Mytilopsis leucophaeata - Conrad's false mussel
SCIENTIFIC NAME
Mytilopsis leucophaeata (Conrad, 1831)
1800 - 1900
NW Atlantic
Shipping (hull fouling)
The genus Mytilopsis was already present in Europe 60 million years ago [2]. Some 30 million years ago, America got colonised by the European populations. In America, Conrad’s false mussel M. leucophaeata occurs in estuaries along the Atlantic coast from the Hudson to the Gulf of Mexico. The Mytilopsis genus got extinct in Europe approximately five million years ago. However, recently, the species returned to our regions due to human activities [3-5].
First observation in Belgium
In 1835, Conrad’s false mussel got observed for the first time in the port of Antwerp. Then, the species was misidentified as a new species, namely Mytilus cochleatus [6]. It took some time before it got observed again. Then, in 1960, the species got observed again near Nieuwpoort [4, 7].
Spreading in Belgium
In Belgium, Conrad’s false mussel is mainly found in the cooling water systems of a few companies in and around the port of Antwerp [8]. The species also occurs in Nieuwpoort [7, 9, 10] and the Ghent-Terneuzen Canal [11].
Spreading in neighbouring countries
The first observation in the Netherlands dates back to 1895 in the Amstel River [12]. More recently, the species got reported in cooling water systems of some companies on the Scheldt and Rhine. Furthermore, the species has been found attached to natural surfaces such as rocks and wood. This was the case in the Ghent-Terneuzen Canal, the North Sea Canal and the Delta region [11, 12]. Nowadays, Conrad’s false mussel is also found in the Waal [4].
The species has been found in France, near Caen and Dunkirk. [4, 5], and in German and Ukrainian waters [13]. Juveniles of this species got discovered in the summer of 2010 in the Gulf of Gdansk, but it is unclear whether they developed into adults and reproduced [14]. In the United Kingdom, it was first observed in Cardiff (South Wales) in 1996 [15] and subsequently also in Kent [16] and Lincolnshire [17]. However, an English museum collection shows that the mussel was already present in the United Kingdom in 1866, maybe even as early as 1833. This is 150 years earlier than the first official observation [18].
The species is mainly noticed when it causes a nuisance, usually in industrial cooling water systems such as in the Dutch North Sea Canal. The species also occurs in the Spanish Guadalquivir River [19], in the Baltic Sea near Finland [20] and in the southern Bothnian Sea near Sweden [21].
Its frequent occurrence in harbours is a strong indication that this species – as larvae or as adults – get dispersed in ship’s ballast water or attached to the hulls of ships. As ballast water was not used before 1870, the initial colonisation must have been by adults attached to ship hulls [22].
As an estuarine species, Conrad’s false mussel is very resistant to fluctuations in salinity [21]. This alien species also seems to tolerate temperature variations well. This makes Conrad’s false mussel a eurytopic species, which means that it thrives in a range of habitats and environmental conditions [23].
One of the main factors influencing the spread of this alien species is the presence of a suitable substrate for attachment. Conrad’s false mussel does not have a buried lifestyle, but lives attached to hard structures (similar to the common blue mussel Mytilus edulis). Scientists call this an ‘epifaunal’ lifestyle. Conrad’s false mussels attach themselves to rocks and aquatic plants, but the largest numbers are found on artificial, slightly irregular surfaces, such as concrete walls and metal pipelines of cooling water installations [4].
The level of salinity tolerance of Conrad’s false mussels depends strongly on the place of occurrence. The individuals occurring in the Scheldt area are found within salinity levels that vary from 0.1 psu (almost fresh water) to 11.7 psu [4]. By comparison, the North Sea has a salinity of approximately 35 psu. In other places, individuals have been found in waters of 31.6 psu. Experimental studies have calculated the optimal (0.2-17.5 psu) and unfavourable (20.0-30.0 psu) salinity ranges of the species [24]. As a consequence, the species does not occur in the high seas, which limits further expansion via natural processes [23]. In freshwater, the mussels have been found in the Lincolnshire Canal [25].
