1. Scientific name: Balanus improvisus Darwin (Balanidae, Cirripeda)
Common name: Acorn barnacle

Photo by Jonne Kotta.

2. Identification
Shell white or grey, wall typically conical, very smooth; walls never folded longitudinally. Shell opening diamond-shaped, slightly toothed. Base radially calcareous, flat and thin, permeated by pores that do not generally run to the very centre of the basal plate (Gollasch et al. 1999).

3. Natural distribution
An Atlantic, Old World or most likely American species, widely distributed around the world but its distribution is patchy.

4. First finding in Estonia
Mid or late XIX century, exact location and time are not known.

5. Invasion history in the Baltic Sea
B. improvisus is likely to have been introduced from North America. B. improvisus is transmitted by ships or as a fouling organism or in ballast water (Gollasch et al. 1999). It was first found in the southeastern Baltic in 1844, i.e. 10 years before being described by Darwin, and became common especially in ports. The introduction of B. improvisus was so successful that nowadays most of the Baltic Sea is colonized (Leppäkoski and Olenin 2000).

6. Distribution and population dynamics
Specific methods are required to estimate the population density, hence, there are a few quantitative data about the species. Barnacle densities have been estimated from some tens to 5000 ind. dm-2.
Extremely euryhaline, tolerates salinities from nearly 0 to 40 ‰. Adults live on rocky shores, boulders, pilings, buoys, ship hulls, wood, seaweed, shells, and crustaceans. Generally inhabits the depth range from the surface down to 6 m. Tolerant to pollution and eutrophication, preference for organically enriched sites in ports with good foraging conditions (filter-feeder on detritus and phytoplankton) (Gollasch et al. 1999).

Broken line shows the eastern distribution limit of B. improvisus in the Estonian coastal sea.

7. Ecological and economical impact
It is capable of marked habitat alteration through the construction of dense crusts on hard surfaces and secondary hard substrates. In dense population of B. improvisus, associated species such as chironomid larvae, ostracods, copepods, and juvenile bivalves increased compared to adjacent sites without crusts. The main effect of the barnacle was facilitating settlement of other organisms (Leppäkoski and Olenin 2000).
The species is a fouling organism on ship's hulls but likely on blue mussels and oysters. Fouling of water intake pipes and heath exchangers. Through filter-feeding slows down eutrophication processes (Gollasch et al. 1999).

8. More information
Jonne Kotta, e-mail: jonne@klab.envir.ee

9. References
Gollasch, S., Minchin, D., Rosenthal, H. and Voigt, M (eds.). 1999. Exotics Across the Ocean Case Histories on Introduced Species. Department of Fishery Biology, Institute for Marine Science, University of Kiel, Germany.
Furman, E.R. 1989. The taxonomic relationship of Balanus improvisus (Darwin) populations in the Baltic and the Atlantic. Ph D thesis, University College of North Wales, 136 pp.
Furman, E. 1990. Geographical variations of Balanus improvisus in biochemical and morphometric characters. J. Mar. Biol. Ass. U.K. 70: 721-740.
Leppäkoski, E. and Olenin, S. 2000. Xenodiversity of the European brackish water seas: the North American contribution. In: Pederson, J. (ed) Marine Bioinvasions. Proceedings of the First National Conference, January 24-27, 1999. Massachusetts Institute of Technology, pp. 107-119.
Leppäkoski, E. and Olenin, S. 2001. The meltdown of biogeographical peculiarities of the Baltic Sea: the interaction of natural and man-made processes. Ambio 30: 202-209.

1. Scientific name: Dreissena polymorpha (Pallas) (Dreissenidae, Bivalvia)
Common name: Zebra mussel, Wandering mussel

Photo by Jonne Kotta.

2. Identification
Shell is triangular (height makes 0.40-0.60 of length), convex. In front view, the dorsal margin of valves meet to from an angle about 90?. The ventral edge is flattened with a clear byssal notch one third of its length from apex. Umbonal ridges very prominent, almost parallel with ventral margin. Up to 50 mm. Greenish, brownish-yellowish with clear dark and light coloured ("zebra") zig-zag banding. Shell shape and coloration are highly variable, depending on substrate, depth, and density of aggregation (Gollasch et al. 1999).

