History of introductions in Central/East Europe and non-European Russia

Chinese mitten crabs were first found in European waters in the North Sea, in Germany (Panning, Reference Panning1939; Herborg et al., Reference Herborg, Rushton, Clare and Bentley2003). Currently it is one of the most widespread alien species in European Russian waters and neighbouring countries (Figure 2). It is found in most of the European basins except the Barents Sea. In the Gulf of Finland, the first specimens were recorded in 1933, but since then the species was seldom observed (Herborg et al., Reference Herborg, Rushton, Clare and Bentley2003; Ojaveer et al., Reference Ojaveer, Gollasch, Jaanus, Kotta, Laine, Minde, Normant and Panov2007). This crab was found in the estuary of river Neva in the 1980s (Berezina & Petryashov, Reference Berezina and Petryashov2012), that coincided with the beginning of its regular findings on the coast of Poland, particularly near Gdansk, in the neighbouring Vistula lagoon (Grabowski et al., Reference Grabowski, Jazdzewski and Konopacka2005 and earlier references therein), on the shores of Kaliningrad region (reported here) and in Lithuania (Bacevičius & Gasiūnaitė, Reference Bacevičius and Gasiūnaitė2008), Latvia, Estonia and Finland (Ojaveer et al., Reference Ojaveer, Gollasch, Jaanus, Kotta, Laine, Minde, Normant and Panov2007).

Fig. 2. Records of Chinese mitten crab (Eriocheir sinensis) in the watersheds and coastal marine waters of Russia and Ukraine. Simplified mapping of the data presented by Kamentseva (Reference Kamentseva2002), Panov (Reference Panov2006), Ojaveer et al. (Reference Ojaveer, Gollasch, Jaanus, Kotta, Laine, Minde, Normant and Panov2007), Shakirova et al. (Reference Shakirova, Panov and Clark2007), Berezina & Petryashov (Reference Berezina and Petryashov2012) and Son et al. (Reference Son, Novitsky and Diadichko2013).

From the 1990s and early 2000s reports of this species have become more common in the Neva estuary, Ladoga and Onega lakes that are also interconnected with the Baltic Sea (Panov, Reference Panov2006). In 1998, E. sinensis has been found in the delta of the Northern Dvina River in the White Sea basin (Berezina & Petryashov, Reference Berezina and Petryashov2012 and earlier references therein).

In the Black Sea, mitten crabs have been recorded since 1998, and from the 2000s they have been commonly sighted on the Ukrainian coast in the north-western part of the Danube River, and in Crimean Sevastopol. Two records (2002–2003) are known from the Dnieper River: in the Dnieper and the Kakhovka Reservoirs. At a similar time (1998), it has also been found in the Molochnyi liman of the Sea of Azov (Son et al., Reference Son, Novitsky and Diadichko2013 and earlier references therein; Anosov, Reference Anosov2016). Nearly simultaneously E. sinensis was reported from the mouths of the rivers Don and Manych (Kamentseva, Reference Kamentseva2002).

In the Caspian Sea basin E. sinensis is known from the Volga River, possibly since the 1970s (Shakirova et al., Reference Shakirova, Panov and Clark2007 and earlier references therein). From 1995 until 2007 this crab was regularly sighted mainly in reservoirs and in the delta of the Volga River (Shakirova et al., Reference Shakirova, Panov and Clark2007, and earlier references therein) (Figure 2) and in a river on the Iranian coast (Robbins et al., Reference Robbins, Sakari, Baluchi and Clark2006).

Since the early 2000s Chinese mitten crabs are sometimes found in the Amur River and its side streams near Khabarovsk (Novomodnyi, Reference Novomodnyi2014). Novomodnyi (Reference Novomodnyi2014 and personal communication) suggests that the observed crabs are escapees from nearby Chinese markets in the Amur Basin.

Genetic structure and origin of alien populations

The European populations of the Chinese mitten crab generally show reduced genetic diversity compared with the samples from the native distribution range. Only one sample of E. sinensis that originated from the vicinity of the studied region (Maelaren, Sweden), has been examined for microsatellite loci (Herborg et al., Reference Herborg, Weetman, van Oosterhaut and Hänfling2007b). It showed a remarkable similarity to the mitten crab populations in the estuaries of the rivers Elbe and Weser in the North Sea. This supports the hypothesis of the recruitment of the central Baltic populations of E. sinensis from the North Sea (Herborg et al., Reference Herborg, Weetman, van Oosterhaut and Hänfling2007b; Ojaveer et al., Reference Ojaveer, Gollasch, Jaanus, Kotta, Laine, Minde, Normant and Panov2007).

Contemporary distribution pattern, habitats and habit

Records in the eastern Baltic indicate a geographic pattern of occurrence, i.e. greater abundance of E. sinensis in the Gulf of Riga and the eastern Baltic compared with the Bothnian Bay (Ojaveer et al., Reference Ojaveer, Gollasch, Jaanus, Kotta, Laine, Minde, Normant and Panov2007). There is also a seasonal pattern of recording of this crab by fishermen in the Gulf of Finland and Latvian waters (Ojaveer et al., Reference Ojaveer, Gollasch, Jaanus, Kotta, Laine, Minde, Normant and Panov2007) that may indicate migratory activity.

The Chinese mitten crab widely occurs throughout watersheds and estuarine areas of the Black and the Caspian basins, but little specific information on habitats is present in the literature for these regions. The species is known for its burrowing habit in tidal estuaries (Panning, Reference Panning1939; Dittel & Epifanio, Reference Dittel and Epifanio2009), but no observations on burrows of mitten crabs has been published for the studied region, which is generally a non-tidal environment.

Size and population characteristics

There is limited information on the biology of the Chinese mitten crab in Central and East Europe. Most of the records in the eastern and central Baltic (Normant et al., Reference Normant, Chrobak and Skora2002; Ojaveer et al., Reference Ojaveer, Gollasch, Jaanus, Kotta, Laine, Minde, Normant and Panov2007) and in the White Sea basin (collection of ZIN RAS) consist of large specimens, including ovigerous females (Normant et al., Reference Normant, Chrobak and Skora2002). The specimens found inland are also large; one sighting of an ovigerous female was reported from the inland reservoir of Rybinsk of the Volga River (Shakirova et al., Reference Shakirova, Panov and Clark2007). Larvae have not been recorded from the central or eastern Baltic (Ojaveer et al., Reference Ojaveer, Gollasch, Jaanus, Kotta, Laine, Minde, Normant and Panov2007); accordingly there are no published records of larvae in the southern seas of East Europe.

