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408 British Wildlife August 2011
Killer Shrimps in Britain: hype or horror?
August 2011 British Wildlife 409
On 3rd September 2010, a fisherman at
Grafham Water, Cambridgeshire, found
a strange-looking shrimp on his waders.
The Environment Agency (EA) team at Bramp-
ton, with help from Dirk Platvoet, a Dutch expert,
soon confirmed its worst fears: the Killer Shrimp
Dikerogammarus villosus had arrived in Britain
(MacNeil et al. 2010). In November 2010, two
further populations were found: one in Cardiff
Bay, and another in Eglwys Nunydd reservoir,
Port Talbot, Wales.
Within weeks the story had hit the local and
national press, with many alarmist headlines.
While the environmental agencies involved
deliberated as to whether, as level-headed public
bodies, they should really be calling it the ‘Killer
Shrimp’, there was little choice. ‘Killer Shrimp’ is
the only widely used common name for Dikerog-
ammarus villosus and the media soon latched on
to it. But does our most recent invasive animal live
up to its name?
Here we present the facts about this species,
based on what we know so far; the story of its
spread, its ecology and the likely threat to conser-
vation interests. We also will consider what, if
anything, can be done about it now it is in Britain.
Genevieve Madgwick and David C Aldridge
Killer Shrimps in
Britain: hype or horror?
The facts about
our latest invasive animal
Killer Shrimps range in size from
1.6mm to 30mm. Environment Agency
408 British Wildlife August 2011
Killer Shrimps in Britain: hype or horror?
August 2011 British Wildlife 409
How to spot it
The Killer Shrimp is a crustacean (order Amphip-
oda, family Gammaridae). Gammarids are the most
common amphipods in European fresh waters, and
in Britain we have half a dozen freshwater species.
Typical of amphipods, the female Killer Shrimp
carries its eggs in a brood pouch until the young
hatch. Individuals range in size from a minimum of
1.6mm at hatching (Devin et al. 2004) to a maxi-
mum size of over 30mm, significantly larger than
our native freshwater shrimp species, whose maxi-
mum size is 20mm. As well as size, the key identifi-
cation feature of the Killer Shrimp is the presence of
two cone-shaped protrusions on the tail. It is often
striped, although this varies. It likes to hide between
stones and in the crevices under rocks.
How much of a ‘killer’ is it?
Experience on the Continent and from scien-
tific studies in the field, in the laboratory and in
controlled mesocosm experiments all stack up
to paint a rather gruesome portrait of a serious
invader (Dick & Platvoet 2000; Dick et al. 2002;
Boets et al. 2010; MacNeil et al. 2011). Like most
freshwater shrimps, it is an omnivore, but, unlike
others, it also has prominent predatory traits, such
as powerful mouthparts, well-developed antennae
to help catch prey and a fast ambush technique.
The Killer Shrimp can establish large populations
very quickly in invaded areas, helped by its high
fecundity and early sexual maturity compared
with other freshwater shrimps (Devin et al. 2004;
Pöckl 2007).
How to identify Dikerogammerus villosus.
Detail from identification factsheet produced by
GB Non-native Species Secretariat with assistance from the
Freshwater Biological Association and Environment Agency.
Available as a download from www.nonnativespecies.org.
Killer Shrimps in Britain: hype or horror?
410 British Wildlife August 2011
Killer Shrimps in Britain: hype or horror?
August 2011 British Wildlife 411
This non-specific feeder consumes prey items
up to 40mm in size, preying on almost anything,
including other invertebrates such as water fleas,
waterboatmen, Water Hoglice Asellus aquaticus,
snails, mayflies, damselflies, leeches and crayfish
larvae (Dick et al. 2002; Boets et al. 2010). In
European lakes that have been invaded by Killer
Shrimps, this species has outcompeted or preyed
on once abundant native freshwater shrimps (e.g.
Gammarus pulex, G. roeseli and Echinogamma-
rus stammeri) to the extent of their exclusion in
suitable habitats (Dick & Platvoet 2000; Casellato
et al. 2006; Kinzler et al. 2009). In fact, inverte-
brate diversity is generally poor in habitats where
the Killer Shrimp has become well established (van
Riel et al. 2006a; Boets et al. 2010).
Killer Shrimps do not confine themselves to
smaller invertebrates and have been observed to
consume fish eggs and fry. It may be no coinci-
dence that the disappearance of native Bullhead
Cottus gobio populations in Lake Gouwzee, in
The Netherlands, occurred with the arrival of this
shrimp (Platvoet et al. 2009).
