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First record of the brackish water dreissenid bivalve Mytilopsis leucophaeata in the northern Baltic Sea


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Conrad's false mussel, Mytilopsis leucophaeata has been found in the central Gulf of Finland, which is the first record of this brackish water dreissenid species in the northern Baltic Sea. In 2003 a strong recruitment of young dreissenid bivalves was observed and in 2004 dense assemblages consisting of adult M. leucophaeata were discovered in an area affected by cooling water discharges from a nuclear power plant. The introduction of the species has obviously taken place via ballast water transport, resulting in a successful establishment in a favourable warm water environment. Based on the wide salinity tolerance, M. leucophaeata might also colonize areas inhabited by functionally similar bivalves if able to survive the cold winter conditions.
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Aquatic Invasions (2006) 1: 38-41
© 2006 European Research Network on Aquatic Invasive Species
First record of the brackish water dreissenid bivalve Mytilopsis leucophaeata
in the northern Baltic Sea
Ari O. Laine1*, Jukka Mattila2 and Annukka Lehikoinen2
1Finnish Institute of Marine Research, P.O. Box 2, FIN-00561 Helsinki, Finland
2STUK - Radiation and Nuclear Safety Authority, Research and Environmental Surveillance,
Laippatie 4, P.O. Box 14, FIN-00881 Helsinki, Finland
*Corresponding author, E-mail:
Received 11 January 2006; accepted in revised form 23 January 2006
Conrad’s false mussel, Mytilopsis leucophaeata has been found in the central Gulf of Finland, which is
the first record of this brackish water dreissenid species in the northern Baltic Sea. In 2003 a strong
recruitment of young dreissenid bivalves was observed and in 2004 dense assemblages consisting of
adult M. leucophaeata were discovered in an area affected by cooling water discharges from a nuclear
power plant. The introduction of the species has obviously taken place via ballast water transport,
resulting in a successful establishment in a favourable warm water environment. Based on the wide
salinity tolerance, M. leucophaeata might also colonize areas inhabited by functionally similar bivalves
if able to survive the cold winter conditions.
Key words: Mytilopsis leucophaeata, Dreissenidae, invasions, Baltic Sea, cooling waters
Conrad’s false mussel, Mytilopsis leucophaeata
(Conrad 1831) (Bivalvia, Dreissenidae) is a
brackish water species with an original dis-
tribution in the subtropical and temperate Gulf of
Mexico area (Marelli and Gray 1983). The
current distribution along the North Atlantic
west coast extends in north to Massachusetts in
the United States (Smith and Boss 1996). The
first record of the species in Europe was made in
Belgium already in 1835 (Nyst 1835, cited in
Verween et al. 2005) where it was most probably
transported by shipping. In north western Europe
M. leucophaeata currently occurs in estuaries
along the North Sea coast from Germany to
France and in Great Britain (Oliver et al. 1998,
Verween et al. 2005). Recently, the species has
also been recorded in Spain (Escot et al. 2003),
and in the Black Sea (Therriault et al. 2004). In
the Baltic Sea area the species was first detected
in Kiel Canal connecting the North and Baltic
Seas (Boettger 1933, Schlesch 1937) and later in
Kaliningrad area (Brohmer 1962) but the latter
population has probably gone extinct. Recently,
a local but obviously established population was
found in river Warnow estuary, northern Ger-
many (Darr and Zettler 2000). In this study we
present the first finding of M. leucophaeata in
the northern Baltic Sea.
M. leucophaeata is a highly euryhaline species
occurring from fresh water to salinities ex-
ceeding well 20 ‰. According to Siddall (1980),
the species is able to complete larval develop-
ment in salinities up to 32 ‰. The species
occurrence in Europe is obviously much concen-
trated to estuarine areas with fluctuating salinity
conditions. In e.g. Belgium, M. leucophaeata has
established vigorous fouling communities in
conditions where salinity during the reproductive
period varies from 0.8 to 10.3 ‰ (Verween et al.