Symbionts, parasites and pathogens

No information on parasites and diseases of E. sinensis in Russia and neighbouring countries is available in the literature. Numerous nematodes, bivalves, crustaceans, oligochaetes and gastropods were found in the mittens of crabs’ claws in brackish waters. In fresh water the epibiota in the mittens of these crabs consisted of chironomids and water mites (Normant et al., Reference Normant, Korthals and Szaniawska2007).

Position in the ecosystem and impact

In the Baltic (Odra/Oder estuary) mitten crabs have been shown to feed on aquatic plants, copepods and chironomids along with detritus, and they have been found to be able to significantly reduce phyto- and zoobenthic biomass in mesocosm experiments (Czerniejewski et al., Reference Czerniejewski, Rybszyk and Wawrzyniak2010). The ability of crabs to carry numerous epibiotic organisms in their mittens may facilitate dispersal of indigenous and non-indigenous species between different watersheds (Normant et al., Reference Normant, Korthals and Szaniawska2007). While the Chinese mitten crab is known for its significant impact on the estuarine and riverine habitats in West Europe and North America, due to burrowing activity (Panning, Reference Panning1939; Dittel & Epifanio, Reference Dittel and Epifanio2009), there is no such information in the literature for East Europe.

Possible vectors and potential for spread

Chinese mitten crabs in the central and eastern Baltic most likely constitute a pseudopopulation, supplied by migration of crabs from the North Sea or the western Baltic (Ojaveer et al., Reference Ojaveer, Gollasch, Jaanus, Kotta, Laine, Minde, Normant and Panov2007). Otto & Brandis (Reference Otto and Brandis2011) also presented evidence of possible reproduction of E. sinensis in the Kiel Fjord in the western Baltic. Few papers (Shakirova et al., Reference Shakirova, Panov and Clark2007; Berezina & Petryashov, Reference Berezina and Petryashov2012) discuss the possible mechanism of introduction of E. sinensis into the watersheds of East Europe. Berezina & Petryashov (Reference Berezina and Petryashov2012) suggest that a possible vector of introduction could be with ballast water release in the estuary region of the Bay of Finland and the White Sea. Shakirova et al. (Reference Shakirova, Panov and Clark2007: 171) proposed that ‘mature mitten crabs are being transported through the calm European waterways as hull fouling on slow moving vessels such as barges’ and point to several indications of such a vector. The specimens found in the Ladoga and Onega Lakes could thus originate from the Gulf of Finland and enter these big lakes via migration and shipping vectors (Panov, Reference Panov2006; Berezina & Petryashov, Reference Berezina and Petryashov2012). The canal system also makes the entry of crabs from the Baltic to the White Sea basin possible (Northern Dvina watershed). The Baltic–Volga canal is probably the main route of E. sinensis entry into the Volga Basin (Shakirova et al., Reference Shakirova, Panov and Clark2007; Berezina & Petryashov, Reference Berezina and Petryashov2012). The occurrence of the Chinese mitten crab in the north-western part of the Black Sea could be due to the migration of this crab via canals from the North Sea watershed through the Rhine–Main–Danube Canal to the Danube River (Paunovic et al., Reference Paunovic, Cakic, Hegedis, Kolarevic and Lenhardt2004).

There is an alternative vector of introduction to the Northern Dvina and the Sea of Azov, via ballast waters from tankers, which were common in the 1990s due to oil export from the Arkhangelsk port and a transit via Rostov-on-Don. Similar circumstances could have resulted in the appearance of the Chinese mitten crab in the southern Caspian Sea. Released juveniles could survive in the estuaries for several years and move up rivers as they increase in size, where they have been found by local fishermen.

There are no reports of reproducing populations of E. sinensis in the eastern Baltic, White, Black, Azov or Caspian Seas at the present time. Shakirova et al. (Reference Shakirova, Panov and Clark2007) suggested that if a substantial number of ovigerous crabs were transported and released into a suitable environment, then this could speed up the establishment of populations in the Black and Caspian Seas.

History of invasion in Central/East Europe

The introduction of the Harris mud crab to the southern and eastern Baltic is relatively well documented, but mostly by researchers from the countries neighbouring to Russia: Poland, Estonia and Finland. The species was first recorded in the Kiel Canal system in 1936 (Neubaur, Reference Neubaur1936). In 1951 it was found both in Russian and Polish parts of the Vistula Bay (Reznichenko, Reference Reznichenko1967 and earlier references herein), and in 1953 in Dead Vistula, an abandoned terminal part of the channel of the Vistula River (Turoboyski, Reference Turoboyski1973 and earlier references therein). Populations of the Vistula Lagoon and Dead Vistula have been maintained since that time, although the latter showed a decline and a recovery (Grabowski et al., Reference Grabowski, Jazdzewski and Konopacka2005). Although Dead Vistula has a direct connection to the Gulf of Gdansk, R. harrisii was recorded there significantly later, in the early 1960s, and an abundant population only became established during the early 2000s (Hegele-Drywa et al., Reference Hegele-Drywa, Normant, Szwarc and Podłuska2014). The area between the Hela Peninsula (Mierzeja Helska) in Poland and the Sambian (Semland) Peninsula (Kaliningrad Area of Russia) remained the easternmost known area in the Baltic populated by R. harrisii until 2000, when two specimens were found in the Klaipeda Port area in Lithuania (Bacevičius & Gasiūnaitė, Reference Bacevičius and Gasiūnaitė2008). In 2009 the species was found, and by 2011 apparently became established, in the skerries area of the Finnish Archipelago Sea in the eastern Baltic (Fowler et al., Reference Fowler, Forsström, von Numers and Vesakoski2013). At the same time in 2011 there were indications of establishing mud crab population in the Estonian waters: Pärnu Bay, the north-eastern part of the Gulf of Riga (Kotta & Ojaveer, Reference Kotta and Ojaveer2012).

In the north-western Black Sea, the initial dispersal of the species could not be well documented because it probably occurred during World War II. In the late 1940s to early 1950s it was found in the entire Dnieper–Bug and the Berezan limans, and around 1954–1957, the Harris mud crab was reported from the open waters near Odessa (Reznichenko, Reference Reznichenko1967). Simultaneously it was found throughout the Romanian and Bulgarian coasts (Ziemiankowski, Reference Ziemiankowski1951; Reznichenko, Reference Reznichenko1967). In 1969–1985 R. harrisii occurred along the entire continental coast of the north-western Black Sea, in the then Ukrainian Republic of USSR: from the Karkinitsky Bay in the very north-west of the Crimean Peninsula to the Danube Delta, both in limans and relatively open waters (Reznichenko, Reference Reznichenko1967; Makarov, Reference Makarov2004).