The larvae of Conrad’s false mussel have the capacity to develop at high salinities (of up to 32 psu). Therefore, it is possible for the larvae to survive for a long time in ballast water with high salinities, crossing the ocean and colonising new places [21, 23]. In such a situation, adults undergo a salinity shock (e.g. during transport in ballast water) and only survive for a few weeks. This is still a barrier for covering long distances, but because ship transport becomes ever faster, this barrier may gradually disappear [24].
The Conrad’s false mussel can withstand low minimum temperatures (e.g. 6.8°C in the Scheldt), but a water temperature of at least 13°C is required for reproduction [8]. This makes it difficult for the species to establish and reproduce in colder areas where the water temperature remains below 10°C during summer. The species can only be found in colder areas where human activity artificially raises the water temperature (e.g. near cooling water systems) [24]. The species is a slow coloniser by nature, which is partly due to the salinity barriers of the open sea and freshwater.
Conrad’s false mussel causes fouling problems in many industrial sites in Europe, such as nuclear power plants and chemical factories [4]. The cooling water systems suck up water to cool down certain industrial processes. The larvae of Conrad’s false mussel present in the water column are pumped up during these processes and can easily pass the 1 mm sieve at the entrance of the system [4].
The inside of the cooling systems provides ideal conditions with regard to oxygen, temperature and salinity. There are also no predators and a constant supply of food. Moreover, algae, which compete with mussel larvae for space, cannot thrive in this dark environment. All these factors result in a continues growth of the Conrad’s false mussel population until they hinder industrial processes and actions are needed. The growth slows down the flow rate in the pipes and can cause obstructions [4, 25].
As a remedy to this proliferation, the application of biocides, such as chlorine, is the cheapest and most efficient solution. A problem here is that adult mussels can close their shells and temporarily isolate their bodies. The larvae are more vulnerable and can be controlled using this method [26]. Biocides are best applied during the spawning period(s) when larvae are massively present in the water column and adults are usually weakened as a result of spawning. The use of biocides during spawning periods is considered to be as effective as year-round use. This method can be combined with a prediction of the presence of larvae in the water column based on a changing water temperature, which allows a very targeted control [4].
The current permitted dose of a chlorine biocide (sodium hypochlorite) is 0.2 mg/l [27], but this appears to be too low for effective control of the larvae. A minimum dose that is effective for controlling larvae for a short period of 30 minutes is 0.6 mg/l. Peracetic acid is recommended as an alternative to chlorine biocide. This substance is more easily biodegradable. In addition, it continues to work under various pH and temperature conditions, and is less corrosive than other commercial peracetic acid products. A dose of 1.5 mg/l for one hour does not differ economically from a 15-minute treatment with a dose of 6 mg/l. However, it is suggested to apply the first strategy, as a longer treatment period will kill a larger number of larvae [28, 29].
Conrad’s false mussel is closely related to the freshwater alien mussel Dreissena polymorpha, known to us as the zebra mussel.
The strong similarities between the two species sometimes cause difficulties in identification. The most reliable identification feature for Conrad’s false mussel Mytilopsis leucophaeata is the presence of a protrusion – called an apophysis – along the inside of the shell at the level of the growing point or ‘umbo’ of the shell (figure 1). This protrusion is shaped like a triangular to sometimes round tooth, to which the retractor muscles are attached [4]. Adult animals can also be distinguished on the basis of shell length: the shell of Conrad’s false mussel tends to be small (1 to 2 cm), whereas the shell of the zebra mussel can usually be 4 cm long [30].
The diet of Conrad’s false mussel consists mainly of phytoplankton and zooplankton. They also feed on bacteria, detritus (dead organic matter) and other organic matter [4].
Figure 1: Top: Conrad’s false mussel Mytilopsis leucophaeata, showing the apophysis (green arrow); Bottom (for comparison): the zebra mussel Dreissena polymorpha. Source: Annick Verween [4].
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