2005). The population in the German Baltic Sea
coast was found in an area where salinity may
fluctuate between 0.3 and 8 ‰ within days (Darr
and Zettler 2000). Due to the wide salinity
tolerance M. leucophaeata has been reported to
coexist both with M. edulis and D. polymorpha
in Europe (Jenner and Janssen-Mommen 1993,
Van der Velde et al. 1998, Darr and Zettler 2000,
Rajagopal et al. 2005). In North America the
A. Laine et al., Mytilopsis leucophaeata in the Baltic Sea
distribution of M. leucophaeata and D. polymor-
pha overlap especially in salinities below 3.0 ‰
(Pathy and Mackie 1993).
In western Europe M. leucophaeata has be-
come a serious biofouling organism in cooling
water systems with economical impacts for water
dependent industries (Jenner and Janssen-Mo-
mmen 1993, Verween et al. 2005). The species
is obviously more tolerant to common control
methods (e.g. chlorination and heat treatment)
than the zebra mussel, Dreissena polymorpha or
the blue mussel, Mytilus edulis (Rajagopal et al.
2002, 2005).
Observations in the Gulf of Finland
In autumn 2003, a pronounced settlement of
young dreissenid mussels was observed in Lo-
viisa archipelago, northern coast of the central
Gulf of Finland, leading to a dense adult popu-
lation thereafter. The species was first identified
as zebra mussel, Dreissena polymorpha, which
had been recently increasing in numbers in the
eastern Gulf of Finland (Antsulevich et al. 2003).
However, an examination of adult mussels col-
lected in December 2004 at the island of Häst-
holmen, Loviisa (60°22.1’ N, 26°21.3’ E) reveal-
ed the species to be the very similar looking
Conrad’s false mussel, Mytilopsis leucophaeata
(Conrad 1831). The species has not been pre-
viously observed in the northern Baltic Sea.
The identification of mussels was based on
characteristics presented in Marelli and Gray
(1983). In general, M. leucophaeata resembles
much D. polymorpha and especially young indi-
viduals are difficult to distinguish. Adult indivi-
duals found are usually brownish in colour
without the stripe patterns that are typical to
young individuals. The shell shape of M. leu-
cophaeata is less angular than in D. polymorpha
and resembles more the form of M. edulis.
M. leucophaeata shells do not have a longitu-
dinal ridge typical to D. polymorpha (Figure 1).
However, the exact species identification of
M. leucophaeata is based on an internal shell
structure. Both D. polymorpha and M. leucop-
haeata have in the anterior end a myophore plate
used for muscle attachment but in M. leuco-
phaeata there is also dorso-laterally closely
attached a tooth-like projection, an apophysis
(Figure 2), which is absent in D. polymorpha.
Microscopic examination of this structure is
needed for species identification.
Figure 1. Shells of Conrad’s false mussel, Mytilopsis
leucophaeata. The length of the largest individual is about
20 mm. The young individuals have typical stripe patterns
that resemble much zebra mussel, Dreissena polymorpha.
The adult individuals are usually even brownish in colour
and the shell shape is less angular than in D. polymorpha.
Figure 2. Conrad’s false mussel, Mytilopsis leucophaeata
with opened shells. The arrows point the tooth-like projec-
tion, apophysis, in the anterior end of the animal.
The occurrence of M. leucophaeata in the Gulf
of Finland has been confirmed so far in the sea
area from Loviisa to archipelago of Pernaja. The
densest assemblages of M. leucophaeata found
since 2003 are clearly located in a coastal semi-
enclosed bight that is directly affected by the
cooling waters of the Loviisa nuclear power
plant. Near the power plant the seawater surface
salinity is about 4.3 ‰ and during summer the
water temperature may exceed 25°C. In winter
the water temperature is about 5°C keeping this
area ice free in contrast to typical winter con-
ditions in the Gulf of Finland. The mussel
assemblages, observed by scuba diving and sam
A. Laine et al., Mytilopsis leucophaeata in the Baltic Sea
pling, covered stones and boulders up to 100 %
and were mainly found in the depth of 1-3 m.