3. Natural distribution
Native to the drainage basins of the Black, Caspian and Aral Sea. Introduced to north-west Russia, central and western Europe, Scandinavia, Britain, Ireland and North-America (Gollasch et al. 1999).

4. First finding in Estonia
Firstly the species was introduced to the estuary of Põlula River (?) in the Gulf of Finland and Pärnu Bay in the Gulf of Riga in the middle of the XIX century. Second introduction took place through Lake Peipsi to the easternmost part of the Gulf of Finland in the 1930s.

5. Invasion history in the Baltic Sea
How the initial expansion from the Black Sea and Aralo-Caspian Sea took place is largely unclear. The zebra mussel may have penetrated from the Black Sea via Dnepr, the Oginskij Canal (completed in 1804) to the River Neman and further to the Curonian Lagoon, SE Baltic Sea. However, it may have come via canals from the Caspian region using the Volga and its tributaries and lakes Onega and Ladoga. In 1990 it was reported from brackish water in the eastern part of the Gulf of Finland after being present for 150-years in the nearby freshwater lake Ladoga (Gollasch et al. 1999).

6. Distribution and population dynamics
There is a great amount of genetic variation within the species. This feature may facilitate the mussel to rapidly colonize new habitats and adapt to new environments. The high reproductive output and ability to extend their plaktonic stage enables rapid dispersal. Larvae and adults can be distributed in ballast water as fouling on ship and boat hulls, navigation buoys, fishing vessel wells; industrial activities such as transport of timber or river gravel; fisheries such as fish stocking water, fishing equipment. The overland transport of zebra mussels by small trailered boats has been repeatedly implicated in inner-lake dispersal (Gollasch et al. 1999).
Nowadays, the species has been established in a few localities of the easternmost Gulf of Finland (Valovirta and Porkka 1996, Kotta et al. 1998) and in less saline parts of the Gulf of Riga (Kotta et al. 1998). D. polymorpha has not been found in the Väinameri.
New findings of D. polymorpha in the coastal area of the Gulf of Finland indicate that a slow expansion of its distribution area is currently taking place. However, the Gulf of Finland is a relatively hostile environment for D. polymorpha, which originates from the warmer areas. The population seems to be unstable and the species is likely to reproduce only occasionally in favourable years (Valovirta and Porkka 1996, Kotta et al. 1998).
D. polymorpha is relatively common everywhere in the Gulf of Riga except in its middle part and at the southern coast of Saaremaa Island (Kotta et al. 1998). The distribution area documented in the literature (Shurin 1953, 1961, Järvekülg 1979) is fairly consistent with recent findings with the exception that Shurin (1961) also found a population of D. polymorpha living on phanerogams in the southern coastal sea off Saaremaa Island.
D. polymorpha inhabits depths down to 15 m when suitable substrate, i.e. hard bottoms or macrovegetation with strong thalli, is present. The biomass of D. polymorpha was significantly higher on stone bottoms followed by vegetated and mixed bottoms. Sediment type did not contribute to the variance of the abundance of D. polymorpha (Kotta et al. 1998).
Generally, the abundances and biomasses of the species were low, around 50 ind. m-2 and 5 g dw m-2, respectively. The values up to 8400 ind m-2 and 1400 g dw m?2 were recorded in the southernmost part of the Gulf of Riga. This could be explained by the inflow of fresh water of the Daugava River resulting in lower salinity and higher nutrient concentrations and, consequently, higher phytoplankton biomass (Kotta et al. 1998).
Hence, the factors favouring the development of a dense population of D. polymorpha are rather low salinity (< 5 psu) and high trophic conditions (chl a values during spring bloom > 20 ?g l-1). In the areas of higher salinity (> 5 psu) lower filtration rates set the limits of its distribution and the species is probably outcompeted by M. edulis.

Crosses show the recent distribution area of D. polymorpha in the Estonian coastal sea.