Along the Crimean coast of the Black Sea it has only been reported from Sevastopol Bay (Shalovenkov, Reference Shalovenkov2005: Mordvinova & Lozovsky, Reference Mordvinova and Lozovsky2009). In the Sea of Azov the Harris mud crab was first observed in the Taganrog Bay in 1948 (Mordukhai-Boltovskoi, Reference Mordukhai-Boltovskoi1952). However taking into account the duration of World War II (1941–1945) a difference of 11 years between the finding time of the species in the north-western Black Sea and in the Sea of Azov has to be taken with caution (Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a). By the 1950s Harris mud crab became a common species throughout the Sea of Azov (Reznichenko, Reference Reznichenko1967).

In the eastern and southern Black Sea, first populations of R. harrisii have been discovered in the estuaries of the Vulan and Shapsukho Rivers and the Bugaz Liman (Krasnodar area of Russia) in 2012–2013 (Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a) and in the estuary of the river Tuapse in 2016 (G.A. Kolyuchkina and U.V. Simakova, personal communication). It is however unknown when this species was introduced and established in these particular localities.

After the construction of the Don–Volga Canal connecting the Azov and Caspian basins (built in 1952), R. harrisii was found in the northern Caspian Sea in 1958 (Nebolsina, Reference Nebolsina1959). By early 1960s it became widespread and abundant at shallow depths throughout the entire Caspian Sea (Reznichenko, Reference Reznichenko1967). In 1963 R. harrisii was recorded in the coastal lagoon of the Gorgan Bay in the south-eastern corner of the Caspian Sea near the Iranian town Bandar-e Gaz (NHM collection).

From the Sea of Azov, Harris mud crab was unintentionally introduced to another land-locked Aral Sea in the 1970s (first recorded 1976). It was probably transported with species intended to improve food resources for fish in the Aral (Filippov, Reference Filippov2005). However, it was restricted to the southern part of the sea (Andreev & Andreeva, Reference Andreev and Andreeva1988). Harris mud crab was still recorded in the 1980s when the sea underwent a strong regression, increased salinity (about 30 g l⁻¹) and separation into the Large and the Small Aral salt lakes (Andreev et al., Reference Andreev, Andreeva, Filippov and Aladin1992; Aladin et al., Reference Aladin, Plotnikov, Letolle, Nihoul, Zavialov and Micklin2004). Desiccation of the Large Aral continued and apparently R. harrisi has not survived the phase of increasing salinity up to 57 g l⁻¹. The Small Aral is maintaining lower salinity (now 8–11 g l⁻¹) owing to the Syr-Daria River discharge (Izhitskiy et al., Reference Izhitskiy, Zavialov, Sapozhnikov, Kirillin, Grossart, Kalinina, Zalota, Goncharenko and Kurbaniazov2016). It seems that the Harris mud crab had failed to reach the Small Aral before its separation in 1989 (Andreev et al., Reference Andreev, Andreeva, Filippov and Aladin1992). In the expeditions of IO RAS in 2014 (Izhitskiy et al., Reference Izhitskiy, Zavialov, Sapozhnikov, Kirillin, Grossart, Kalinina, Zalota, Goncharenko and Kurbaniazov2016) and 2015 the coastal areas of the Small Aral near village of Akbasty (46°16.5′N 60°54.7′E) was surveyed specifically for R. harrisii, however there have been no sightings.

Genetic structure and origin of alien populations

European populations of R. harrisii are genetically heterogeneous (Projecto-Garcia et al., Reference Projecto-Garcia, Cabral and Schubart2010; Hegele-Drywa et al., Reference Hegele-Drywa, Thiercelin, Schubart and Normant-Saremba2015; Simakova et al., Reference Simakova, Zalota and Spiridonov2017). Populations from the North Sea and some other European waters are more similar in their haplotypes constitution to the New Jersey population, although haplotype A, which is most common in European populations, has not been found in its native populations (Projecto-Garcia et al., Reference Projecto-Garcia, Cabral and Schubart2010). The establishment and dispersion of European populations have been largely determined by the founder effect; nevertheless the inflow of genetic information to the established populations has been substantial. In addition, the possibility of several introduction events should not be excluded (Projecto-Garcia et al., Reference Projecto-Garcia, Cabral and Schubart2010; Hegele-Drywa et al., Reference Hegele-Drywa, Thiercelin, Schubart and Normant-Saremba2015). Rhithropanopeus harrisii in the Black, Azov and Caspian Seas is characterized by low genetic diversity, two unique haplotypes, and most probably originated from some unknown European population (Simakova et al., Reference Simakova, Zalota and Spiridonov2017). The Caspian population is directly related to the Sea of Azov population and probably had multiple episodes of introduction through the Volga-Don canal (Simakova et al., Reference Simakova, Zalota and Spiridonov2017).

Contemporary distribution pattern, habitats and habit

In the north-western Black Sea (from the Danube delta to the isthmus of the Crimean Peninsula: Figure 3) R. harrisii shows a semi-continuous distribution along the coast in: limans, lagoons and estuaries, offshore banks and marine ports (Reznichenko, Reference Reznichenko1967; Makarov, Reference Makarov2004; Vinogradov et al., Reference Vinogradov, Bogatova and Sinegub2012). In the Sea of Azov, the Harris mud crab distribution pattern appears to be nearly continuous over the entire basin, including the northern part of Kerch Strait and the Taman Bay (Figure 3). There, this species occurs practically everywhere at various depth (0–10 m) and habitats, with the exception of oxygen depleted mud habitats in the deepest (10 m and deeper) area (Mordukhai-Boltovskoi, Reference Mordukhai-Boltovskoi1952; Reznichenko, Reference Reznichenko1967; Makarevich et al., Reference Makarevich, Lyubin and Larionov2000; Sergeeva & Burkatsky, Reference Sergeeva and Burkatsky2002; Ivanov & Sinegub, Reference Ivanov and Sinegub2008; Nabozhenko et al., Reference Nabozhenko, Shokhin, Bulysheva, Matishov and Boltachev2010; Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a). In some areas, i.e. the entrance to the Taganrog Bay, according to the data of a grab survey, R. harrisii has even been recognized as a dominant species in the community (Makarevich et al., Reference Makarevich, Lyubin and Larionov2000). The information on the distribution in the Caspian Sea is fragmented (Figure 3), but there are several indications of significant continuity of its population over vast coastal shallow areas (Reznichenko, Reference Reznichenko1967; Yablonskaya, Reference Yablonskaya1985; Malinovskaja et al., Reference Malinovskaja, Filippov, Osadchikh and Aladin1998; Karpinsky, Reference Karpinsky2002; collection of ZMMU).

