and can settle as far upstream as the salinity remains within its range of tolerance. Whether it constructs mats depends on the presence of its predators.