The Chinese Midden Crab - Eriocheir sinensis




Species Directory

















Potential areas of origin for species which might become established in new regions, are characterised by comparable environmental conditions. Germany appertains to the temperate climate zone. Similar environmental conditions can be found in the Ponto-Caspian area as well as on the coasts of South and North America, Japan, Tasmania and New Zealand. However, the long distance prevents an intensive natural exchange of species. Therefore, the majority of organisms from remote areas are dependent on human-mediated vectors, such as ornamental trade, aquaculture, and shipping.


Updated: 24-02-2014




Number of established alien species in German inland and coastal waters, known or probable

introduction vectors, main watersheds, important shipping canals and their opening date.



In Germany the common cord-grass Spartina anglica was planted at several sites in the East and North Frisian Wadden Sea in 1927-1937 (König 1948). In all probability, more than 70,000 shoots were imported from Britain. The introduction of the common cord-grass to Germany was succesful and the species has apparently become part of the German coastal flora. The rate of spread at individual sites has varied greatly but in many areas has involved the rapid spread from transplants or other propagating units to form a continuous sward. Within several decades this alien species was frequently observed along the entire German Wadden Sea coast (Nehring & Hesse 2006).



Ornamental trade

Ornamental trade is an important factor for the introduction of alien species in aquatic habitats. These species are spread throughout the world in a generally unregulated industry. And, expansion of the trade resulted in one of the fastest growing hobbies in history. Emptying of aquaria and the disposal of organisms from private garden ponds into lakes etc. appear to be important sources of introduction for especially water weeds and snails in Germany (Kowarik 2003). In total 36 alien species are believed as being introduced via the ornamental trade into German waters yet. The number known for this vector is considerably higher than that attributable to ballast water (see figure above and text below), which receives so much more notoriety and concern.


Aquarium trade - Photo: S. Nehring

The aquarium trade - an important source of introduction for

water weeds and snails in German waters.



Within German inland waters, 21 alien species are assumed to be introduced via stocking. Stocking of alien fish for commercial purposes has a long history in Germany. Despite of many governmental regulations, fisheries practice, especially stocking, is still the main source for non-indigenous fish species/populations. Also some invertebrate species being stocked for fisheries (e.g. the invasive North American crayfish Orconectes limosus). Several invertebrate species were also introduced in creeks and rivers as potential fish food (e.g. the North American amphipod Gammarus tigrinus). About 1990 the Italian Crested Newt Triturus carnifex was stocked in the German alps.





Since more than 100 years commercial shellfish transfers across the globe to Northern Europe are carried out (Wolff & Reise 2002).

The substantial imports of shellfish for aquaculture activities provided a suitable vector for attached macroalgae and their spores, for invertebrates as well as phytoplankton species. Fifteen alien species are believed as being imported as associated organisms with American or Japanese oysters on the German North Sea coast yet.

Since 1985 commercial farming activities started up with the invasive Pacific oyster Crassostrea gigas in the northern area of the Wadden Sea near the island of Sylt (Nehring 2003). These oysters reproduced successfully, and in 1991 the first oysters were found outside the culture plot. Spat settle on any hard substrate in the intertidal zone but preferentially upon wild banks of the native blue mussel Mytilus edulis (Reise 1998). Near Sylt development has locally advanced from solitary oysters to coherent reefs. These calcareous reefs are a completely new biogenic structure in the intertidal zone and it can be expected that Crassostrea will considerably change the biotic and abiotic characteristics of the Wadden Sea (Reise et al. 2005).



Pacific oyster farming near the German island of Sylt.

Oysters are cultivated in plastic mesh bags, called 'poches',

fastened onto steel trestles with rubber bands.



International shipping represents the most important introduction vector of aquatic alien species in Germany. About half of the 141 species introduced to German waters since 1492 are associated with this acting mechanism. In the following special attention is given to the main components associated with shipping: canals, ships’ hull and ships’ ballast water and sediments.



The natural barriers between the river and sea basins, as they had stabilised themselves in Europe since the end of the Pleistocene, were eliminated by the canals built during the last centuries. This enables mobile species as well as organisms that are transported by the drag force of water, by biovectors (particularly fishes) or by ships, to spread into new water systems.

