112
cies, but often also unintentionally introduces organisms,
which are associated with them if they are not vigorously
cleaned before transportation. Worldwide, the introductions
of more than 40 marine species can be directly linked to the
translocation of bivalves used in aquaculture (Padilla et al.
2011), and in total 206 NIS have been linked to this vector in
Europe alone (Katsanevakis et al. 2013; Fig. 1). Particularly
invasive ecosystem-engineers like reef-building oysters (esp.
M. gigas and Crassostrea virginica) promote the establishment of NIS they brought along by providing favorable habitats, which eventually further enhances community shifts in
the invaded systems (e.g., Ruiz et al. 2000; Markert et al.
2010; Padilla et al. 2011). Another important taxon in this
context are macroalgae, which are regularly introduced as
‘blind passengers’ with aquaculture organisms. They can
likewise change existing or form new habitats, thus affecting
both native and other alien species (e.g., Jones and Thornber
2010; Salvaterra et al. 2013; Thomsen et al. 2016).
Floating (plastic) litter is a vector recently gaining attention. While the marine litter problem is mostly discussed
under the aspect of pollution and the hazardous effects of
microplastic accumulation, larger pieces of litter are also a
possible habitat for fouling organisms, which might then be
transported over large distances by oceanic currents. Recent
studies found a variety of species from different taxonomic
groups (including bryozoans, barnacles and mollusks) settling on macroplastic, with a considerable proportion of
marine NIS among them (Barnes and Milner 2005; Gregory
2009; Gil and Pfaller 2016). While driftwood and other
debris may already historically have played a role in the cosmopolitan distribution of species like Teredo navalis
(Bivalvia, Myoida) or Lepas anatifera (Crustacea,
Pedunculata), the recent extreme increase in amounts of
marine litter may lead to a future increase in numbers on
marine NIS dispersed by this vector (Gregory 2009).
Trade of ornamental and aquarium-kept organisms has
been widely neglected by scientists and policy makers as an
introduction pathway, although it bears a high potential for
species invasions (Padilla and Williams 2004). Introductions
of aquarium organisms to natural environments may occur
accidentally, when organisms escape during transport or, for
example, from public aquaria with in-/outflow from/to natural
water bodies, or intentionally, when hobbyists or traders
release single individuals or discard the contents of whole
aquaria into the wild. Fish and macroalgae are the taxa with
the highest numbers of species (potentially) introduced by
aquarium trade. Zenetos et al. (2016) list 19 introduced fish
species with a potential link to this vector in the Mediterranean
Sea alone, and Vranken et al. (2018) identified at least 23 seaweed species commonly found in aquaria across Europe,
which have the potential to thrive European natural waters,
with the highly invasive Caulerpa taxifolia as the most striking example (compare also Padilla and Williams 2004;
Fig. 1). Besides the usually ornamental target species, aquarium trade may also account for unintentional introductions of
associated species, especially epibionts on seaweeds and live
rock used for aquarium decoration, such as macro- and microalgae, (hemi)sessile cnidarians, crustaceans, polychaetes or
mollusks (Padilla and Williams 2004). Aquarium trade is a
strongly growing economy, and commercial and private
online retailers make exotic species easily available worldwide via the internet, rendering this vector extremely difficult
to control and regulate (Padilla and Williams 2004; Mazza
et al. 2015; Vranken et al. 2018). Today, the Mediterranean
and southern European Atlantic are the regions within Europe
which are most affected by this introduction pathway, due to
the (sub)tropical origin of most of the traded species. In the
light of ongoing ocean warming, also more temperate regions
might get invaded by these species in the future (Vranken
et al. 2018). Thus, the number of marine invasions promoted
by aquarium trade are very likely to increase in the future.
Although not thought of as a ‘classical’ vector, canals are
a major introduction pathway for marine NIS. The best
known example is the Suez Canal, connecting the
Mediterranean Sea to the Red Sea and Indian Ocean, which
accounts for the vast majority of species invasions to the
Mediterranean by migration through the canal (Lessepsian
migration) (Galil 2009). The Baltic Sea, as another example,
was invaded by numerous ponto-caspian species since it is
connected to the Black Sea by a system of canals and rivers
(Leppäkoski et al. 2002; Katsanevakis et al. 2013).
Additionally, the Kiel Canal provides a shortcut route
between the southwestern Baltic and the southeastern North
Sea. It likely served as an invasion pathway for numerous
species native or invasive to the Atlantic, like the crabs
Rhithropanopeus harrisii and H. takanoi (Fowler et al. 2013;
Geburzi et al. 2015). An interesting case in this context is the
shrimp Palaemon elegans, of which an Atlantic type invaded
the Baltic Sea from the west, and a Mediterranean/Black
Sea-type invaded from the southeast (Reuschel et al. 2010).
Besides opening routes for the active migration or natural
(e.g., larval) dispersal processes, canals also increase the
probability for successful ship-mediated introductions, as
they shorten transportation times, thus increasing survival
probabilities, e.g., for organisms in ballast water tanks.
Human activities not only provide vectors and pathways
for species’ introductions, but they also impact the environment in ways that can promote the establishment success of
marine NIS, in particular by changing natural habitats. The
‘invasibility’ of a community or habitat, i.e., its receptivity
towards invasive species, can be strongly influenced by
human activities. Apart from the propagule pressure of
invaders, it largely depends on the availability of suitable
niches and resources. Anthropogenic habitat changes (addition or depletion of different niches) and disturbances leading to a reduction in native diversity (increasing resource
J. C. Geburzi and M. L. McCarthy
cies, but often also unintentionally introduces organisms,
which are associated with them if they are not vigorously
cleaned before transportation. Worldwide, the introductions
of more than 40 marine species can be directly linked to the
translocation of bivalves used in aquaculture (Padilla et al.