Fig. 3. Distribution of Harris mud crab (Rhithropanopeus harrisii) in the Black Sea – Caspian region. Shadowed areas indicate basins with more or less continuous distribution of the species (shortage of data for the Iranian waters). Quadrates indicate localized records. For sources see text.

In contrast to this, the distribution pattern in the eastern Black Sea is different and is dominated by localized discontinuous populations associated with river estuaries and probably harbours (Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a; Figure 3). Localized discontinuous populations of R. harrisii are also presently characteristic for the Baltic (Kotta & Ojaveer, Reference Kotta and Ojaveer2012; Fowler et al., Reference Fowler, Forsström, von Numers and Vesakoski2013; Hegele-Drywa et al., Reference Hegele-Drywa, Thiercelin, Schubart and Normant-Saremba2015; Figure 4).

Fig. 4. Distribution of Harris mud crab (Rhithropanopeus harrisii) in the central and eastern Baltic Sea. For sources see text. Quadrates indicate the areas of occurrence of Harris mud crab.

Within the areas of its distribution Harris mud crabs shows a highly opportunistic habitat choice. In the Azov and Black Seas the populated habitats include practically all existing types of substrate from muddy sediments to sand with shell debris and stones, in the seagrass meadows and between roots of reed, or without aquatic vegetation at all (Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a). In the Baltic, they live on: various plant debris; mud and sand with shell; and among Fucus macroalgae on hard bottom (Turoboyski, Reference Turoboyski1973; Fowler et al., Reference Fowler, Forsström, von Numers and Vesakoski2013), which has been its preferred habitat during the experiments performed by Nurkse et al. (Reference Nurkse, Kotta, Orav-Kotta, Pärnoja and Kuprijanov2015). However, the presence of various kinds of shelters appears to be the key factor (Petersen, Reference Petersen2006; Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a), while the patterns of shelter use and hiding habits vary significantly between different sizes and sex groups and localities (Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a).

Few studies have been undertaken on the distribution of its larvae. In the tidal environment (typical for the native area of distribution along the south-eastern coast of North America) larvae of R. harrisii can maintain their distribution in estuarine areas by a complex mechanism of hatching synchronization with the tidal cycle and vertical migration superimposed on differences in velocity and directions of tidal flow (Forward, Reference Forward2009 and references therein). The Baltic, Black, Azov and Caspian Seas are non-tidal basins (Zalogin & Kosarev, Reference Zalogin and Kosarev1999). Field surveys indicate that in the Black Sea, Harris mud crab larvae have restricted distribution in limans, inlets and harbours (Makarov, Reference Makarov2004; Selifonova, Reference Selifonova2012). In the Sea of Azov they occur over the entire area (Makarov, Reference Makarov2004).

Size and population characteristics

Rhithropanopeus harrisii attains greater maximum size (20–26 mm carapace width) in the Baltic, Black and Azov Seas than in its native distribution range; i.e. Chesapeake Bay, Louisiana where males larger than 18 mm, and females larger than 16 mm have not been reported (Fowler et al., Reference Fowler, Forsström, von Numers and Vesakoski2013; Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a). This agrees with a general pattern revealed for most studied marine invasive species, the majority of which become significantly larger in the introduced range compared with the native range with little evidence for any decrease in size following the invasion (Grosholz & Ruiz, Reference Grosholz and Ruiz2003).

Size and age composition, reproductive traits and dynamics of the introduced populations of Harris mud crab have not been much studied. During the laboratory studies of the Baltic Sea specimens, moulting ceased at temperatures below 14°C (Turoboyski, Reference Turoboyski1973). Thus, the individual growth should be seasonal. Two or three size groups (except newly settled juveniles) are usually present in populations of the Dead Vistula, the Azov and Black Seas, which may correspond to year classes; some females reproduce in the 2nd year of their life and most of females in the 3rd year, depending on the temperature (Turoboyski, Reference Turoboyski1973; Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a). Laboratory experiments have shown that they could potentially reproduce within 2 months after hatching (Morgan et al., Reference Morgan, Goy and Costlow1983). Some populations, such as the one in the Dead Vistula, show high pulses of abundance: from being extremely abundant in the 1950–60s to nearly collapsed condition in the 1970–80s and back up in the 2000s (Grabowski et al., Reference Grabowski, Jazdzewski and Konopacka2005). There are also indications of significant changes of this species abundance and biomass in the northern Caspian Sea in the 1960–1980s (Yablonskaya, Reference Yablonskaya1985; Malinovskaja et al., Reference Malinovskaja, Filippov, Osadchikh and Aladin1998; Karpinsky, Reference Karpinsky2002).

Symbionts, parasites and pathogens

The distribution range of native populations of R. harrisii is constrained by the presence of the rhizocephalan parasite Loxothylaxus panopaei whose larvae develop well at 10–15 psu but survive poorly in salinities below 10 psu (Reisser & Forward, Reference Reisser and Forward1991). No indication of this parasite has been found in the Baltic (Fowler et al., Reference Fowler, Forsström, von Numers and Vesakoski2013) nor in the Azov and Black Seas (authors’ data). In the Black Sea Harris mud crab appears to have a specific parasitic gregarine species Cephaloidophora rhithropanopei Belofastova, Reference Belofastova1996 which has not been recorded in other Black Sea crab species (Belofastova & Lozovsky, Reference Belofastova and Lozovsky2008; Mordvinova & Lozovsky, Reference Mordvinova and Lozovsky2009). No other parasite species, including trematode cercaria and native rhizocephalans (common for indigenous decapod species), have been found in R. harrisii (Mordvinova & Lozovsky, Reference Mordvinova and Lozovsky2009).

Position in the ecosystem and impact

Not many publications have dealt with the ecosystem relationships of R. harrisii in spite of its long history as an alien species in Europe. Gut content analyses in the Vistula (Baltic Sea) and the Taman (Sea of Azov) Bays suggest an opportunistic feeding mode with importance of plant detritus and plant remains in the stomach content and indication of common consumption of benthic invertebrates such as polychaetes, crustaceans and molluscs (Czerniejewski & Rybczyk, Reference Czerniejewski and Rybczyk2008; Hegele-Drywa & Normant, Reference Hegele-Drywa and Normant2009; Kolesnichenko, Reference Kolesnichenko2014; Kolesnichenko et al., Reference Kolesnichenko, Burukovsky and Marin2014). However, a stable isotopes analysis of the Taman Bay specimens indicates that Harris mud crab is using mainly animal food, that originates from all main primary producers’ chains, i.e. phytoplankton, benthic algae and seagrass (Zalota et al., Reference Zalota, Spiridonov and Tiunov2013, Reference Zalota, Kolyuchkina, Tiunov, Birjukova and Spiridonov2017). Similar results were obtained, at least for adult crabs, in the Finnish Archipelago Sea (Aarnio et al., Reference Aarnio, Törnroos, Björklund and Bonsdorff2015). In the newly colonized area of the Bay of Finland predatory behaviour of R. harrisii has been shown, but the realized predation pressure in the field is lower than could be expected from laboratory experiments (Forsström et al., Reference Forsström, Fowler, Manninen and Vesakoski2015).