In these populations densities and biomass up
to 28 000 individuals/m2 and 9.8 kg/m2 (wet
weight), respectively, were observed. These
numbers are clearly higher than those found in
the southern Baltic Sea (50 ind./m2; Darr and
Zettler 2000). In Loviisa, high numbers of
M. leucophaeata were also found attached on the
macroalgae Fucus vesiculosus. The maximum
length of mussels sampled in December 2004
was 21 mm and most of the individuals were
larger than 10 mm. M. leucophaeata may reach a
maximum size of about 22 to 25 mm in length
(Siddall 1980, Pathy and Mackie 1993). Based
on measurements of individuals from the 2005
recruitment the growth rate of M. leucophaeata
may be high. A maximum length of 17 mm was
recorded for first-year mussels in the end of the
growing season.
Due to the long distance to the nearest abundant
M. leucophaeata populations in the North Sea
area and local populations in the southern Baltic
Sea, it is obvious that M. leucophaeata has been
transported to the Gulf of Finland as larvae in
ships ballast water as spread by natural means is
unlikely over such long distances. The first find-
ings were located only a couple of kilometres
from the port of Loviisa. The cooling water
discharge area has obviously offered a favour-
able environment for survival and reproduction
of the species and can act as a bridgehead for
further spread of the species. The present find-
ings also demonstrate that cooling water areas
are potential hot spots for introduction of alien
species originating from warm waters, as pre-
dicted by Gollasch and Leppäkoski (1999). The
current occurrence of M. leucophaeata is located
at the high-salinity tolerance limit of D. poly-
morpha and low-salinity tolerance of M. edulis.
Thus it seems that M. leucophaeata has found an
open ecological niche in the area as a hard
bottom filter-feeding bivalve. Due to the wide
salinity tolerance, M. leucophaeata would be
able to widen its distribution, which could lead
to resource competition with two other mussel
species in the Gulf of Finland. Based on settle-
ment studies with artificial plates in 2004-2005 a
wider distribution of young, first-year M. leucop-
haeata has been observed. After the successful
reproduction in 2005, mussels were also found
on natural substrates of rocks and stones in the
sea area of Pernaja about 15 km southwest from
Loviisa. Future studies will show if M. leucop-
haeata is able to establish adult populations also
in this area, which is not affected by the cooling
water discharges.
Based on its original distribution M. leucop-
haeata is considered a warm water species able
to live in temperate areas also (Marelli and Gray
1983). It is however possible that the majority of
the mussels do not survive harsh winter condi-
tions since the distribution of old individuals
found in this study seems to be much restricted
to the area affected by warm cooling waters from
the power plant. Temperature affect e.g. repro-
duction and according to Verween et al. (2005)
the gamete maturation starts at about 13°C,
which is slightly higher than for D. polymorpha.
The lower temperature limit for the survival of
juvenile and adult M. leucophaeata is not known.
This factor might limit the establishment of
permanent populations in a highly seasonal envi-
ronment where winter temperatures fall close to
0°C, as in the northern Baltic Sea. However,
more field investigations are needed to verify the
current distribution of M. leucophaeata and its
dependence on warm water discharges. Also,
special attention should be paid on species iden-
tification of especially young dreissenid mussels
to reveal the large-scale distribution of M. leu-
cophaeata in the Baltic Sea.
S. Rajagopal (University of Nijmegen, The Net-
herlands) and A. Verween (University of Gent,
Belgium) are kindly acknowledged for help
with species identification. We are also grateful
to K. Huusela (STUK, Finland) for his invol-
vement in the studies.
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... During the winter period the temperature is far below the lower temperature tolerance of M. leucophaeata (below 5 °C) (Zhulidov et al. 2015). These temperatures indicate that the lower temperature limits for M. leucophaeata are not defined well enough (Laine, Mattila, and Lehikoinen 2006) or that M. leucophaeata is very adaptable to temperature. They are very important for future habitat predictions. ...
