Ari O. Laine / FIMR
In 2003, a pronounced local settlement of young dreissenid mussels was observed in Loviisa archipelago, northern coast of the central Gulf of Finland, leading to a dense adult population thereafter. The invasion was first connected to zebra mussels, Dreissena polymorpha, but a later examination of adult mussels collected in late 2004 revealed the species to be the very similar looking Conrad’s false mussel, Mytilopsis leucophaeata (Conrad 1831). The species has not been previously identified in the northern Baltic Sea.
M. leucophaeata is an invasive brackish water dreissenid species with an original distribution in the subtropical and temperate Gulf of Mexico area. The mussel was recorded in Western Europe as early as in the 19th century, but it has recently expanded its distribution in European estuaries. The species now also occurs in the Black Sea. In the Baltic Sea area recent local populations have been found only in northern Germany. M. leucophaeata is a euryhaline species with an optimum salinity between 1.4 and 12.7 ‰. The species has been reported to coexist both with blue mussel, Mytilus edulis and zebra mussel in Western Europe. The distribution of Mytilopsis and Dreissena overlap especially in a salinity of 0.2-3.0 ‰. In Western Europe M. leucophaeata has become a serious biofouling organism in cooling water systems and is more tolerant to common control methods (e.g. chlorination and heat treatment) than zebra mussel or blue mussel.
M. leucophaeata resembles the zebra mussel closely and especially young individuals can be difficult to distinguish. Adult individuals are usually even brownish in colour without the zigzag pattern or stripes that are typical to young individuals. The length of adult mussels is usually less than 2 cm. The shell shape is less angular than in the zebra mussel and resembles more the form of the blue mussel. M. leucophaeata shells do not have a longitudinal ridge typical to zebra mussels (Figure 1). However, the exact identification of M. leucophaeata is based on an internal shell structure. Both Dreissena and Mytilopsis have anteriorly a myophore plate used for muscle attachment but in M. leucophaeata there is also dorso-laterally a closely attached tooth-like projection, an apophysis (Figure 2), which is absent in the zebra mussel. Microscopic examination is needed in species identification.
The occurrence of M. leucophaeata in the Gulf of Finland has been confirmed so far only in a relatively small area near the nuclear power plant in Loviisa. In 2004 dense adult mussel assemblages were observed in the area directly affected by the cooling waters of the power plant. In these populations densities and biomass up to 28 000 individuals and 9.8 kg per square meter (wet weight) were observed, respectively. Based on settlement studies with artificial plates in 2004-2005 a wider distribution of young individuals has been observed. It is possible that the mussels do not survive the harsh winter conditions since the distribution of older individuals seems to be much restricted to the area affected by warm cooling waters from the power plant. However, more field investigations are needed to verify the current distribution of M. leucophaeata.
Mytilopsis leucophaeata identification and ecology, see e.g. the Global Invasive Species Database