The occurrence of 27 alien macrozoobenthic species and their area of expansion in German waters is facilitated primarily by the construction of canals. For instance: the early and frequent occurrence of Ponto-Caspian species in northern Europe (e.g. the invasive zebra mussel Dreissena polymorpha), the opening of the Dnjepr-Bug-Canal in 1784, which connects the Pripyat system to the rivers Bug and Vistula, was of crucial importance. After the opening of the Main-Danube Canal in Germany in 1992, connecting the Rhine and Danube basins, this southern corridor is today the most important link between the Ponto-Caspian area and Western Europe. Recently, several Ponto-Caspian species have been found increasingly in the German rivers Main and Rhine (e.g. the polychaete Hypania invalida and the isopod Jaera istri, Schöll & Banning 1996). In 1995 the Ponto-Caspian amphipod Dikerogammarus villosus arrived the river Main. Since then this new invader has dispersed over large distances in a short time and in 2000 first organisms were observed in the river Oder. This dynamic geographic extension of D. villosus in Germany was fascilitated by the existence of several man-made canals in northern Germany (e.g. Mittelland-Canal), which creates artificial connections between all large river systems (Rhine, Weser, Elbe, Oder) (Nehring 2002). The quickly increasing population density of this invasive amphipod has enabled it to become a major component of the macrobenthic assemblages in German freshwaters, eliminating both native and another alien amphipod species (Tittizer et al. 2000). Some species from the Rhine can also be found in the Danube now (e.g. the clam Corbicula fluminalis, Tittizer & Taxacher 1997).



Spreading of the invasive alien amphipod Dikerogammarus villosus

in German waters (Tittizer et al. 2000, Nehring 2005).


The specific role of inland waterway shipping in the faunal exchange between Danube and Main was clarified by examining the macroinvertebrate colonisation of inland vessels (Reinhold & Tittizer 1999). Six alien species were found on hulls and in cooling water filters, but not in ballast water. These investigations show clearly that the passive transport on ships’ hulls represents an important vector for limnic macroinvertebrate aliens (Reinhold & Tittizer 1999).

In 1999 the Ponto-Caspian goby Proterorhinus marmoratus reached the upper River Main via the Main-Danube-Canal (Schadt 2000). In 2002, this species was already present in the German and Dutch part of the River Rhine and is considered as an established alien species (Freyhof 2003).

More Ponto-Caspian species, mainly invertebrates and fishes, are expected to migrate into the North Sea basin via the Main-Danube-Canal within the following years, especially those species that have already been observed in the upper and middle Danube. One example is the goby Neogobius kessleri, an invasive species from the lower Danube, which has reached the German part of this river and was first recorded in 1999 (Freyhof 2003).

To reduce the uncontrolled range extensions of alien species by the interconnection of river and sea basins through canals, existing and future constructions, the installation of migration barriers of species (deterrent electrical systems, salt water locks etc.) should be considered (Nehring 2005).


Ships’ hull

Ships are capable of carrying a wide range of sessile species, their epibionts and paraites on their hulls. Prior to the introduction and widespread use of anti-fouling paints containing Tributyltin (TBT) about one half of the alien animal species introduced into German coastal waters came from fouled hulls of oversea trade ships. With the introduction of the effective biocide TBT in 1970, there has been a reduction of fouled hulls, considerably reducing the introduction of organisms via ship fouling (Nehring 2001). Since then the discussion on the role of ballast water as the important carrier of organisms from overseas has increased (e.g. Carlton 1985; Minchin & Sheehan 1995; Gollasch 2002). However, for the North Sea the vectors ballast water and ship hulls have equivalent importance since 1970 (Nehring 2001). This is probably based on the fact that fouling still occurs today, especially in situations where anti-fouling paint is damaged, difficult to apply, or otherwise ineffective. And, due to the recent discussion on the ecological effects of harmful biocides and their ban in antifoulants (IMO 1999), the pathway of invasive species introductions via ship fouling can attain a new dimension.



Number of introduced non-indigenous free-living brackish and marine

species (flora: phytoplankton, phytobenthos; fauna: zooplankton,

zoobenthos) which became established in the North Sea including the

Channel and Kattegat region (pre- and since 1970, the time of

introduction of the effective antifouling coatings containing tributyltin)

and their known or probable vector of introduction (Nehring 2001).