... (Verween, Vincx, and Degraer 2010). Jaeckel 1962;Steusslof 1939;Zilch andJaeckel 1962 1993 Guadalquivir River, Spain Escot et al. 2003Escot et al. 1999 Thames River Estuary, E Coast, United Kingdom Bamber and Taylor 2002 2002 Black Sea Basin, Dniester Liman, Ukraine Therriault et al. 2004Therriault et al. 2003 Loviisa Archipelago, Gulf of Finland Laine, Mattila, andLehikoinen 2006 2009 Caspian Sea, Iran Heiler, Nahavandi, andAlbrecht 2010 2010 Gulf of Gdańsk (southern Baltic Sea), Poland Dziubińska 20112011Southern Bothnian Sea, Sweden Florin et al. 2013 ...
... Population densities of Dark Falsemussel in newly invaded European habitats generally greatly exceed population densities in their native North American range, a pattern that is perhaps characteristic of newly invaded versus native habitats (Kennedy, 2011). Similarly, Laine et al. (2006) observed up to 28,000 individuals m -2 near a power plant cooling water system in Finland. In contrast, the Hudson River has reported density ranges from 1-25 (at 0-3 ppt), 100-200 (5-9 ppt) to 1,000-2,000 (2-6 ppt) individuals m -2 (Walton, 1996). ...
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... The bivalve family Dreissenidae includes two genera that have a long history of becoming invasive species, Dreissena (Beneden, 1835) has invaded fresh waters of Eurasia and North America (Locard 1893;Hebert et al. 1989;Mills et al. 1996;Bij de Vaate et al., 2002;Carlton, 2008;Benson 2009;Higgins and Vander Zanden 2010). Mytilopsis (Conrad, 1831) has invaded brackish waters of Western Europe, Western equatorial Africa, South America, the Indo-Pacific region and, recently, Eastern Europe (Laine et al., 2006;Nuttall, 1990;Verween et al. 2010;Mumladze et al., 2019;Tan and Tay, 2018;Fernandes et al. 2020). In 1967, a species of Mytilopsis was discovered in Visakhapatnam, India and identified as Mytilopsis sallei (Récluz, 1849) based on unreported morphological criteria (Ganapati et al. 1971;Morton 1970). ...
An invasive bivalve of the genus Mytilopsis has been present in the Indo-Pacific region since at least 1870, although most of the literature on this species states that it invaded the Pacific from the Caribbean through the Panama Canal after 1914. Although species of Mytilopsis are difficult to identify using morphological characters, difficult does not mean impossible. Comparisons of recently collected specimens as well as museum holdings suggest that the invasive Indo-Pacific bivalve is Mytilopsis adamsi, although further research including genetic analyses can help to answer this question. Review of literature and historical records support the interpretation that the invasive species of Mytilopsis in the Indo-Pacific probably originated in the eastern Pacific and not the Caribbean.. If phylogenetic constructions within the family Dreissenidae have used genetic sequences from a misidentified species they must be revised.
... This is a characteristic not previously apparent in its dispersal history. The water temperature in the south-eastern part of the Black Sea (see Figure 1, Locality #3 and #4) and in the middle part of the Caspian Sea (Figure 1, Locality #5) is within commonly known tolerance limits of M. leucophaeata (5-30 °C) (Simonov and Altman 1991;Terziev et al. 1992;Laine et al. 2006; unpublished data of the authors). However, mortality peaks at temperatures close to 0 °C, up to almost complete extinction of the population, were noted in experimental studies and field observations (Van der Gaag et al. 2016 and literature therein). ...
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... This species has its original distribution in the subtropical and temperate Gulf of Mexico area and is tolerant to the entire salinity range found in the Baltic Sea. In Western Europe, it has become a serious biofouling organism in cooling water systems, with economic impacts for water-dependent industries (Laine et al. 2006). The species may benefi t from the future expected climate changeassociated temperature increase. ...