7. Ecological and economical impact
Owing to its large filtration capacity and mass occurrence D. polymorpha outcompetes the native species of similar feeding type. The species slows down the eutrophication processes, indirectly favours the blooms of blue green algae, increases water transparency and ameliorate the conditions for benthic macrovegetation. Through biodeposition D. polymorpha increase the density of benthic deposit feeders.
Economic impacts are the fouling of intake pipes, ship hulls, navigational constructions, cages of aquaculture, cuts to bathers feet, reduced angling catches (Gollasch et al. 1999).

8. More information
Jonne Kotta, e-mail: jonne@klab.envir.ee

9. References
Gollasch, S. and Leppäkoski, E (eds.). 1999. Initial Risk Assessment of Alien Species in Nordic Coastal Waters. Nordic Council of Ministers, Copenhagen.
Järvekülg, A. 1979. Benthic Fauna of the Eastern Baltic Sea. Valgus, Tallinn (in Russian).
Kotta, J. 2000. Impact of eutrophication and biological invasions on the structure and functions of benthic macrofauna. Dissertationes Biologicae Universitatis Tartuensis, 63, Tartu University Press, pp. 1-160.
Kotta, J and M¸hlenberg, F. 2002. Grazing impact of Mytilus edulis and Dreissena polymorpha (Pallas) in the Gulf of Riga, Baltic Sea estimated from biodeposition rates of algal pigments. Ann. Zool. Fenn. 39: 151-160.
Kotta, J., Orav, H. and Kotta, I. 1998. Distribution and filtration activity of Zebra mussel, Dreissena polymorpha (Pallas) in the Gulf of Riga and the Gulf of Finland. Proc. Estonian Acad. Sci. Biol. Ecol. 47: 32-41.
Kotta, J., Orav, H. and Vuorinen, I. 2001b. In situ estimates of the seasonal variability in the benthic-pelagic coupling of Dreissena polymorpha, Mytilus edulis and Balanus improvisus in the NE Baltic sea. 36th European Marine Biology Symposium. A marine science odyssey into the 21st century. Maó, 17-22.9.2001. Abstracts, 163.
Mikelsaar, N.Õ. and Vinkel, R. 1936. Uusi andmeid rändkarbi Dreissena polymorpha Pall. esinemisest Eestis. Eesti Loodus 4: 142-145.
Orlova, M. 2002. Dreissena (D.) polymorpha: Evolutionary origin and biological peculiarities as prerequisites of invasion success. In: Leppäkoski et al. (eds.) Invasive Aquatic Species of Europe. Kluwer Academic Publishers, Dordrecht, pp. 127-134.
Shurin, A.T. 1953. Bottom fauna in the Gulf of Riga. Tr. Latv. Odteleniya VNIRO, 1, 77-113 (in Russian).
Shurin, A.T. 1961. The grouping of bottom fauna in the Gulf of Riga. Tr. NIIRH SNH Latv. SSR, 3, 343-368 (in Russian).
Timm, T. 1960. Rändkarp. Eesti Loodus 4: 211-215.
Timm, T., Kangur, K., Timm, H. and Timm, V. 1996a. Macrozoobenthos of Lake Peipsi-Pihkva: long-term biomass changes. Hydrobiologia 338: 155-162.
Timm, T., Kangur,K., Timm, H. and Timm, V. 1996b. Macrozoobenthos of Lake Peipsi-Pihkva: taxonimical composition, abundance, biomass, and their relation to some ecological parameters. Hydrobiologia 338: 139-154.
Timm, V. 1990. Peipsi järve suured karbid (Bivalvia). Eesti TA Toim. Biol. 39: 46-54.
Valovirta, I. and Porkka, M. 1996. The distribution and abundance of Dreissena polymorpha (Pallas) in the eastern Gulf of Finland. Mem. Soc. Fauna Flora Fenn. 72: 63-78.

1. Scientific name: Eriocheir sinensis Milne-Edw. (Brachyura, Decapoda)
Common name: Chinese Mitten Crab, Mitten Crab, Chinese Freshwater Edible Crab

Figure in Gollasch et al. 1999.