Upon establishing, these crabs became an important component of the diet of some fishes in the north-western Black Sea, the Azov and Caspian Seas (Reznichenko, Reference Reznichenko1967; Yablonskaya, Reference Yablonskaya1985; Karpinsky, Reference Karpinsky2002).

In the Sea of Azov, an indigenous brachyuran species, Brachynotus sexdentatus (Risso, 1824) was common and relatively abundant prior to the introduction of R. harrisii (Vorobyev, Reference Vorobyev1949). Presently this species is not reported in the Sea of Azov where its characteristic habitats are populated by Harris mud crab (Makarevich et al., Reference Makarevich, Lyubin and Larionov2000; Sergeeva & Burkatsky, Reference Sergeeva and Burkatsky2002; Ivanov & Sinegub, Reference Ivanov and Sinegub2008; Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a). However, it is not known if the decline of B. sexdentatus in the Sea of Azov has been related to competitive/predatory pressure from R. harrisii or other factors.

Possible vectors and potential for future spread

Surprisingly, not much is known about the vectors of introduction of Harris mud crab. Initial introduction took place either owing to the transportation on ship hull fouling (Buitendijk & Holthuis, Reference Buitendijk and Holthuis1949) or with American oyster export (Projecto-Garcia et al., Reference Projecto-Garcia, Cabral and Schubart2010). Further spread in European seas was most probably associated with shipping, including traffic via canal systems, such as the Kiel Canal, which connects the North and the Baltic Seas and the Don–Volga Canal, which connects the Azov and the Caspian basins (Reznichenko, Reference Reznichenko1967). In the latter case, it is assumed that R. harrisii is capable of surviving in ship fouling submerged in fresh water and can tolerate such conditions for up to 10 days of the journey. Adaptations of osmotic regulation (Normant & Gibowicz, Reference Normant and Gibowicz2008), established populations in the Texas freshwater lakes (Boyle et al., Reference Boyle, Keith and Pfau2010) and nearly freshwater conditions of the Panama Canal (Roche & Torchin, Reference Roche and Torchin2007) along with a recent report of R. harrisii in the Dnieper reservoir (Son et al., Reference Son, Novitsky and Diadichko2013) indicate that it might well be the case. However, it is also possible that multiple vectors have been involved in the transportation of mud crabs from the Sea of Azov to the Caspian Sea: the shipping via the Don–Volga Canal that commenced in 1952 and unintentional transfer with the clam Abra segmentum (Récluz, 1843). This was introduced to the Caspian Sea from the Sea of Azov as an additional food resource for commercial fish in 1947–1948 (Karpevich, Reference Karpevich1975).

In recent decades, transportation of several invasive crabs in ship fouling was apparently complemented with ballast water (Carlton & Cohen, Reference Carlton and Cohen2003). In this respect an ovigerous female of R. harrisii was found in the ballast water (Briski et al., Reference Briski, Chabooli, Bailey and McIsaac2012).

The present review demonstrates the contrast between the spread of the Harris mud crab in the Black–Caspian Seas region and the Baltic. There was a rapid dispersal in the former region with the formation of practically continuous areas of distribution along the coast of the north-western Black Sea, the entire Sea of Azov and the Caspian coastal area that took about 20 years from the presumed time of its arrival in the region. Colonization of the Baltic has been proceeding about three times slower and resulted in a quite patchy distribution. These differences may result from multiple vectors of spread in the first region: various forms of shipping and dispersal of larvae throughout the extensive shallow basins with wind-driven circulation and unintended artificial introductions (Zalota et al., Reference Zalota, Spiridonov and Kolyuchkina2016a; Simakova et al., Reference Simakova, Zalota and Spiridonov2017). In the Baltic, shipping appears to be the main vector, because larval dispersal may be effective only within continuous shallow areas with local water circulation which are limited in this basin.

Consequently an establishment of new Harris mud crab populations via shipping may be expected in protected shallow areas of the western Gulf of Finland, where salinity is still suitable for the species.

In the Black Sea–Caspian region Harris mud crab reached its physical limits of marine dispersion. Transportation with ship fouling most likely provides a continuous supply of crabs to such water bodies as Tsimlyanskoe Reservoir in the Don River, which has mineralized water with tendency to increase in the last decades (Nikanorov & Khoruzhaya, Reference Nikanorov and Khoruzhaya2014). It is unknown whether limited genetic diversity in the regional population is sufficient to provide the basis for adaptation and an establishment of freshwater populations (similarly to the Panama Canal and Texas reservoirs), but special studies that focus on this possibility are worth considering.

History of introductions in East Europe

The initial introduction area was the so-called Western Murmansk Coast between the entrance of the Kola Bay and Rybachiy Peninsula (Orlov & Ivanov, Reference Orlov and Ivanov1978; Kuzmin & Gudimova, Reference Kuzmin and Gudimova2002). The expansion of the population is relatively well documented (see Kuzmin & Gudimova, Reference Kuzmin and Gudimova2002; Berenboim, Reference Berenboim and Berenboim2003; Sundet, Reference Sundet and Stevens2014). It seemed to be explosive, since in the early 1990s the king crabs became abundant in the area of introduction, in 1996–97 its abundance increased by an order of magnitude, and from 1998 they spread further east along the Kola Peninsula coast of the Barents Sea and west, to Varanger Fjord (shared by Russia and Norway), Tana, Lakse and finally Porsanger Fjord and some smaller fjords in northern Norway (Kuzmin & Gudimova, Reference Kuzmin and Gudimova2002; Berenboim, Reference Berenboim and Berenboim2003; Sundet, Reference Sundet and Stevens2014). By 2000 in the Russian Barents Sea P. camtschaticus occupied the entire shelf zone of the Kola Peninsula up to the area of Kolguev Island and Gusinaya Banks (Kuzmin & Gudimova, Reference Kuzmin and Gudimova2002; Berenboim, Reference Berenboim and Berenboim2003), which is under direct influence of Atlantic waters (Boitsov, Reference Boitsov and Berenboim2003; Figure 5). A detailed trawl survey in 2011 did not indicate further spread of the king crab to the eastern Barents Sea or to the north (Zimina et al., Reference Zimina, Lyubin, Jørgensen, Zakharov and Lyubina2015).

