Top 5 marine invaders

Invasive alien species are considered to be one of the greatest threats to global biodiversity. There is an urgent need to anticipate
which invasive species are likely to cause future problems so that preventative action can be taken. Work has recently been undertaken to determine
which species are most likely to invade British shores.

Below are five species that are considered to present a significant risk to native habitats, fauna and flora.

Asian shore crab (Hemigrapsus sanguineus)

The Asian shore crab is thought to have been introduced to European waters in ships’ ballast water. This species can spread easily through dispersion of
its larvae. This species is reported to cause significant reductions in the common shore crab (Carcinus maenas) by competing with it and displacing
it from its habitat. Invertebrates such as mussels, snails, barnacles and polychaetes may also be threatened due to increased predation. It is thought
that this species could also pose a predation threat to commercial mussel and oyster beds. And it could compete with juvenile edible crabs (Cancer pagurus)
which is another species of economic value. A few individuals have already been recorded in Kent and South Wales.

Comb jelly (Mnemiopsis leidyi)

The comb jelly, or sea walnut, was introduced to the Black Sea in ballast water in the 1980s and since then has spread and been recorded in Danish and
Dutch waters. The comb jelly is a voracious predator of zooplankton, fish eggs and larvae. Declines in planktivorous fish species, such as anchovy,
sprat and horse mackerel, had a devastating effect on fisheries in the Black Sea, costing hundreds of millions of US dollars. This has been attributed
to the comb helly. Impacts have been noticed higher up the trophic levels . Declines in dolphin numbers in the Black Sea, and seal numbers in the Caspian
Sea, have also been attributed to the presence of comb jellyfish.

Veined rapa whelk (Rapana venosa)

The veined rapa whelk is a very large, active predatory snail which can grow to up to 18 cm in length. It has not been recorded on British shores; however,
it has been recorded in the North Sea and the Channel. This whelk could outcompete the native common whelk (Buccinum undatum) as it is able to consume
large quantities of prey. The reduction in prey could also impact other predators including crabs, birds, fish and starfish. Due to its predatory impact
this species is considered to be one of the most unwelcome invasive species worldwide.

American lobster (Homarus americanus)

American lobsters have been introduced to British waters through accidental or deliberate release by the catering industry. It is not thought to have become
established in British waters yet;however, recordings of individuals date back to 1988. American lobsters could outcompete the native European lobster
(Homarus gammarus) as they are larger, more aggressive and can inhabit a broader range of habitats. They could also compete for resources
with other environmentally and economically important species. The American lobster carries diseases which can be transferred to our native European
lobster. The blood disease gaffkemia causes a rapid 100% mortality in European lobsters within a few days of exposure. This could have significant
impacts on wild and commercial populations.

Celtodoryx ciocalyptoides

C. ciocalyptoides is a species of sponge which originates from the North West Pacific. It is thought that it was introduced to European waters through
the transfer of Pacific oysters for aquaculture in France and the Netherlands. C. ciocalyptoides is an encrusting sponge and can cover a large area.
Consequently it competes successfully with other microbenthic organisms and can overgrow other sessile invertebrates. It is now the dominant benthic
megafauna in Dutch inshore waters, displacing native benthic communities.

Communication and cross-boundary collaborations are essential to ensure that early warnings of and knowledge about invasive species are shared between
neighbouring countries. Early identification of species that could pose a threat to our native species and habitats can allow measures to be put in
place to prevent or manage their arrival. Once a species is established, there is no guarantee it can be eradicated.

1 Roy et al. (2014) Horizon scanning for invasive alien species with the potential to threaten biodiversity in Great Britain. Global Change Biology 20:






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