Alternative ship coatings showed that in general fouling is more intensive on these materials than on TBT-containing paints (Watermann et al. 1999a). However, fouling on the alternative products is not as strong as on completely untreated ship hulls and can usually be removed easily. In some cases, it has even been observed that organisms having grown to a certain size on the silicone coating were removed by the water currents in dependence on the speed of the ship. A striking finding was that the two barnacles Balanus improvisus and Elminius modestus, which had been introduced in former times in the fouling of ships from overseas into the North Sea, occurred in high abundances on the biocide-free coatings. In quantitative terms, Elminius ranked in a top position (Watermann et al. 1999a).

However, these facts have received little attention so far, although they show that increasing introduction of alien animal and plant species into North Sea coastal waters by intercontinental shipping has to be expected if the anti-fouling effectiveness of TBT-free alternatives is lower than in conventional organotin anti-fouling paints (Nehring 2001). Especially the biocide-free silicone coatings, from which fouling growth is easily removable, have an extraordinary high potential for the introduction of alien species. Up to 90% of the organisms on the silicone coating were directly removed through the water current depending on the speed of the ship (Watermann et al. 1999b). In spite of this "cleaning effect", a silicone coating is about 50% less efficient compared to TBT paints (Nehring 2001). Consequently, this ecological problem will continue to grow because above all, silicone will be painted on FastFerries, cruise vessels and other high-speed ship types with wide operating ranges in the future. The survival of the organisms in the fouling will indeed increase noticeably when they are transported faster to their new potential colonization areas. In addition, as organotins become replaced by environmentally less toxic paint coatings, coastal areas will become less contaminated by organotins. Such conditions may promote the effectiveness of alien species inoculations (Nehring 2001).

Also the biocide-free mechanically resistant coating of ship hulls will lead to an increasing transportation of fouling species. A crucial precondition for the regular mechanical underwater hull cleaning is the reliable collection and environmentally compatible disposal of the brushed-off biofouling, which is not functional yet. If the removed organisms sink to the bottom of the sea floor several species may survive and successfully establish new populations. Hull cleaning may also encourage the release of spores, cysts and gametes from crushed organisms. Certainly, a controlled disposal of the biofouling can not stop the introduction of alien species, but would probably minimize it.



Ships’ hull with a) effective anti-fouling paint containing ecologically harmful

Tributyltin, and  b) with ineffective anti-fouling paint and typical clusters

of hydroids, barnacles, serpulid worms, and colonies of bryozoa.


Even if the substitution of TBT by the envisaged alternative ship paints would not mean "to jump out of the frying pan into the fire", their unreserved ecological acceptance necessitates a multitude of investigations and interdisciplinary analyses. More recent technologies concentrate on the natural anti-fouling strategies of marine organisms. Presently a ship coating which simulates the skin of seals is tested in coastal waters on the German North Sea. Numerous short fibres kept in permanent movements through the water currents are expected to prevent that organisms can settle. Moreover, certain biologically active substances, which occur in marine plants and animals and protect them from being colonized, have promising prospects in modern anti-fouling technologies (Wahl 1997). Here might be a potential for a future-oriented, environmentally compatible fouling control (Nehring 2001).


Ships’ ballast water and sediments

Ballast water has been used since the 1870s to stabilise and trim the vessel and to submerge the propeller when ships are not fully loaded. With the intake of ballast water, organisms and sediments suspended in the water are pumped on board into the ballast tanks. Typically some 30-40% of ship deadweight tonnage can be carried as ballast water, although ballast capacities on a large bulk carrier may be as much as 60% of deadweight tonnage. The amount of sediment can reach several hundred tons. The maximum thickness of sediment known to be transported in ballast tanks was more than 50 cm (Gollasch 1996). Whereas the ballast water favours mainly pelagic species, the sediment hosts ground-dwelling species and increases the number of transported species. If the ballast water is discharged, parts of the sediment and organisms, which survived the cruise will also be discharged. It has been demonstrated that up to 4,000 pelagic and benthic species are transported between continents by ships each day (Gollasch 1996). Each species discharged with ballast water outside their native range has the potential to establish a self-sustaining population, coupled with all ecological and economical consequences. Recent calculations of Gollasch (1996) on the individual entry through ballast water discharges from overseas areas into the ports on the German North Sea coast revealed that 2.7 million individuals are released daily.

The first account attributing the introduction of an aquatic organism to ballast water transport was published by Ostenfeld (1908) after a mass occurrence of the phytoplankton algae Odontella (Biddulphia) sinensis in the German Bight of the North Sea in November 1903. Within a few years this alien diatom, which had previously been known only in the Indo-Pacific and the China Sea, had spread over the entire North Sea.

