Ctenophores of the Baltic and adjacent Seas – the invader Mnemiopsis is here !

The invasive ctenophore Mnemiopsis leidyi has been detected during late summer and autumn 2006 in northern Europe. So far it has been found in the southern North Sea area, along the Swedish west coast, in the southwestern Baltic and along the south and southwestern Norwegian coasts. From the large populations observed, it is obvious that it must have been introduced before 2006, but remained unrecorded until this autumn.

The only comb jelly known to be widespread in the Baltic is the "Sea Gooseberry" Pleurobrachia pileus (O.F. Müller, 1776) belonging to the order Cydippida.This species, which may reach a length of 25 mm except its tentacles, is common in the marine waters of almost all northern Europe and can tolerate salinities down to ca 6.5 psu.In the southernmost part of the Baltic two other species of the group may regularly be found as a result of water inflow from the Kattegatt.One of these is Bolinopsis infundibulum (O.F.Müller, 1776) belonging to the order Lobata, i.e. species with two oral lobes.B. infundibulum may reach a length of 15 cm.The other is the less than 16 cm long, somewhat elongate rugby ball shaped Beroe cucumis O. Fabricius, 1780, belonging to the only completely atentaculate order, Beroida.While cydippids and lobates prey upon small planktonic organisms, the sack shaped beroids are predators on predominantly large gelatinous plankton organisms.A second and smaller (usually less than 3 cm long) species of the latter genus, B. gracilis Künne, 1939, is common in the North Sea and the Skagerrak (and likely also in the Kattegatt) and may possibly also enter the southwestern part of the Baltic occasionally.This species is feeding almost exclusively on Pleurobrachia, while the up to 16 cm long B. cucumis may consume other larger organisms as well, mainly Bolinopsis sp., and -if availablealso salps.These species are easy to separate from each other.B. cucumis has a branched gastrovascular system between each of the 8 longitudinal rows of comb plates, while no trace of such a system is seen in B. gracilis.No other pelagic ctenophores are known to occur in western European shallow waters.
In autumn 2006 (October 31) a ctenophore species of the genus Mnemiopsis L. Agassiz, 1860 was for the first time reported from the Baltic, by Jamileh Javidpour who studied Pleurobrachia sp. in the Kiel Bay (54°40'N, 10°30'E) (Javidpour 2006, see also Javidpour et al. 2006, this issue).Javidpour reported "considerable numbers" of less than 5 cm long specimens from October onwards.Densities up to 92 individuals per cubic meter were found (Javidpour et al. 2006, this issue).I had myself observed a few specimens from a plankton haul in the vicinity of the Tjärnö Marine Biological Laboratory (TMBL) at the northern part of the Swedish west coast (58°52'N, 11°06'E) in mid September and again on the 8th of October.A marine botanist colleague, Jan Karlsson, snorkeling in the area during late August, later told me that he observed several P. pileus, many specimens of a very long and thin, almost completely hyaline species of Beroe -likely B. gracilis -together with thousands of lobed ctenophores in the warm surface water.As B. infundilum is a cold water species, it must have been Mnemiopsis.
The temperatures in the uppermost water masses outside TMBL have been exceptionally warm during the late summer and autumn in 2006, approximately 19-20°C in September when Mnemiopsis first was observed, approximately 15°C (down to a depth of ca 50 meters) during most of October, and by the end of November approximately 9°C.This is at least 3°C more than the usual temperatures for the area.
Another exceptional circumstance during this time was the virtual lack of the usually very common pelagic scyphomedusae in the region.Not a single Aurelia aurita have been observed during all late summer and autumn and only rather few Cyanea capillata and exceptional specimens of Rhizostoma pulmo and Chrysaora hysoscella were found.It is not clear if the few scyphomedusae may have supported the occurrence of Mnemiopsis, but the high water temperatures may have been beneficial.
Overwintering has probably occurred in estuaries along the North Sea coast (see Faasse and Bayha 2006, this issue), British Islands or the Atlantic part of Europe, from where offspring has drifted north-and eastwards.Mnemiopsis sp. may occur in temperatures from -0.7 to 32 (or even 35) °C (Miller 1974) and in salinities between 3.4-70 ppt (Miller 1974), but for short times slightly lower or higher values are tolerated.In low salinity areas, they die at higher temperatures (Shiganova et al. 2001).They breed only in temperatures between ca 19-23°C during darkness and when plenty of food is available (GESAMP 1997).
Whether the introduction to northern Europe happened through ships ballast water discharge from North America or the Black Sea remains unclear.Both Mnemiopsis sp. and B. ovata (see below) have -after their introduction to the Black Sea -also spread to the eastern parts of the Mediterranean, but no occurrence in the western parts of this sea has been published (Shiganova and Panov 2003).