2. Identification
The square shaped carapace of adult crabs, clearly distinguishes it from other European brachyuran crabs. It can attain a carapace width of 5 cm. Males have a hair-like covering on the claws forming mitten-like claws. The colour varies from yellow to brown, rarely purple and red (Gollasch et al. 1999).

3. Natural distribution
Areas of origin are waters in temperate and tropical regions between Vladivostock (Russia) and South-China, including Japan and Taiwan. Centre of occurrence is the Yellow Sea. Records outside its native regions are Europe, Mediterranean Sea, Black Sea, Hawaii, Great Lakes and San Francisco Bay (Gollasch et al. 1999).

4. First finding in Estonia
Adjacent to Vormsi Island, Väinameri Archipelago Sea in 1933.

5. Invasion history in the Baltic Sea
In Europe the crab was first recorded from the German river Aller in 1912 and may have been released with ballast water discharges. The species has probably spread into the Baltic Sea via the Kiel Canal (1926) (Gollasch et al. 1999).

6. Distribution and population dynamics

Up to date there are little information about the distribution and population dynamics of the crab in the Estonian coastal sea as these knowledge are obtained only from irregular reports of fishermen.
In general the crabs live on different types of sediment, preferring hard bottom substrate. The species feeds on a wide variety of plants, invertebrates, fishes and detritus. Snails and clams are the main food.
On their migrational routes crabs may be exposed to air for several hours.

7. Ecological and economical impact
Crabs are preying upon native species and after mass occurrence may exterminate the native species. They are competing for space and food with other species. Burrowing activities of crabs result in damages of dikes, river banks and port installations. Crabs feed on fishes caught in traps and nets. Nets will be damaged. Clogging of water intake filters during mass occurrence. In some countries the crabs are imported for human consumption. Crabs have been used as bait for eel fishing, to produce fish meal, cosmetic products (Gollasch et al. 1999).
The larvae of the crab develop only in marine water. Therefore a migration is the only way of the species to colonize brackish or fresh waters. This also explains why the species is not able to attain self-sustaining populations in the Baltic Sea. We believe that owing to the low density of the crab the species is likely to have little effect on the local ecosystem of the Estonian coastal sea.

8. More information
Jonne Kotta, e-mail: jonne@klab.envir.ee

9. References
Gollasch, S., Minchin, D., Rosenthal, H. and Voigt, M (eds.). 1999. Exotics Across the Ocean Case Histories on Introduced Species. Department of Fishery Biology, Institute for Marine Science, University of Kiel, Germany.
Leppäkoski, E. 1991. Introduced species - resource or threat in brackish-water seas? Examples from the Baltic and the Black Sea. Mar. Poll. Bull. 23: 219-223.

1. Scientific name: Marenzelleria cf. viridis (Verrill) (Polychaeta)
Common name: red gilled mud worm

Photo by Jonne Kotta.

2. Identification
Body long, worm-like without any dorsal scales. Neoropodium (ventral branch of parapodia) bearing also short chaetae at least in some parts of body; no shift of parapodia to dorsal side in hind body. Anterior end with various long appendages. More than one pair of long tentacles or gills on the anterior end. Gills beginning from the first setiger. Hooked chaetae in both notopodia and neuropodia beginning far backwards of chaetal segments XX. In I or I-II some hair chaetae considerably longer than the rest. Numerous small appendages surrounding anus (Timm 1999).

3. Natural distribution
The world-wide distribution of the genus Marenzelleria is restricted to the northern Hemisphere. The native region of M. cf. viridis is the Atlantic coast of North-America: Currituck Sound (North Carolina), Trippe Bay (Chesapeake Bay), Chester River (Virginia and Delaware) and Ogeechee River (Georgia) (Gollasch et al. 1999).