Despite the mass release of alien organisms through ballast water, only 20 established marine species in German waters were attributed to this introduction vector. This is due to the fact that a number of individual factors, primarily related to the biological characteristics of the species and the environmental conditions in the new region, such as temperature, salinity, nutrients, availability of food or native competitors, regulates successful establishment. Nevertheless, as each introduced species poses the potential of unwanted and uncontrollable consequences, their introduction as well as their spreading should be minimized wherever possible.



Two invasive species of the polychaete genus Marenzelleria were introduced

in ballast water to German coastal waters. Both species show distinct

occurrences due to the salinity range (Nehring & Leuchs 2001).


It is apparent that no single or simple universal solution for shipboard treatment or management to prevent the transfer of viable alien organisms in ballast water and sediments presently exists. Currently, heat treatment, mechanical removal of organisms in combination with UV treatment, and chemical treatment of ballast water are considered the most promising approaches (Taylor et al. 2003). In the meantime some countries, such as Australia, have introduced national ballast water regulations. Given the global nature of shipping, the need for the implementation of an international binding and effective act is obvious. An important measure is the new guideline of the International Maritime Organization on minimizing current risks and side effect to the environment and human health arising from the transfer of species in ships’ ballast water and sediments, which was actually adopted as a convention by IMO member States (IMO 2004).

The implementation of this new convention will significantly minimize the unintentional introduction of aquatic species, however, a comprehensive prediction of future introductions is impossible.

In addition, shipping activities have increased over the past decades with no end in sight and with corresponding increases of amounts of transported and released ballast water. The port of Hamburg has the greatest growth rates among the most important sea ports of Northern Europe today. And up to 2015 a doubling of goods traffic is expected for this biggest German port (PoH 2004).

Secondly, the duration of ship voyages will decrease due to technical improvements resulting in faster ships, and consequently will increase survival of organisms transported in ballast tanks (and on ships' hulls).

Thirdly, the increasing trade by ships enforced the construction of new ports, as the plannings for the new big German sea port Jade-Weser show, causing additional introductions of species and/or introductions from new regions.





Carlton, J.T. (1985): Transoceanic and interoceanic dispersal of coastal marine organisms: the biology of ballast water. - Oceanogr. Mar. Biol. Annu. Rev. 23: 313-317.

Freyhof, J. (2003): Immigration and potential impacts of invasive freshwater fishes in Germany. - Berichte des IGB 17: 51-58.

Gollasch, S. (1996): Untersuchungen des Arteintrages durch den internationalen Schiffsverkehr unter besonderer Berücksichtigung nichtheimischer Arten. - Verlag Dr. Kovac, Hamburg.

Gollasch, S. (2002): The importance of ship hull fouling as a vector of species introductions into the North Sea. - Biofouling 18: 105-121.

IMO (1999): 21st Assembly Session, Resolution A.895(21) Anti-fouling systems used on ships. - International Maritime Organization, IMO News, 4/1999: 7.

IMO (2004): International Convention for the Control and Management of Ships' Ballast Water and Sediments. - International Maritime Organization:

König, D. (1948): Spartina townsendii an der Westküste von Schleswig-Holstein. - Planta 36: 34-70.

Kowarik, I. (2003): Biologische Invasionen - Neophyten und Neozoen in Mitteleuropa. - Ulmer: Stuttgart, 380 S.

Minchin, D. & Sheehan, J. (1995): The significance of ballast water in the introduction of exotic marine organisms to Cork Harbour, Ireland. - ICES CM 1995/O:1: 1-15.

Nehring, S. (2001): After the TBT era: Alternative anti-fouling paints and their ecological risks. - Senckenbergiana maritima 31: 341-351.

Nehring, S. (2002): Biological invasions into German waters: an evaluation of the importance of different human-mediated vectors for nonindigenous macrozoobenthic species. - In: Leppäkoski, E., Gollasch, S. & Olenin, S. (Eds.), Invasive Aquatic Species of Europe - Distribution, Impacts and Management. Kluwer, Dordrecht: 373-383.

Nehring, S. (2003): Pacific oysters in the European Wadden Sea - an irreversible impact in a highly protected ecosystem. - Aliens 17: 20-21.

Nehring, S. (2005): International shipping - A risk for aquatic biodiversity in Germany. - In: Nentwig, W.,Bacher, S., Cock, M.J.W., Dietz, H., Gigon, A. & Wittenberg, R. (Eds.), Biological Invasions - From Ecology to Control. Neobiota 6: 125-143.