... In marine and estuarine harbors worldwide, invasive Mytilopsis spp. exert similar ecological and economic fouling problems as do zebra and quagga mussels (Laine, Mattila, & Lehikoinen, 2006;Sousa, Gutiérrez, & Aldridge, 2009). In addition to the well-known zebra and quagga mussels, two Dreissena species native to the Balkans (D. blanci and D. carinata) recently expanded their ranges and may become invasive throughout Europe (Wilke et al., 2010). ...
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Water Mussel, is a mytiliform bivalve (Mollusca, Bivalvia, Veneroida, Dreissenidae), which produces strong byssus to attach to hard substrates. Mytilopsis leu-cophaeata is a typical estuarine species, and thus resistant to a wide range of oligo-to mesohaline conditions (1). The species originates from the southern coast of the U.S. to Tampico, Mexico (2). In 1835, it was first detected in Europe, in the harbour of Antwerp (3). After a period of apparent absence, M. leucophaeata is currently found along the coast of the North Sea from Germany into France and recently in Great Britain (4). Ballast water discharges from ships were identified as a major vector in the transfer of nui-sance aquatic species, such as M. leucophaeata, from one area of the world to another. The fact that the species was not detected in Belgian waters over more than 50 years does not necessarily indicate the absence of M. leu-cophaeata along the European coast. Because of the mor-phological resemblance with the closely related Dreis-sena polymorpha, the Zebra Mussel, species-confusion may have arisen. When M. leucophaeata became an eco-nomic problem in the nineties as an important industrial fouler, attention was brought back to this relatively unknown species. Any surface exposed to untreated water provides an opportunity for the settlement and subsequent growth of organisms. Because of the high temperature and the con-stant supply of food and oxygen, cooling water systems are an ideal habitat for M. leucophaeata. Given these per-fect conditions, settlement occurs readily and growth can be rapid until it causes fouling at the heat exchangers and the tubes in the conduits and finally leads to the failure of the operational systems. This phenomenon is known as biofouling (5). Of all organisms causing fouling in cool-ing systems, mussels are known to cause the most serious problems (6). The freshwater Zebra Mussel D. polymorpha causes major fouling problems in freshwater lakes and great riv-ers in the U.S.. Hence, the biology and possible control methods of the species are well examined throughout the years. Brackish water species, on the other hand, are far more resistant to environmental changes, which makes them particularly robust fouling species. The most effec-tive and cheap control measure is the use of chlorination. It was only when the legislation on biocide draining became stricter (VLAREM II, 4.2.4., VLAREM II, annex 2.3.1.), that the magnitude of the biofouling problem by M. leucophaeata in the harbour of Antwerp became clear. In the near future, specific research on cooling water draining will be conducted and standard concentrations will be lowered. When the legislation on biocide draining in Belgium will get stricter, the use of merely chlorine will no longer be effective against biofouling. Other, (more expensive) methods have to be searched for to pre-vent fouling problems, caused by M. leucophaeata. Adult mussels can shut their protective shell valves and stop byssus production to isolate their body from changes in the external environment (7), such as biocide-passage. The planctonic larvae and plantigrades are the most vul-nerable life stages, and thus susceptible to the biocides. Hence, knowledge on the cyclic presence of M. leu-cophaeata larvae provides a basis for an ecologically and economically proper use of these detrimental chemicals (8).
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Though the Conrad's false mussel, Mytilopsis leucophaeata, is an important fouling animal in industrial cooling water systems, there are no published reports on the tolerance of this species to chlorination. A series of experiments was conducted to determine the effects of mussel size (2-20 mm shell length), season (breeding versus nonbreeding), nutritional status (fed versus starved) and acclimation temperature (5-30 degrees C) on the mortality pattern of M. leucophaeata under continuous chlorination (0.25-5 mg/L). The effect of mussel size on M. leucophaeata mortality in the presence of chlorine was significant, with 10 mm size group mussels showing greater resistance. At 0.25 mg/L residual chlorine, 2 mm size group mussels took 89 days to reach 100% mortality, whereas 10 mm size group mussels took 109 days. M. leucophaeata collected during nonbreeding season (December-April) was more tolerant to chlorine than those collected during breeding season (June-October). Nutritional status of the mussel had no significant influence on the chlorine tolerance of the mussel: fed and starved mussels succumbed to chlorine at equal rates. The effect of acclimation temperature on M. leucophaeata mortality in the presence of chlorine was significant. At 0.5 mg/L residual chlorine, mussels acclimated at 5 degrees C required 99 days to reach 95% mortality, whereas mussels acclimated at 30 degrees C required 47 days. A comparison of present data with previous reports suggests that resistance of M. leucophaeata to chlorination is higher than other mussel species causing fouling problems in The Netherlands ( Mytilus edulis and Dreissena polymorpha).