4. First finding in Estonia and adjacent waters
M. viridis was observed for the first time in the Gulf of Riga, near the mouth of the Daugava in 1988 (Lagzdins and Pallo 1994). The following four years the polychaete densities rose more than 100 times reaching the values of 1400 ind m-2. In the northern part of the Gulf of Riga and the Väinameri M. cf. viridis was found in 1995. The salinity values were relatively stable at the beginning of the 1990s whereas average temperatures were much higher in 1994 than in previous years. Probably, higher summer temperature resulted in a higher reproductive output of M. cf. viridis, which enhanced its invasion ability towards the northern part of the Gulf of Riga and the Väinameri (Kotta and Kotta 1998).
The first observation of M. cf. viridis at the northern coast of the Gulf of Finland was made in 1990 (Norkko et al. 1993, Stigzelius et al. 1997). During 1990-93 M. cf. viridis expanded its distribution into the eastern parts of the Gulf (Stigzelius et al. 1997). However, anti-clockwise circulation of the currents would not permit M. cf. viridis to spread from the northern side of the Gulf of Finland towards its southern side. In addition, the larvae of the polychaete are unable to complete their development at salinities below 5 psu (George 1966), which may frequently occur in the easternmost part of the Gulf of Finland. Only one specimen was recorded near the Pühajõgi River, south-eastern coast of the Gulf of Finland, in 1994. Until 1997 this polychaete was not observed along the southern coast of the Gulf of Finland. Some occasional findings of M. cf. viridis in the westernmost bays of the Gulf of Finland suggest the Väinameri as a donor region. Nowadays M. cf. viridis is slowly expanding its distribution range towards the eastern parts of the Gulf of Finland, being established as far as in Narva Bay.

5. Invasion history in the Baltic Sea
In 1985 the first specimens of M. cf. viridis were found in the Baltic Sea. By 1992 it occurred at several coastal areas along the Baltic Proper up to the northern side of the Gulf of Finland. The distribution of the species is restricted to coastal waters, estuaries and shallow bays.

6. Distribution and population dynamics
The species is often described as motile. But its spread is often associated with dispersal and/or long development of planktonic larvae. The transport as larvae in the ballast water of ships is likely.
Depth does not correlate with the abundance and biomass of M. cf. viridis but sediment type is a significant factor for both. In shallower areas (< 10 m) M. viridis prefers sand or gravel bottoms. Its abundance is higher in more densely vegetated areas. Deeper down (> 10 m) M. cf. viridis is confined to silty clay bottoms.
In the Väinameri the polychaete is restricted to deeper parts of the archipelago (7-11 m). The area is homogeneous both in terms of sediment and macrovegetation: the sandy clay substrate is covered with a loose layer of the red algae Furcellaria lumbricalis. Among benthic invertebrates mainly Macoma balthica, Cerastoderma glaucum and Nereis diversicolor are found in the sediment while Mytilus edulis is more abundant in the layer of F. lumbricalis. Higher biomass of the polychaete under the mat of F. lumbricalis agrees with the hypothesis that uniformity of assemblage facilitates the establishment of introduced species (Carlton, 1996). Also, it is likely that a thick mat of F. lumbricalis protects infauna effectively from fish predation. On the other hand, intermediate disturbance (Connel 1978) due to possible temporary hypoxia under F. lumbricalis may be beneficial for the establishment of opportunistic species such as M. cf. viridis.
As compared to the Latvian side of the Gulf of Riga, the abundances of M. cf. viridis in the Estonian coastal sea are rather low, seldom surpassing 100 ind m-2. Hence, an increase in the abundances of M. cf. viridis is expected in the coming years.

Relationship between the areal coverage of the red alga Furcellaria lubricalis and the biomass of M. viridis.

Crosses show the recent distribution area of M. viridis in the Estonian coastal sea.

7. Ecological and economical impact
The polychaete is a deep burrowing deposit feeder - a new function, which has not previously been observed in the northern Baltic Sea. We estimated whether the appearance of the new function affected the native benthic invertebrate communities (Kotta et al. 2001, Kotta and Olafsson 2003).
Experimental studies gave further information on that (1) the introduced polychaete M. cf. viridis negatively influence the native polychaete species Nereis diversicolor and the amphipod Monoporeia affinis. (2) This effect is likely to decrease with the increasing density of adult specimens of the bivalve M. balthica. (3) Competitive interactions between M. cf. viridis and M. balthica appear a key factor determining the distribution pattern of M. cf. viridis in the Baltic Sea. (4) Competitive superiority of M. balthica over M. cf. viridis is likely due to more efficient feeding regime of the bivalve. The pilot experiment indicated that M. balthica consumed sedimented phytodetritus from wider surface area and its consumption rates were higher than that of M. cf. viridis.