Nehring, S. & Hesse, K.-J. (2006): The common cord-grass Spartina anglica: An invasive alien species in the Wadden Sea National Park. – Verhandlungen der Gesellschaft für Ökologie 36: 333.

Nehring, S. & Leuchs, H. (2001): Das BfG-Ästuarmonitoring Makrozoobenthos in Eider, Elbe, Ems, Jade und Weser. – Jber. 2000 Bundesanstalt für Gewässerkunde, Koblenz: 93-95.

Ostenfeld, C.H. (1908): On the immigration of Biddulphia sinensis Grev. and its occurrence in the North Sea during 1903-1907 and on its use for the study of the direction and rate of flow of the currents. - Medd. Komm. Havunders. Ser. Plankton 1(6): 1-44.

Petersen, K.S., Rasmussen, K.L., Heinemeier, J. &  Rud, N. (1992): Clams before Columbus? – Nature (Lond.) 359: 679.

PoH (2004): Data and facts. - Port of Hamburg,

Reise, K. (1998): Pacific oysters invade mussel beds in the European Wadden Sea. - Senckenbergiana marit. 28: 167-175.

Reise, K., Dankers, N. & Essink, K. (2005): Introduced species. - In: Essink, K., Dettmann, C., Farke, H., Laursen, K., Lüerßen, G., Marencic, H. & Wiersinga, W. (Eds.), Wadden Sea Quality Status Report 2004. Wadden Sea Ecosystem No. 19: 155-161.

Reinhold, M. & Tittizer, T. (1999): Verschleppung von Makrozoen durch Kühlwasserfilter eines Schiffes. - Wasser und Boden 51 (1/2): 61-66.

Schadt, J. (2000): Neue Fischart im Main entdeckt: Marmorierte Grundel (Proterorhinus marmoratus). - Fischer & Teichwirt 6/2000: 217-218.

Schöll, F. & Banning, M. (1996): Erstnachweis von Jaera istri (Veuille) (Janiridae, Isopoda) im Rhein. - Lauterbornia 25: 61-62.

Taylor, A., Rigby, G., Gollasch, S., Voigt, M., Hallegraeff, G., McCollin, T. & Jelmert, A. (2003): Preventive treatment and control techniques for ballast water. - In: Leppäkoski, E., Gollasch, S. & Olenin, S. (Eds.): Invasive Aquatic Species of Europe: Distributions, Impacts and Management. Kluwer, Dordrecht: 484-507.

Tittizer, T. & Taxacher, M. (1997): Erstnachweis von Corbicula fluminea/fluminalis (Müller 1774) (Corbiculidae, Mollusca) in der Donau. - Lauterbornia 31: 103-107.

Tittizer, T., Schöll, F., Banning, M., Haybach, A. & Schleuter, M. (2000): Aquatische Neozoen im Makrozoobenthos der Binnenwasserstraßen Deutschlands. - Lauterbornia 39: 1-72.

Wahl, M. (1997): Natürliche Verteidigungssysteme gegen Aufwuchs. - DGM-Mitteilungen, 4/1997: 8-12.

Watermann, B., Michaelis, H., Daehne, B., Haase, M. & Isensee, J. (1999a): Alternativen zu TBT - Erprobung von umweltverträglichen Antifoulinganstrichen auf Küstenschiffen im niedersächsischen Wattenmeer. - Report Phase I, Umweltstiftung WWF-Deutschland Frankfurt: 1-125.

Watermann, B., Haase, M., Isensee, J., Sievers, S., Dannenberg, R., Rohweder, U., Bauer, O. H. & Wohnout, R. (1999b): Alternativen zu TBT - Chemisch-analytische und ökotoxikologische Untersuchungen an biozidfreien Unterwasseranstrichen. - Final Report Umweltbehörde Hamburg and Umweltstiftung WWF-Deutschland Frankfurt: 1-105.

Wolff, W.J. & Reise, K. (2002): Oyster imports as a vector for the introduction of alien species into northern and western European coastal waters. - In: Leppäkoski, E., Gollasch, S. & Olenin, S. (Eds.): Invasive Aquatic Species of Europe - Distribution, Impacts and Management. Kluwer, Dordrecht: 193-205.



© 2005 by Stefan Nehring / / Disclaimer Impressum