Mnemiopsis has inner structures of the lobes originating from a position at the same level as the apical organ, not far from the top of the animal, while these structures in the similar genus Bolinopsis originate from a much lower level, almost halfway down between the top and the mouth side.Both of these genera have eight meridianal canals.
Diving colleagues have observed rather large amounts of the new species in the vicinity of TMBL during mid October 2006 and several have also been caught here in plankton nets in November.Because of the huge amount of specimens, it is unlikely that the introduction to north European waters happened this year and it is assumed that the species occurred before, although undetected until recently.The first to positively identify the taxon from this region was likely Keith M. Bayha, Dauphin Island Sea Laboratory, USA.Photos of the individuals caught (Figure 1) and samples from northern Europe were analysed in the first half of September 2006.After sequencing the material for rDNA and mitochondrial DNA, it was confirmed that it was definitively Mnemiopsis sp.(Faasse and Bayha 2006, this issue).Mnemiopsis was originally described from Charleston Harbor, South Carolina, where the type species, named M. gardeni L. Agassiz, 1860 was found.Five years later the author's son Alexander described a new species from the Woods Hole area, M. leidyi A. Agassiz, 1865.Another -and last -description of a new species of this genus was named M. mccradyi A.G. Mayer, 1900, also found in the Charleston Harbor area.The species in this genus are originally known from around 42° N to around 46° S along the American coast.Seravin (1994) revised the genus, and concluded that all 3 species M. mccradyi, M. leidyi and M. gardeni are one and the same, but are polymorphic because of environmental adaptations.Most specialists seem to agree that there is only one species (e.g.Bayha et al. 2003), i.e.Mnemiopsis leidyi (A. Agassiz, 1865).The specimen seen from the Swedish west coast has been lacking surface papillae.Papillae may be a sign of age rather than a species difference, because very large specimens found off the US Atlantic seaboard are often very "warty" and muscular, while smaller specimens from the same waters are "smooth" (Moss pers.comm.).
In coming years we have to be very observant regarding the new invader.In the Black Sea the introduction had severe consequences in the beginning, especially for the anchovy and European and Caspian kilka fishery.The decline of the anchovy in the Black Sea was likely due to a combination of overfishing, eutrophication and the introduction of Mnemiopsis (Bilio and Niermann 2004).After the introduction of the predatory ctenophore Beroe ovata sensu Mayer, 1912Mayer, , during 1997 which also originates from the Atlantic American waters, the situation has changed (Gordina et al. 2005).
Mnemiopsis has a broad food spectrum which includes fish eggs and larvae, different kinds of smaller holoplanktonic animals, and pelagic larvae of different benthic invertebrates.Even protozoan plankton (e.g.Ciliophora) may be consumed, but it cannot survive on passively ingested phytoplankton only.In areas with dense populations of this ctenophore, it may locally affect populations of species with late summer or autumn larvae (Burrell andVan Engel 1976, Bullard et al. 1999).
"B. ovata" does not reach the full size of the native B. cucumis, but is very similar.While B. cucumis usually is approximately two times longer than its diameter with a rounded oral side, "B.ovata" usually is a bit shorter, with a more flattened oral side and is also somewhat flatter seen from the sides.An adult specimen is around 1.2 -3.3 times higher as width.It cannot be exclude that this predator may find its way also to northern Europe like it did to the Black Sea and it is a species to look out for in the future.Likely, however, "B.ovata" may have lower tolerance limits for salinity and temperature than Mnemiopsis and it remains unclear how far north along the coast of Europe it may spread.Along the US coast small specimens of Beroe may "bite off" small pieces from larger Mnemiopsis specimens (Moss pers.comm.).
The coastal waters in northern Europeexcept for a few summer months -are in the lower limit of the temperature tolerance of Mnemiopsis, so hopefully their impact will be less severe than in the Black and Caspian Seas.However, knowing the potential impact of Mnemiopsis this new record from the Baltic Sea is of concern.

Figure 1 .
Figure 1.A specimen of Mnemiopsis, a little smaller than 20 mm long, from the Baltic Sea.Two long comb plate rows can be seen along the lobe to the left and two shorter rows along the central body.Iridescens is visible from the two long rows along the edge of the body.The inner structures of the lobes reaching almost to the apical organ is also fairly evident.Photo: Nicklas Wijkmark