Fig. 5. Roughly schematic limits of regular occurrence of the Kamchatka and snow crabs in the area of introduction. Based on the data presented by Jørgensen & Spiridonov (Reference Jørgensen and Spiridonov2013), Sokolov (Reference Sokolov2014), Sundet (Reference Sundet and Stevens2014), Spiridonov et al. (Reference Spiridonov, Zalota, Vedenin and Flint2015) and Zimina et al. (Reference Zimina, Lyubin, Jørgensen, Zakharov and Lyubina2015).

Genetic structure of alien populations

The gene of cytochrome oxidase (COI) and five nuclear microsatellite loci of king crab populations have been studied at four localities in the Barents Sea and two donor populations from the Western Kamchatka and Primorye. No decrease in the genetic diversity of the introduced populations has been detected (Zelenina et al., Reference Zelenina, Miuge, Volkov and Sokolov2008)

Contemporary distribution pattern, habitats and habit

Being the result of an intentional introduction, P. camtschaticus in the Barents Sea makes a unique case of nearly continuous monitoring and assessment of a non-indigenous decapod population from the termination of the latent phase of establishment, using fishing gears (Kuzmin & Gudimova, Reference Kuzmin and Gudimova2002; Berenboim, Reference Berenboim and Berenboim2003; Sokolov & Miljutin, Reference Sokolov and Miljutin2008; Sundet, Reference Sundet and Stevens2014; Bakanev et al., Reference Bakanev, Dvoretsky, Pavlov, Pinchukov, Zacharov and Zolotarev2016).

Larval settlement of Kamchatka crab in the Barents Sea occurs mainly on red and brown algae in the coastal zone; subsequently, fingerlings are found on algae or algal offal. Yearlings inhabit rocky substrates with a developed microrelief that provides shelter from predators (Pereladov, Reference Pereladov and Sokolov2003). Immature individuals (older than 1 year) concentrate mainly on solid substrates offshore, frequently in kelp fronds (Matyushkin, Reference Matyushkin and Berenboim2003b; Pereladov, Reference Pereladov and Sokolov2003; Sokolov & Miljutin, Reference Sokolov and Miljutin2008; V. Spiridonov, personal communication); and far from the coast, on scallop settlements in Voronka, White Sea (Zolotarev, Reference Zolotarev2010). The preferred habitats of adult crabs in the Barents Sea are mainly silty-sandy grounds with rich infauna, but during the period of inhabiting coastal zones, they can also be found in other various biotopes, including vertical rock walls (Pereladov, Reference Pereladov and Sokolov2003).

The Kamchatka crab is a migratory species (Vinogradov, Reference Vinogradov1945). Adult crabs undergo seasonal migrations (male and females often separately). They mate and breed in the coastal waters in spring. In autumn they migrate back from the coast to depths greater than 100 m. In the Barents Sea a significant change in the migration pattern has been observed: the extent of migrations is much lower than in the Pacific (Talberg, Reference Talberg2005), with a portion of males remaining at depth throughout the year (Matyushkin, Reference Matyushkin and Berenboim2003a). In some fjords of the Barents Sea, partially isolated by sills (for instance the western arm of Ura Guba Fjord) both males and females maintain a relatively settled mode of life, reproduce at depth, and spend a greater part of the year at a temperature of 2.0–2.5°C (Matyushkin, Reference Matyushkin and Berenboim2003a). In general, distribution and migration patterns in the area of introduction are an adapted version of the pattern which exists in its native areas (Talberg, Reference Talberg2005; Sundet, Reference Sundet and Stevens2014). In a relatively well-studied area of Varanger Fjord, king crab distribution and migration is most similar to Alaskan populations, but adult crabs seem to move to the shore for hatching, spawning and mating earlier than in Alaska, in late winter or early spring (Pereladov, Reference Pereladov and Sokolov2003; Talberg, Reference Talberg2005; Sundet, Reference Sundet and Stevens2014).

Size and population characteristics

Several studies deal with the differences in size characteristics and growth of the king crab in the native and introduced populations. Pinchukov & Berenboim (Reference Pinchukov, Berenboim and Berenboim2003) reported that males attain commercial size (150 mm carapace breadth) at about the same age (10 years) in the Barents Sea as in the Russian Far East. However, size of female maturity is somewhat greater than in the native range (Hjelset et al., Reference Hjelset, Sundet and Nilsen2009). On the West Kamchatka shelf the growth of females to carapace breadth larger than 120 mm has not been recorded, which might be the result of either negligible growth increment per moult, non-annual moulting in large specimens or both (Buyanovsky, Reference Buyanovsky2004). Contrary to this, in the Barents Sea, both size composition data and tagging experiments indicated continuous growth of females up to carapace breadth of at least 160 mm (Pinchukov, Reference Pinchukov2015). The maximum size attained by males in the West Kamchatka shelf population (one of the largest in the North Pacific) in the 1990s did not exceed 200 mm, however this population was under strong pressure from fishery, including illegal catch (Slizkin & Safonov, Reference Slizkin and Safonov2000; Buyanovsky, Reference Buyanovsky2004). In the Barents Sea males over 200 mm have been a usual component of commercial catches (Dolgov et al., Reference Dolgov, Matyushkin and Sennikov2006). The maximum longevity of Kamchatka crabs is about 20 years, both in the West Kamchatka shelf (Buyanovsky, Reference Buyanovsky2004) and in the Barents Sea (Pinchukov & Berenboim, Reference Pinchukov, Berenboim and Berenboim2003; Pinchukov, Reference Pinchukov2015). Even though no standard characteristics of size and growth are consistently reported in literature to be able to perform a fully conclusive comparison, king crabs in the Barents Sea appear to be effectively growing to larger sizes than in many native areas.

Population characteristics, such as abundance, relative composition of different age groups and fecundity have been varying strongly over the last two decades with regard to area, years, possibly food supply and fishery pressure (Sokolov & Miljutin, Reference Sokolov and Miljutin2008; Bakanev, Reference Bakanev2009; Hjelset et al., Reference Hjelset, Nilsen and Sundet2012; Sundet, Reference Sundet and Stevens2014).

