BSEP116B Biodiversity in the Baltic Sea - Helcom

Box 6.7.1. Invasion status of the Baltic Sea as of April 2008
Number of species 1)
Number of alien species recorded 120
Number of established 2) species 77
– of which ship-mediated species 40
Major groups 3)
Crustaceans (shrimps, crabs, etc.) 23
Molluscs (snails, mussels, clams, etc.) 9
Fish 8
Oligochaetes 7
Polychaetes (bristle worms) 4
1)
Including the Kattegat.
2)
A few species with unknown status are to be added (in total 25
species).
3)
Only species known to be established were taken into account.
Cergopagis pengoi
Source: Baltic Sea Alien Species Database (http://www.corpi.ku.lt/
nemo; update: 10 April 2008)
some of the most recent invaders have shown a
very rapid expansion. One of the best-documented
invasions is that of the benthic bristle worm Marenzelleria
spp. that currently occurs in the entire Baltic
Sea and has become common in many soft-bottom
habitats and is even a dominant species in some
bottom communities; this has occurred in only the
roughly ten years since its first appearance (Zettler
1996, Cederwall et al. 1999, Perus & Bonsdorff
2004, Figure 6.7.2). Recent genetic studies have also
revealed that the invasion has been made by three
different species (M. viridis, M. neglecta and M.
arctia) that obviously are still expanding their range
(Blank et al. 2008). Other recent alien species with
a rapid invasion over large sea areas in the Baltic
include pelagic species such as the fishhook water
flea Cercopagis pengoi and the American comb
jelly Mnemiopsis leidyi. However, in 2009 scientists
carried out genetic analyses of samples of M. leidyi
and found out that specimen recently identified
as M. leidyi were in fact Mertensia ovum, an arctic
comb jelly (Maiju Lehtiniemi, pers. comm.). American
comb jelly M. leidyi, nevertheless, occurs at least
in the Southern Baltic Sea.
In the Baltic and elsewhere, the increased invasion
rate (Figure 6.7.1, Box 6.7.2) can be related
to several factors: (i) increased number and size
of ships, (ii) increased speed of ships, resulting in
better survival of organisms during the voyage,
(iii) use of separate tanks instead of cargo tanks
for ballast water (less polluted ballast water), (iv)
Box 6.7.2. How do we know which species is a human-mediated newcomer? *)
120
For most of the alien species recorded in the Baltic Sea, there
is evidence of their origin achieved through studies of:
• paleontological and archaeological records (absence of
shells and other remnants),
• historical data (absence in previous surveys and check lists,
documented first collection or first release),
• biogeographical patterns (discontinuous distribution, known
as introduced from other regions),
• dispersal mechanisms (links to human-mediated vectors,
direct evidence of transport),
• molecular genetic evidence, and
• ecological evidence (short larval survival time, community
association (e.g., fouling on ship hulls), post-introduction
range expansion).
However, it is often difficult to determine whether a
species is native or introduced; such species with an
unknown origin are termed cryptogenic (Carlton 1996).
*) Based on lectures given in 1997 by Prof. James T. Carlton
(Williams College, Mystic, Connecticut, USA) at Åbo Akademi
University.