Mnemiopsis leidyi - American comb jelly
SCIENTIFIC NAME
Mnemiopsis leidyi A. Agassiz, 1865
2001 - 2010
NW Atlantic
SW Atlantic
Shipping (ballast water)
The original habitat of the warty comb jelly is the temperate to subtropical eutrophic estuaries and coasts along the Atlantic side of North and South America. In North America, the species occurs from Rhode Island to the Caribbean, while in South America, it occurs along the Brazilian and Argentinian coasts [2, 3].
First observation in Belgium
The presence of the warty comb jelly in our study area got demonstrated by genetic analysis of samples of comb jellies taken between August and November 2006 in the Western Scheldt near Borssele (the Netherlands) [4]. The first observation in Belgium dates from 2007, in the port of Zeebrugge [5].
Spreading in Belgium
Two years after the first observation of the warty comb jelly, it was present along the entire Belgian coast, both in the ports and near the offshore windfarm C-Power (Thornton Bank), 27 km seaward. Observations of adult individuals in the cold winter months – even during the exceptionally cold winter of 2010 – indicate that the species can easily survive our winters. This alien species can occur locally in large numbers: in October 2010, 17 warty comb jellies per m3 were seen swimming in the Sluice Dock of Ostend, while at sea, a maximum of one individual per m3 was reported [6, 7].
In 2011 and 2012, a big sampling campaign was launched: monthly plankton samples were taken at several stations in the Belgian part of the North Sea [8]. Results showed that M. leidyi mainly occurs from August to December. This comb jelly has been found in the ports of Ostend, Zeebrugge and the Western Scheldt, but never more than 30 km offshore. Population densities were – in comparison to the Black Sea – relatively low (≤ 0.5 individuals per m³). Relatively high densities were observed in September 2012 in the port of Ostend (up to 18 individuals per m³) and in October 2012 in the Western Scheldt estuary (up to two individuals per m³). It is not a coincidence that these locations are half-open basins with mild hydrodynamics since these conditions stimulate the growth of large populations. The Western Scheldt basin and the ports of the Belgian coast function as breeding grounds for the warty comb jelly. This was confirmed by the findings of the larvae of M. leidyi during the autumn and winter months in these areas [8].
In the fall of 2014, the spring and fall of 2016, and the fall of 2018, large numbers of the warty comb jellies were found with a 22 mm bottom trawling net at the Thornton and Hinder Banks (respectively 30 and 60 km seawards) during the Belgica monitoring campaign of ILVO [9].
Areas with high densities of this alien ctenophore can potentially suffer from its predation and competition pressure [8]. It might be that the North Sea is just as vulnerable as the Black Sea due to the high anthropogenic pressure [10-15]. A slight change in environmental conditions can cause an exponential increase of this species [16]. Comb jellies that feed on other comb jellies, such as the native Beroe gracilis (i.e. a natural enemy of the sea gooseberry Pleurobrachia pileus), could serve as a natural enemy of M. leidyi [17]. Due to the predation by the comb jelly Beroe ovata was an M. leidyi outbreak prevented in the Black Sea [8, 18, 19].
Spreading in neighbouring countries
The first introduction to Europe occurred in the Black Sea. Here, the warty comb jelly arrived via the ballast water of ships. The species was observed in this region for the first time in 1982 [20]. By the summer of 1988, the species had spread across the Black Sea and established average densities of 310 comb jellies per m² (about 1 kg wet weight per m3) [20]. The species further migrated to adjacent basins via straits [21] (figure 1).
Figure 1: The distribution of Mnemiopsis leidyi in western Eurasian waters from 1990 to 2016 based on 12,400 geo-referenced observations (black dots). Individual observations of single animals or environmental DNA in the period 2014-2015 are indicated by orange dots. The species is absent in the regions marked in blue and present in the red regions (Source: Jaspers et al. 2018) [21].
The exponential growth of M. leidyi got facilitated by the local overfishing of European anchovy, causing more zooplankton to be available for the comb jellies. Moreover, extensive agriculture caused anoxic bottom waters. These conditions are advantageous to the tolerant M. leidyi, whose prey gets more sensitive and thus more easily caught [22].
In the Black Sea, the warty comb jelly caused a shift in the benthic biodiversity. This comb jelly preys on the larvae of bivalves. When its population collapsed in 1999 due to the predation of Beroe ovata, the bivalves did not return. Instead, an ecosystem dominated by polychaetes arose [23].
The warty comb jelly was first observed in the Aegean Sea (Mediterranean Sea) in 1990 [23]. From there, the species quickly spread to the Levantine Sea (eastern part of the Mediterranean Sea) [24]. In 2005, M. leidyi was present in the Adriatic Sea and the coastal waters of France (figure 1) [25-27]. Meanwhile, the warty comb jelly had reached the Caspian Sea (1999) [23]. In 2009, large blooms of this species were reported from several places in the Mediterranean, such as Italy [28] and Spain [29]. Its fast spread in the Mediterranean Sea is probably the result of the combination of transport with ballast water and natural currents [29].