Schematic view of the competitive interactions between the introduced polychaete and the native fauna in the study area.

Other known effects of M. cf. viridis are the increased benthic production, higher burrowing activity improves the oxygen content of the sediment. The species is a potential food source for ground living fish.

8. More information
Jonne Kotta, e-mail: jonne@klab.envir.ee

9. References
Carlton, J.T. 1996. Pattern, process, and prediction in marine invasion ecology. Biol. Conserv., 78, 97-106.
Connell, J.H. 1978. Diversity in tropical rain forests and coral reefs. Science, 199, 1302-1310.
George, J.D. 1966. Reproduction and early development of the spionid polychaete, Scolecolepides viridis (Verrill). Biol. Bull. 130: 76-93.
Gollasch, S. and Leppäkoski, E (eds.). 1999. Initial Risk Assessment of Alien Species in Nordic Coastal Waters. Nordic Council of Ministers, Copenhagen.
Gollasch, S., Minchin, D., Rosenthal, H. and Voigt, M (eds.). 1999. Exotics Across the Ocean Case Histories on Introduced Species. Department of Fishery Biology, Institute for Marine Science, University of Kiel, Germany.
Kotta, J. 2000. Impact of eutrophication and biological invasions on the structure and functions of benthic macrofauna. Dissertationes Biologicae Universitatis Tartuensis, 63, Tartu University Press, pp. 1-160.
Kotta, J. and Kotta, I. 1998. Distribution and invasion ecology of Marenzelleria viridis (Verrill) in the Estonian coastal waters. Proc. Estonian Acad. Sci. Biol. Ecol. 47: 210-217.
Kotta, J. and Ólafsson, E. 2003. Competition for food between the introduced exotic polychaete Marenzelleria viridis and the resident native amphipod Monoporeia affinis in the Baltic Sea. J. Sea Res., in press.
Kotta, J. and Orav, H. 2001. Role of benthic macroalgae in regulating macrozoobenthic assemblages in the Väinameri (north-eastern Baltic Sea). Ann. Zool. Fennici 38: 163-171.
Kotta, J., Orav, H. and Sandberg-Kilpi, E. 2001a. Ecological consequence of the introduction of the polychaete Marenzelleria viridis into a shallow water biotope of the northern Baltic Sea. J. Sea Res. 46: 273-280.
Kotta, J., Simm, M., Kotta, I., Kanoðina, I., Kallaste, K. and Raid, T. 2003. Factors controlling the long-term changes of the eutrophicated ecosystem of Pärnu Bay, the Gulf of Riga. Hydrobiologia, in press.
Lagzdins, G. and Pallo, P. 1994. Marenzelleria viridis (Verrill) (Polychaeta, Spionidae) - a new species for the Gulf of Riga. Proc. Est. Acad. Sci. Biol. 43: 184-188.
Norkko, A., Bonsdorff, E. and Boström, C. 1993. Observations of the polychaete Marenzelleria viridis (Verrill) on a shallow sandy bottom on the South coast of Finland. Mem. Soc. Fauna Flora Fenn. 69: 112-113.
Stigzelius, J., Laine, A., Rissanen, J., Andersin, A.-B. and Ilus, E. 1997. The introduction of Marenzelleria viridis (Polychaeta, Spionidae) into the Gulf of Finland and the Gulf of Bothnia (northern Baltic Sea). Ann. Zool. Fenn. 34: 205-212.
Timm, T. 1999. Eesti rõngusside määraja. Teaduste Akadeemia Kirjastus, Tartu.
Zettler, 1997. Bibliography on the genus Marenzelleria and its geographical distribution, principal topics and nomenclature. Aq. Ecol. 31: 233-258.
Zettler, M.L., Daunys, D., Kotta, J. and Bick, A. 2002. History and success of an invasion into the Baltic: the case of the polychaete worm Marenzelleria cf. viridis, development and strategies. In: Leppäkoski, E., S. Gollasch and S. Olenin (eds.), Invasive Aquatic Species of Europe - Distributions, Impacts and Management. Kluwer Academic Publishers, Dordrecht, pp. 66-75.

