You have a Mytilopsis leucophaeata (Conrad, 1831) – dark false mussel or Conrad’s false mussel
Synonyms: Mytilus leucophaetus Conrad, 1831; Congeria leucophaeata (Conrad, 1831); Congeria cochleata (Kickx in Nyst, 1835); Dreissena cumingiana Dunker, 1855; Mytilus americanus Reeve, 1858; Mytilus tenebrosus Reeve, 1858; Tichogonia cochleata Dunker, 1853. The species name is often misspelled as M. leucophaeta.
Common names: Dark false mussel, Conrad's false mussel, Brackish water mussel (USA, U.K.), Brackwasserdreiecksmuschel (GE), Brakwatermossel (NL), valekirjosimpukka (FI)
Mytilopsis leucophaeata has a thick, usually dark periostracum covering the shell (FIG). The ventral margin of the shell is almost straight. Internally there is a "shelf" or septum near the umbo, and partly covered by this is a small triangular apophysis (FIG). True mussels (Mytilidae) do not have this septum or apophysis. In Dreissena polymorpha there is a septum, but no apophysis (FIG), and the outside of the shell has the characteristic "zebra-striped" pattern. M. leucophaeata is an epifaunal species, which attaches to hard substrates by byssus, as do other dreissenids as well as most mytilids. Juvenile M. leucophaeata may have the zebra-striped appearance of D. polymorpha and thus cause confusion about identification.
Dreissena polymorpha - interior (left) & external shell (right)
Mytilopsis leucophaeata - interior (left) & external shell (right)
For further information see: The US Environmental Laboratory
Dreissena polymorpha (Pallas, 1771) (Zebra mussel):
Shell up to 6 cm long; externally striped; usually in freshwater
Dreissena bugensis Andrusov, 1897 (Quagga mussel):
Shell up to 6 cm long. May have striped pattern, but usually dark; usually in freshwater. This species has also been recorded under the name(s) Dreissena rostriformis (Deshayes, 1838) or D. rostriformis bugensis.
Mytilopsis leucophaeata (Conrad, 1831):
Shell usually smaller than 2 cm; with thick, dark periostracum, but no stripes except in juveniles; euryhaline, but usually in brackish water.
Native distribution: North America from Massachusetts to Mexico (Marelli & Gray, 1983). There is some debate about the northern limit because apparently its appearance north of the Chesapeake Bay, including rivers entering this, seems to be relatively recent and thus may be human assisted (Verween et al., 2006). The specimens used for illustration in this factsheet have been collected near Baltimore in 1862 and donated to the Zoological Museum in Copenhagen by W. Stimpson, a well-known American marine biologist.
Invasive distribution and history: The first record in Europe is from 1835, when it was found in the Scheldt estuary near Antwerp, Belgium, and described as a new species, Congeria cochleata. It slowly dispersed within Belgium, being recorded from Ostende in 1866 and from Brussels in 1868. In 1872 it was found near Dunkerque in France (Boettger, 1933), and in 1895 it was first found living in the Netherlands (Wolff, 2005). It was first found in the Kiel Canal (Nordostseekanal) in 1928 (Boettger, 1933), but further dispersal in the Baltic Sea was not detected until 2000, when it was found near Rostock at a salinity of 0.7-2.5ppt (Darr & Zettler, 2000). In 2003 it had further dispersed to the Gulf of Finland (Laine et al., 2006). In 1996 it was found at a salinity of 15ppt in Wales, UK (Oliver et al., 1998), and in 2003 it was identified from the estuary of the river Guadalquivir in Spain. However, it was assumed that it had been present for several years at that time (Escot et al., 2003). Finally, in 2001 M. leucophaeata was found in Dniester Liman in the Ukrainean part of the Black Sea (Grigorovich et al., 2002; Therriault et al., 2004). In all cases where localities are described these have been man-made, such as ports and canals, or very close to ports. Also, the associated fauna is often dominated by other alien species (Boettger, 1933; Oliver et al., 1998; Darr & Zettler, 2000; Escot et al., 2003; Laine et al., 2006).
Vector The slow dispersal of Mytilopsis leucophaeata in European waters indicates that its dispersal needs human interference, and its association with man-made structures and other alien species indicates that shipping is the main agent. Wolff (2005) concludes that hull fouling is the most likely vector, whereas Laine et al. (2006) claims that ballast water has been the vector.
Mytilopsis leucophaeata is strongly euryhaline and also fairly eurythermic (Oliver et al., 1998; Darr & Zettler, 2000; Rajagopal et al., 2005; Laine et al., 2006; Verween et al., 2005, 2007). It has been recorded from salinities ranging from <1ppt to almost full strength seawater, and is capable of surviving in full strength seawater in the laboratory, though most often it is found at salinities below 10ppt. Heat has been tried as a controlling measure because a temperature of 37° C will cause 100% mortality in 2h (Rajagopal et al., 2005). Metabolic processes are affected at temperatures above 27° C. Filtration rates are rather low compared to blue mussels of similar size.
Reproduction: Reproduction and larval development was studied in the native Florida (Siddall, 1980), and more recently in Belgium (Verween et al., 2005, 2007). Near Antwerp larvae occur in the plankton between May and October with peak density usually in August (Verween et al., 2005). The minimum temperature for spawning appears to be 13° C. In Florida temperature was above this level all year round, and spawning in the field was apparently triggered by decrease in salinity from 22 to 8ppt following heavy rains (Siddall, 1980). Development is unaffected by salinities between 10 and 32 ppt at 26° C, and only very young embryos (4h) were susceptible to salinities above 20 PSU (practical salinity units, almost identical to ppt) and temperatures below 15°C (Verween et al., 2007). The planktonic stage lasts between 6 days and 2 weeks, depending on temperature (Siddall, 1980). As a curiosity it can be mentioned that norfluoxetine, the active ingredient in some antidepressant drugs, is also active in inducing spawning in M. leucophaeata as well as Dreissena polymorpha. This drug has also been detected in discharged sewage (Fong & Molnar, 2008).
It was only with strict regulations on the use of biocides during the 1990s that Mytilopsis leucophaeata became a serious fouling problem for cooling water systems along the coasts of Belgium, Netherlands and France (Verween et al., 2005). No information is available on competition or tolerance of anoxia or environmental contaminants. Hence nothing is known about its effects on native ecosystems.