Symbionts, parasites and pathogens

Symbionts and pathogens of Kamchatka crabs in the Barents Sea have been studied from relatively early stages of introduction (Bakay, Reference Bakay and Berenboim2003; Dvoretsky & Dvoretsky, Reference Dvoretsky and Dvoretsky2012). Overall, 16 common parasitic species and 31 non-parasitic symbiotic species have been found in P. camtschaticus from the Sea of Okhotsk within its native distribution. In the Barents Sea, king crabs are hosting 12 and 15 species of these groups respectively; both in the native distribution range and in the area of introduction crabs suffer from the ‘shell disease’ caused by several bacterial taxa (Matishov et al., Reference Matishov, Karmanova, Dvoretsky and Utevsky2014). Thus the parasitic/symbiotic community based on the Kamchatka crabs appears yet to be well established in the Barents Sea.

Position in the ecosystem and impact

The impact of king crabs was first of all considered in the context of their feeding habit as a generalist and opportunistic predator with a wide food spectrum, which includes more than 170 species of invertebrates as well as seaweed, detritus and carrion. However, bivalves and echinoderms are often preferred, while crustaceans appear to play a minor role in their diet (review in Jørgensen & Spiridonov, Reference Jørgensen and Spiridonov2013). Crabs show selective feeding on epifauna and less selective feeding on infauna. Experiments have shown that crabs are destroying more epibenthic animals than they eat (review in Jørgensen & Spiridonov, Reference Jørgensen and Spiridonov2013).

A limited number of case studies elucidate on the impact of Kamchatka crabs from foraging on benthic communities and habitats (review in Britayev et al., Reference Britayev, Rzhavsky, Pavlova and Dvoretskij2010; Jørgensen & Spiridonov, Reference Jørgensen and Spiridonov2013; Hjelset, Reference Hjelset2014). In several cases a measurable effect associated with the removal of large echinoderms or bivalves can be inferred in fjords or inlets where juvenile crabs are present round the year and migration activity of adults may be restricted (Britayev et al., Reference Britayev, Rzhavsky, Pavlova and Dvoretskij2010; Jørgensen & Spiridonov, Reference Jørgensen and Spiridonov2013; Pereladov et al., Reference Pereladov, Spiridonov Vassily, Anosov, Bobkov, Britayev, Deart, Labutin, Simakova and Spiridonov Victor2013; Hjelset, Reference Hjelset2014 and references herein). There are also indications of significant predation pressure of king crabs on broods of some fish spawning in the coastal zone (Mikkelsen & Pedersen, Reference Mikkelsen and Pedersen2012; Hjelset, Reference Hjelset2014).

Commercial fishery of king crabs in the Barents Sea commenced in 2004. While in Russia the fishery is primarily managed with the aim to protect the stock, in Norway fishery itself is considered as a management tool for regulating dispersal and impact of this non-indigenous species.

Vectors and potential for future spread

The mechanisms of expansion of king crabs from the area of their initial release have been studied and discussed. Tagging experiments revealed cases of directional movement of adult crabs to the west following the increase in temperature and salinity (Berenboim, Reference Berenboim and Berenboim2003; Sundet et al., Reference Sundet, Nilsen and Hjelset2009; Pinchukov, Reference Pinchukov2009). Dispersal of crabs to the west and east could proceed via different mechanisms: an invasion to the Varanger Fjord and to the fjords of Finmark might be proceeding as a cumulative effect of directional migration of adult crabs and transfer of larvae by local currents (Berenboim, Reference Berenboim and Berenboim2003; Starikov et al., Reference Starikov, Spiridonov, Naumov and Zuev2015). Dispersal to the east is most likely caused by transport of larvae with prevailing currents of eastern direction (Pinchukov, Reference Pinchukov2009).

Several records in Norway, to the west of Porsanger Fjord, may originate either from migratory behaviour of adults or from releases by fishermen (Sundet, Reference Sundet and Stevens2014). Thus it is possible that new populations will be established in particular Norwegian fjords.

During the second half of the 2000s, Kamchatka crab juveniles have been regularly recorded, and indications of the species’ ongoing reproduction has been observed in the northern part of the White Sea – i.e. in the Voronka (Zolotarev, Reference Zolotarev2010). This part of the White Sea has similar oceanographic characteristics to the Barents Sea (Berger & Naumov, Reference Berger, Naumov, Berger and Dahle2001). In 2013, an ovigerous female was caught in the Kandalaksha Bay of the White Sea, which could have been due to intentional transfer by humans (Starikov et al., Reference Starikov, Spiridonov, Naumov and Zuev2015). However, another Kamchatka crab female has been recorded in the same place in 2016 (A.D. Naumov, Kartesh White Sea Biological station, personal communication). The spread and successful establishment of this species in the inner part of the White Sea is unlikely, whether it is done through natural spread or intentional transportation. This will most likely be restricted due to low salinity of the near shore waters, where reproduction occurs, and below zero temperatures of the whole water column in winter, which is unacceptable for adults during the winter time (Starikov et al., Reference Starikov, Spiridonov, Naumov and Zuev2015).

History of introductions in East Europe

In 1996 the snow crab was found in the central part of the Barents Sea (Kuzmin et al., Reference Kuzmin, Akhtarin and Menis1998), which seems to be the result of human mediated introduction. Based on the size of the crab caught in 2004, it is presumed that the introduction could have occurred in the mid 1980s (Pavlov, Reference Pavlov2006).

Due to the research of the Polar Research Institute of Marine Fisheries and Oceanography (PINRO) the growth and dispersal of the population has been well documented (Kuzmin et al., Reference Kuzmin, Akhtarin and Menis1998; Jørgensen & Spiridonov, Reference Jørgensen and Spiridonov2013; Bakanev et al., Reference Bakanev, Dvoretsky, Pavlov, Pinchukov, Zacharov and Zolotarev2016). In less than 15 years since the first record, this species has spread to the east of the Barents Sea and became a common and abundant species in an area with cold transformed Barents Sea water (Pavlov & Sundet, Reference Pavlov, Sundet and Jakobsen2011; Sokolov, Reference Sokolov2014; Zimina et al., Reference Zimina, Lyubin, Jørgensen, Zakharov and Lyubina2015) (Figure 5).

It is worth noting that the range expansion of the alien population in the Barents Sea has been determined by the transfer of larvae with currents (Pavlov, Reference Pavlov2006; Pavlov & Sundet, Reference Pavlov, Sundet and Jakobsen2011; Sokolov, Reference Sokolov2014). Tagging (Goryanina, Reference Goryanina2015) has shown that adults often walk long distances (up to 102 km in 87 days), although overall movement and the resulting speed are usually much lower, especially for crabs caught more than a year after tagging. Overall, the resulting dominant direction of such movements has not been determined (Goryanina, Reference Goryanina2015).