1. Scientific name: Mya arenaria L. (Bivalvia)
Common name: the soft shell clam, sand gaper

Photo by Jonne Kotta.

2. Identification
Oval, umbones just posterior to mid-line, anterior regularly rounded, posterior somewhat tapered. Up to 150 mm long. Off-white, yellowish, or fawn, dark greyish-brown about the umbones; periostracum light brown, often stained by iron deposits. Left valve with a prominent spatulate chondrophore, projecting at a right angle to hinge line, with a distinct tooth-like ridge along its posterior edge. Right valve with a concave, spatulate chondrophore recessed beneath umbo (Hayward and Ryland 1995).

3. Natural distribution
In sand, often mixed with mud or gravel, lower shore and offshore to about 20 m. Often very common in estuaries, where it may occur in extensive beds. Circumboreal, not reaching the Mediterranean (Hayward and Ryland 1995).

4. First finding in Estonia
Not known.

5. Invasion history in the Baltic Sea
Characteristically, the recent stage (since ca 500 BP) of the Baltic Sea has been described as the "Mya Sea" decades before the status of the nominator species was recognized as a globally successful marine invader from North America, probably transported by the Vikings in the 13th century (Leppäkoski and Olenin 2001). Within the Baltic Sea the further range expansion took place through pelagic dispersal of larvae. Although the invasion history of M. arenaria in the Baltic Sea is not known, this species is known as one of the most common shallow-water molluscs of western Europe and as a naturalized species in the most of the Baltic Sea. Its impact in the Baltic Sea has been benign (Leppäkoski and Olenin 2000).

6. Distribution and population dynamics
M. arenaria is widely distributed bivalve species in the whole Estonian coastal sea. It often plays a dominating role in local benthic communities.
M. arenaria has a broad spectrum of habitat and food preference. It is found in soft sea bottoms ranging from hard, stony sand to pure mud. Although it may be more abundant in some substrates than in others, the type of soil seems to have little influence on the presence or absence of M. arenaria. Apparently, the only requirement are substrates loose enough for M. arenaria to dig in and yet stable enough to prevent their burrows from being destroyed too frequently (Strasser 1999).
M. arenaria tolerates a wide range of salinities and temperatures and has high resistance to the presence of H2S and to O2 deficiency. The lowest mean salinity tolerance of M. arenaria is 4.5-5 ‰ in the northern Baltic Sea. Under natural conditions M. arenaria tolerates water temperatures down to -2?C. However, exceptionally severe winters may cause substantial mortality. The critical upper temperature is about 28 ?C (Strasser 1999).

Broken line shows the eastern distribution limit of M. arenaria in the Estonian coastal sea.

7. Ecological and economical impact
The effects of M. arenaria on the sublittoral community ecology are many and varied. It acts as an effective biofilter utilizing the increased resources of particulate organic matter, which have increased due to eutrophication (Leppäkoski and Olenin 2000). M. arenaria shows high niche overlap with other common bivalves. For instance, they all rely on the same food source, most of them spawn roughly at the same time, and the early stages are heavily preyed upon by the same set of predators. No information is available on the effects of M. arenaria on the native communities immediately after its invasion several hundred years ago. Today no negative effects on other species have been observed (Strasser 1999). M. arenaria also produces a great quantity of planktonic larvae, which forms a food basis for fish. Mya shells entered sedimentary processes changing the granulomentry and chemistry of the sediment (Leppäkoski and Olenin 2000).