2011), and in total 206 NIS have been linked to this vector in
Europe alone (Katsanevakis et al. 2013; Fig. 1). Particularly
invasive ecosystem-engineers like reef-building oysters (esp.
M. gigas and Crassostrea virginica) promote the establishment of NIS they brought along by providing favorable habitats, which eventually further enhances community shifts in
the invaded systems (e.g., Ruiz et al. 2000; Markert et al.
2010; Padilla et al. 2011). Another important taxon in this
context are macroalgae, which are regularly introduced as
‘blind passengers’ with aquaculture organisms. They can
likewise change existing or form new habitats, thus affecting
both native and other alien species (e.g., Jones and Thornber
2010; Salvaterra et al. 2013; Thomsen et al. 2016).
Floating (plastic) litter is a vector recently gaining attention. While the marine litter problem is mostly discussed
under the aspect of pollution and the hazardous effects of
microplastic accumulation, larger pieces of litter are also a
possible habitat for fouling organisms, which might then be
transported over large distances by oceanic currents. Recent
studies found a variety of species from different taxonomic
groups (including bryozoans, barnacles and mollusks) settling on macroplastic, with a considerable proportion of
marine NIS among them (Barnes and Milner 2005; Gregory
2009; Gil and Pfaller 2016). While driftwood and other
debris may already historically have played a role in the cosmopolitan distribution of species like Teredo navalis
(Bivalvia, Myoida) or Lepas anatifera (Crustacea,
Pedunculata), the recent extreme increase in amounts of
marine litter may lead to a future increase in numbers on
marine NIS dispersed by this vector (Gregory 2009).
Trade of ornamental and aquarium-kept organisms has
been widely neglected by scientists and policy makers as an
introduction pathway, although it bears a high potential for
species invasions (Padilla and Williams 2004). Introductions
of aquarium organisms to natural environments may occur
accidentally, when organisms escape during transport or, for
example, from public aquaria with in-/outflow from/to natural
water bodies, or intentionally, when hobbyists or traders
release single individuals or discard the contents of whole
aquaria into the wild. Fish and macroalgae are the taxa with
the highest numbers of species (potentially) introduced by
aquarium trade. Zenetos et al. (2016) list 19 introduced fish
species with a potential link to this vector in the Mediterranean
Sea alone, and Vranken et al. (2018) identified at least 23 seaweed species commonly found in aquaria across Europe,
which have the potential to thrive European natural waters,
with the highly invasive Caulerpa taxifolia as the most striking example (compare also Padilla and Williams 2004;
Fig. 1). Besides the usually ornamental target species, aquarium trade may also account for unintentional introductions of
associated species, especially epibionts on seaweeds and live
rock used for aquarium decoration, such as macro- and microalgae, (hemi)sessile cnidarians, crustaceans, polychaetes or
mollusks (Padilla and Williams 2004). Aquarium trade is a
strongly growing economy, and commercial and private
online retailers make exotic species easily available worldwide via the internet, rendering this vector extremely difficult
to control and regulate (Padilla and Williams 2004; Mazza
et al. 2015; Vranken et al. 2018). Today, the Mediterranean
and southern European Atlantic are the regions within Europe
which are most affected by this introduction pathway, due to
the (sub)tropical origin of most of the traded species. In the
light of ongoing ocean warming, also more temperate regions
might get invaded by these species in the future (Vranken
et al. 2018). Thus, the number of marine invasions promoted
by aquarium trade are very likely to increase in the future.
Although not thought of as a ‘classical’ vector, canals are
a major introduction pathway for marine NIS. The best
known example is the Suez Canal, connecting the
Mediterranean Sea to the Red Sea and Indian Ocean, which
accounts for the vast majority of species invasions to the
Mediterranean by migration through the canal (Lessepsian
migration) (Galil 2009). The Baltic Sea, as another example,
was invaded by numerous ponto-caspian species since it is
connected to the Black Sea by a system of canals and rivers
(Leppäkoski et al. 2002; Katsanevakis et al. 2013).
Additionally, the Kiel Canal provides a shortcut route
between the southwestern Baltic and the southeastern North
Sea. It likely served as an invasion pathway for numerous
species native or invasive to the Atlantic, like the crabs
Rhithropanopeus harrisii and H. takanoi (Fowler et al. 2013;
Geburzi et al. 2015). An interesting case in this context is the
shrimp Palaemon elegans, of which an Atlantic type invaded
the Baltic Sea from the west, and a Mediterranean/Black
Sea-type invaded from the southeast (Reuschel et al. 2010).
Besides opening routes for the active migration or natural
(e.g., larval) dispersal processes, canals also increase the
probability for successful ship-mediated introductions, as
they shorten transportation times, thus increasing survival
probabilities, e.g., for organisms in ballast water tanks.
Human activities not only provide vectors and pathways
for species’ introductions, but they also impact the environment in ways that can promote the establishment success of
marine NIS, in particular by changing natural habitats. The
‘invasibility’ of a community or habitat, i.e., its receptivity
towards invasive species, can be strongly influenced by
human activities. Apart from the propagule pressure of
invaders, it largely depends on the availability of suitable
niches and resources. Anthropogenic habitat changes (addition or depletion of different niches) and disturbances leading to a reduction in native diversity (increasing resource
J. C. Geburzi and M. L. McCarthy