By 2007–2008 C. opilio had been observed at the northern tip of Novaya Zemlya and at Karskie Vorota Strait, which is the boundary between the Barents and the Kara Seas. Nevertheless, research expeditions of PINRO and the Institute of Oceanology of the Russian Academy of Sciences (IO RAS) in 2007 have failed to find any specimens in the western part of the Kara Sea (Spiridonov et al., Reference Spiridonov, Zalota, Vedenin and Flint2015).

In 2011–2012 adults, immature specimens and larvae were found for the first time in the north- and south-eastern parts of the Kara Sea (Zimina, Reference Zimina2014; Spiridonov et al., Reference Spiridonov, Zalota, Vedenin and Flint2015). In 2013 the snow crab was observed in 20% of trawling samples of PINRO in the western part of the sea (Sokolov, Reference Sokolov2014). One year later, in the vicinity of St Anna Trough, near the eastern coast of Novaya Zemlya and near Yamal Peninsula the frequency of findings was 60% and 75% in the western part of the sea (Figure 5). Juveniles have also been collected in the eastern bays of Novaya Zemlya in 2014–2016; their distribution and size composition suggested that they are the result of both local recruitment and possible transport of larvae from the Barents Sea (Spiridonov et al., Reference Spiridonov, Zalota, Vedenin and Flint2015; Zalota et al., Reference Zalota, Spiridonov and Vedenin2016b).

Genetic structure and origin of alien population

Only preliminary results are available for the genetic structure of the snow crab population in the Barents Sea. The analysis of 14 microsatellites indicates that the non-indigenous population of the snow crab is more similar to the cluster of samples combining the Bering Sea and the Canadian east coast specimens than to the populations from Greenland (Dahle et al., Reference Dahle, Agnalt, Farestveit, Sevigny, Parent and Hjelset2014).

Contemporary distribution pattern, habitats and habit

During the 2000s the western and southern borders of the snow crab distribution area in the Barents Sea were well determined by the 2°С isotherm (Jørgensen & Spiridonov, Reference Jørgensen and Spiridonov2013). However this species (up until 2016) has not been reported from the northern and north-western part of the Barents Sea (such as Spitsbergen and Franz Josef Land Archipelagos) where the temperature does not exceed this limit. Currently the snow crab occurs mainly in the eastern part of the Barents Sea within the depth range of 40–360 m, temperature range from −1.0 to +3.0°С and salinity 33.4–35.0 (Zimina et al., Reference Zimina, Lyubin, Jørgensen, Zakharov and Lyubina2015). Current (2011–2015) maximum abundance zone of snow crab is located to the north-west of Admiralty Peninsula in North Island of Novaya Zemlya Archipelago and south of it along the western coast (Bakanev et al., Reference Bakanev, Dvoretsky, Pavlov, Pinchukov, Zacharov and Zolotarev2016).

In the Kara Sea the snow crab inhabits muddy sediments of the western shelf and troughs within the depth range 50–450 m where temperature and salinity range from −1.45 to +0.4°С, and from 33.9 to 34.9‰ respectively (Spiridonov et al., Reference Spiridonov, Zalota, Vedenin and Flint2015).

Size and population characteristics

Chionoecetes opilio does not reach its maximum size in the introduced area. The maximum carapace width of male adults registered in the Barents Sea is 166 mm, whereas in its native range, near Sakhalin Island, it can reach 178 mm (Pavlov, Reference Pavlov2006). During the first half of the 2010s the maximum size of 50% of males during their terminal moult was 82 mm (Bakanev & Pavlov, Reference Bakanev and Pavlov2015). In contrast to this, crabs from the Chukchi Sea do not exceed the Barents Sea specimens in size having maximum carapace width of only 95 mm (see Chuchukalo et al., Reference Chuchukalo, Nadtochy, Fedotov and Bezrukov2011).

During the short period that the Barents Sea population has been observed, it underwent substantial changes in size-age group structure; in particular, since 2010 the recruitment has been obtained through ongoing multiple generations, which lead to a much younger population by 2015 (Bakanev & Pavlov, Reference Bakanev and Pavlov2015).

Position in the ecosystem and impact

The diet of snow crabs in the Barents Sea has been studied since the early 2000s and has shown characteristic features that differentiate it from the Kamchatka crabs. Major portions (by weight) of the diet are (decreasingly): crustaceans, polychaetes, molluscs and echinoderms as well as fishes (Pavlov & Pinchukov, Reference Pavlov and Pinchukov2015). In its native range of the Chukchi Sea crabs predominantly feed on similar food items, but molluscs constitute a greater proportion and there are recorded incidents of cannibalism (Chuchukalo et al., Reference Chuchukalo, Nadtochy, Fedotov and Bezrukov2011).

It is hard to estimate the impact of the new incomer on the Barents Sea community at the present time, due to the small time lapse since the invasion. Although there is a potential for good future understanding of such impacts, since the dispersal of the snow crab has coincided with the beginning of regular joint Russian–Norwegian ‘ecosystem’ research events with the help of standardized trawling tools (Jørgensen et al., Reference Jørgensen, Lyubin, Skjoldal, Ingvaldsen, Anisimova and Manushin2015; Zimina et al., Reference Zimina, Lyubin, Jørgensen, Zakharov and Lyubina2015).

Commercial fishery of the snow crab commenced in 2014 in the international enclave of the Barents Sea. Exploitation in the Russian exclusive economic zone of the Barents Sea started in 2016.

Possible vectors and potential for future spread

Possible origin and vector of introduction of snow crabs to the Barents Sea are speculative, but most likely larvae have been transported in ballast waters from the North-west Atlantic (Pavlov, Reference Pavlov2006; Pavlov & Sundet, Reference Pavlov, Sundet and Jakobsen2011; Sokolov, Reference Sokolov2014). This vector might have been associated with floating factories and tankers serving the Soviet fishing fleet which operated in the 1970s and 1980s on both sides of the North Atlantic.

The area of mass occurrence of C. opilio in the Kara Sea coincides with the distribution of waters that originate from the Barents Sea (Spiridonov et al., Reference Spiridonov, Zalota, Vedenin and Flint2015). Consequently, the introduction of these crabs has occurred in the second half of the 2000s from the north- and south-eastern borders of the Barents Sea as a result of the expansion of the Barents Sea population. It is possible that the migration took place via both larvae transportation and adult migration. Further eastward expansion of this species seems to be possible along the salinity gradient of the Siberian Seas (Sokolov, Reference Sokolov2014; Spiridonov et al., Reference Spiridonov, Zalota, Vedenin and Flint2015). It is most likely that it will be subject to the ice coverage regimes in summer that affect larval development as well as food supply in these seas (Spiridonov et al., Reference Spiridonov, Zalota, Vedenin and Flint2015).