8. More information
Jonne Kotta, e-mail: jonne@klab.envir.ee

9. References
Kotta, I. and Kotta, J. 1997. Changes in zoobenthic communities in Estonian waters between the 1970´s and 1990´s. An example from the southern coast of Saaremaa and Muuga Bay. In: Proceedings of the 14th Baltic Marine Biologists Symposium (Ojaveer, E., ed.). Estonian Academy Publishers, Pärnu, Estonia, pp. 70-79.
Kotta, J. 2000. Impact of eutrophication and biological invasions on the structure and functions of benthic macrofauna. Dissertationes Biologicae Universitatis Tartuensis, 63, Tartu University Press, pp. 1-160.
Kotta, J. and Orav, H. 2001. Role of benthic macroalgae in regulating macrozoobenthic assemblages in the Väinameri (north-eastern Baltic Sea). Ann. Zool. Fennici 38: 163-171.
Leppäkoski, E. and Olenin, S. 2000. Xenodiversity of the European brackish water seas: the North American contribution. In: Pederson, J. (ed) Marine Bioinvasions. Proceedings of the First National Conference, January 24-27, 1999. Massachusetts Institute of Technology, pp. 107-119.
Leppäkoski, E. and Olenin, S. 2001. The meltdown of biogeographical peculiarities of the Baltic Sea: the interaction of natural and man-made processes. Ambio 30: 202-209.
Strasser, M., 1999. Mya arenaria - an ancient invader of the North Sea coast. Helgoländer Meresunters. 52: 309-324.

1. Scientific name: Potamopyrgus antipodarum (Gray) (Gastropoda, Hydrobiidae)
Common name: Jenkin's spire shell

Photo by Jonne Kotta.

2. Identification
With six tumid whorls, often encrusted with black deposit. Last whorl large; aperture ear-shaped or oval; inner lip occludes umbilicus. Some shells, notably from brackish water populations, have a spiral keel, with or without periostracal bristles; some have a spiral row of bristles but lack keel. Beneath black encrustations shell is pale horn colour. Up to 6 ´ 3 mm. Snout evenly and darkly pigmented, with pale transverse band anteriorly. Cephalic tentacles pale with darker base; mantle edge without pallial tentacle (Hayward and Ryland 1995).

3. Natural distribution
Native to New Zealand but is widespread throughout eastern Australia and Europe. Since 1987 P. antipodarum has been found in North America. Common in freshwater and brackish (< 16‰) habitats, often with Hydrobia ventrosa in the latter. Most population comprise females only, which reproduce parthenogenetically, and males are rare. Widely distributed throughout Europe (Hayward and Ryland 1995).

4. First finding in Estonia
Late XIX century.

5. Invasion history in the Baltic Sea
Late XIX century via ship traffic.

6. Distribution and population dynamics
Established populations are located in the sea areas adjacent to ports and river estuaries. Very dense populations are found in Tallinn and Kolga bays, middle Gulf of Finland and in the southern coastal sea of Saaremaa Island and Pärnu Bay, northeastern Gulf of Riga. The densities may reach to 37,000 ind. m-2.
The snail inhabits mainly sandy, gravel bottoms but likewise filamentous green algae (Cladophora glomerata) and attached aquatic macrophytes. It can successfully exploit both eutrophic and nonpolluted waters.

Crosses show the recent distribution area of P. antipodarum in the Estonian coastal sea.

7. Ecological and economical impact
Resource competition with native molluscs.

8. More information
Jonne Kotta, e-mail: jonne@klab.envir.ee

9. References

Hayward, P. J. and Ryland, J. S. 1995. Handbook of the marine fauna of North-West Europe. Oxford Univeristy Press, New York.
Järvekülg, A. 1970. Väinamere põhjaloomastik. In: Kumari, E. (ed) Lääne-Eesti rannikualade loodus. Loodusuurijate selts, Valgus, Tallinn.
Järvekülg, A. and Veldre, I. 1963. Elu Läänemeres. Eesti Riiklik Kirjastus, Tallinn.
Kotta, I. and Kotta, J. 1997. Changes in zoobenthic communities in Estonian waters between the 1970´s and 1990´s. An example from the southern coast of Saaremaa and Muuga Bay. In: Proceedings of the 14th Baltic Marine Biologists Symposium (Ojaveer, E., ed.). Estonian Academy Publishers, Pärnu, Estonia, pp. 70-79.