The warty comb jelly was likely also transported with ballast water from the northwestern Atlantic Ocean to the Baltic Sea and the North Sea, where it was first observed in the early 2000s [4, 30, 31].
In the Netherlands, this comb jelly was first photographed in 2004 and later identified as the warty comb jelly [32]. In July 2005, this species of comb jelly was found again – and found even more frequently since August 2006 – in Zeeland (e.g. in Lake Grevelingen) [4]. In late July 2006, comb jellies caught in Lauwersoog (Wadden Sea) were identified as warty comb jellies [33]. The species was very abundant in the Wadden Sea, and it is believed that the warm summer of 2006 was the reason for the bloom of the warty comb jelly.
It might be that the warty comb jelly was already present earlier on in Dutch waters. However, due to negligence and lack of knowledge, it could not be distinguished from similar comb jellies [4]. Young individuals can be confused with sea gooseberries Pleurobrachia pileus. Larger individuals might look like the common northern comb jelly Bolinopsis infundibulum, although this species is quite rare in our regions [4].
The German waters are also inhabited by this comb jelly (figure 1). The species was found in several locations in the North Sea and the Baltic Sea [34-37]. It was remarkable that this alien species had been able to survive the winter in these areas. Between January and May 2007, it appeared that 80% of the captured individuals were young animals, with a body length of less than one millimetre. This means that this alien species may have established itself permanently in the southern Baltic Sea.
The species spread further north in the Baltic Sea, up to the west coast of Sweden [4]. The comb jellies are also observed in the Oslofjord (Norway) [35]. In England and along the Atlantic shores of France and Spain, the warty comb jelly has not yet been observed (figure 1) [35].
It is believed that this non-indigenous comb jelly got passively transported in the ballast water of cargo ships [35]. Genetic analyses indicated that multiple (primary) introductions occurred in Eurasian waters. The comb jellies in the Black Sea originate from Central America (Gulf of Mexico), and those in the north and west European seas originate from North American waters [30, 31].
The warty comb jelly is a hermaphrodite and can self-reproduce, which entails that a single comb jelly can give rise to a whole new population [38, 39]. This comb jelly can produce up to 8,000 eggs in 23 days, and offspring reach sexual maturity after only 13 days [38]. Such a high development rate requires a high food intake. The warty comb jelly is not very selective about its food [40]. Their prey consists mainly of zooplankton, but also includes fish eggs and larvae [41].
The warty comb jelly has few demands regarding its environment. The species has a high tolerance to varying environmental factors like temperature, salinity, and pollution. In combination with their rapid development, the warty comb jelly has the potential to outcompete other species (like zooplanktivorous fishes) and exerts a high predation pressure on its prey [40-42]
The warty comb jelly occurs in waters where the average year temperature fluctuates between 0 and 32°C [43-45] and where the salinity varies from 2 to 40 PSU [29, 46, 47]. By comparison, the North Sea has a salinity of about 35 PSU. The salinity of the North Sea in combination with a water temperature of about 20°C make for the ideal conditions for the occurrence and survival of this species [48].
The invasive success of the warty comb jelly appears to be influenced by the intensity and the duration of winter [49]. Despite its high tolerance to environmental conditions (temperature, salinity, and oxygen (up to 1 mg/l)) [29, 45, 48], unfavourable combinations constrain this comb jelly’s survival. The species cannot survive the winter in the Sea of Azov, where the salinity near the surface varies between 0 and 14 PSU and the temperature is less than 4°C [25].
Very cold winters or too low temperatures in the open sea obstruct the settlement of this species in our regions. Biological factors, such as food supply and the presence of predators, also influence the distribution of the warty comb jelly [29, 46].
Furthermore, the warty comb jelly displays many characteristics typical of pest species: (1) extensive native range with high and low temperature extremes, (2) the ability to reproduce very rapidly, (3) the ability to thrive in both marine and estuarine (brackish) waters as well as in eutrophic and polluted waters, and (4) good dispersal capacities that allow the species to spread further after establishment [50]
In the Black Sea, M. leidyi is kept under control by two factors. The first one is its natural enemy – the comb jelly Beroe ovata – which colonised the area. On the other hand, fertilisation reduced dramatically due to the fall of communism, which reduced the inflow of nutrients [22].
The warty comb jelly grows up to 18 cm in its original habitat, which is quite large for a comb jelly [32]. Until now, in our regions, no individuals larger than 7 cm – measured from the oral to aboral side (without lobes) – were observed [8]. This comb jelly is cylindrical, with two moving spoon-like lobes [33]. Each of these lobes has four comb rows, which give these animals their name (comb jellies). These combs consist of groups of cilia, which are necessary for locomotion. When touched, these combs can intensely phosphorate and glow green (bioluminescence) [51]. There is a central gelatinous column, from the mouth on the posterior end – where the gut begins – to the anterior end of the animal. To orient themselves in the water column, comb jellies use a special balance organ, called the statocyst.
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