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The dark false mussel, Mytilopsis leucophaeata is an important mussel colonising the brackish-water systems of temperate and subtropical regions. Of late it has earned notoriety as a biofouling species in industrial cooling water systems. However, there are no published data on the temperature tolerance of this species. This paper presents data on the upper temperature tolerance of this mussel from the view point of biofouling control using thermal methods. In addition to mortality, response of physiological activities such as oxygen consumption, filtration rate, foot activity and byssus thread production were also studied at temperatures varying from 5 to 35°C. Experiments were also carried out to understand the effect of mussel size, breeding condition, nutritional status and acclimation conditions (temperature and salinity) on the mortality pattern. The physiological activities were significantly reduced at temperatures beyond 27.5°C and ceased at 35°C. In 20 mm size group mussels exposed to 37°C, 50% mortality was observed after 85 min and 100% mortality after 113 min. The effect of mussel size on mortality at different temperatures was significant, with the larger size group mussels showing greater resistance. M. leucophaeata collected during the non-breeding season (December-April) were more tolerant to temperature than those collected during the breeding season (June-October). Nutritional status of the mussel had no significant influence on the thermal tolerance of the mussel: fed and starved (non-fed) mussels succumbed to temperature at comparable rates. The effect of acclimation temperature and acclimation salinity on M. leucophaeata mortality at different temperatures was significant. Survival time increased with increasing acclimation temperature and decreased with increasing salinity. In comparison with other co-occurring species such as Mytilus edulis and Dreissena polymorpha, M. leucophaeata appears to be more tolerant to high temperature stress.
Mytilopsis leucophaeta (Conrad, 1831) is recorded for the first time from British waters. It has been found in a brackish dock in south Wales and is associated with other alien taxa.
Considerable uncertainty exists in determination of the phylogeny among extant members of the Dreissenidae, especially those inhabiting the Ponto-Caspian basin, as multiple systematic revisions based on morphological characteristics have failed to resolve relationships within this group of bivalves. In this study we use DNA sequence analyses of two mitochondrial gene fragments, 16S rRNA and cytochrome c oxidase subunit I (COI), to determine phylogenetic relationships among Dreissena rostriformis, D. bugensis, D. polymorpha, D. stankovici, Congeria kusceri, and Mytilopsis leucophaeata. Dreissena stankovici was determined to represent a sister taxa to D. polymorpha and both are more closely related to other extant Dreissena species than Congeria or Mytilopsis. Sequence divergence between D. rostriformis and D. bugensis was relatively low (0.3-0.4%), suggesting that these two taxa constitute a single species. However, environmental differences suggest two races of D. rostriformis, a brackish water race (rostriformis) and a freshwater race (bugensis). Spread of bugensis-type individuals into habitats in the Caspian Sea that are occupied by rostriformis-type individuals may create novel hybridization opportunities. Species-specific molecular markers also were developed in this study since significant intraspecific variation in morphological features complicates dreissenid identification. Using two gene fragments (nuclear 28S and 16S), we identified restriction fragment length polymorphisms (RFLPs) that distinguish among D. rostriformis/bugensis, D. polymorpha, and D. stankovici and revealed the presence of a cryptic invader to the Black Sea basin, Mytilopsis leucophaeata. This is the first report of this North American native in southern Europe.