Studies also suggest that there may be wider
ecosystem impacts, such as a reduction in leaf
shredding in the invertebrate community, an
important route for the introduction of finer
organic particles into the aquatic food chain
(MacNeil et al. 2011). In Lake Constance, the
exclusion of the native freshwater shrimp Gamma-
rus roeseli resulted in an observed shift in fish
diets from the native to the invader (Eckmann et
al. 2008). It is not yet known what the full conse-
quences of this might be, but any simplification of
the food chain caused by a decrease in invertebrate
diversity is likely to result in a less resilient system.
This in turn may increase vulnerability to future
invasive species, referred to as ‘invasional melt-
down’ by Simberloff & von Holle (1999).
Some of the ecosystem-level effects of a high
Killer Shrimp biomass may already be evident in
Britain, although research is still required to quan-
tify these. In Grafham Water, it is striking that
the distribution of Brown Trout Salmo trutta and
Rainbow Trout Oncorhynchus mykiss seems to
have shifted to dominate marginal habitats. Fish-
ermen reported trout guts full of the shrimp, and
angling tactics had to change, as rocky margins
and the dam wall yielded the best catches, and flies
that resembled Killer Shrimps became bestsellers.
In some locations at Grafham, once abundant
populations of native (Gammarus spp.) and inva-
sive (Chelicorophium curvispinum) crustaceans
are now conspicuous by their
absence (Aldridge pers. obs.).
All these possible impacts are
alarming for the conservation
of our freshwater habitats and
species. We risk extirpation of
familiar common species such
as Gammarus pulex, as well
as potentially increasing the
pressure on already threatened
Biodiversity Action Plan species
such as White-clawed Crayfish
Austropotamobius pallipes,
Spined Loach Cobitis taenia
and Vendace Coregonus albula.
In reality, we would expect
Killer Shrimp, to become domi-
nant only in optimal micro-
The Zebra Mussel and Killer Shrimp have similar patterning, suggesting
these invaders co-evolved. Dirk Platvoet, Amsterdam University
Grafham Water reservoir is an SSSI for breeding
and overwintering bird species, as well as a popular
fishing and sailing venue. Gen Madgwick
Killer Shrimps in Britain: hype or horror?
410 British Wildlife August 2011
Killer Shrimps in Britain: hype or horror?
August 2011 British Wildlife 411
habitats. In particular, they seem to thrive only
where there is a stony, rocky, gravelly or artificial
substrate (Devin et al. 2003). One substrate espe-
cially favoured is the shells and ‘beards’ (byssus)
of another invader, the Zebra Mussel Dreissena
polymorpha. These species are likely to have
co-evolved in their native range, as is evident when
the two are viewed together, as the stripes often
found on Killer Shrimp offer camouflage against a
Zebra Mussel background.
Previous and future spread
The Killer Shrimp originates from the Ponto-
Caspian basin in eastern Europe. It began expand-
ing up the Danube in the late 1980s, facilitated by
increased connectivity of waterways and intensi-
fication of boat traffic. Following the opening of
the Main-Danube canal in 1992, it quickly spread
throughout western Europe via the Rhine and
connected waterways, invading rivers and lakes in
France, Germany, The Netherlands and Belgium.
In the late 1990s, the risks of invasion across the
sea to the UK and USA were highlighted, as the
species has a high salinity tolerance and is able to
survive incomplete water exchange of ship saline
ballast (Bruijs et al. 2001; Ricciardi & Rasmussen
1998; Dick & Platvoet 2000).
Downstream dispersal is likely to be achieved
through drift (van Riel et al. 2006b) and in
the Rhine has occurred at a rate of 124km/
year (Leuven et al. 2009). Upstream dispersal
(30-40km/year; Josens et al. 2005) is likely to be
facilitated by shipping. The idea that the role of
human-mediated transport is important for its
dispersal is reinforced by the shrimp’s arrival since
2000 in Lake Constance (bordering Austria, Swit-
zerland and Germany), Lake Geneva (Switzer-
land) and Lake Garda (Italy), despite its not being
found in their immediate adjoining rivers. This
finding led the environmental bodies to conclude
that the Killer Shrimp was also being transported
overland, probably via pleasure craft and other
recreational water-users (H Löffler pers. comm.,
Landesanstalt für Umwelt, Messungen und Natur-
schutz Baden-Württemberg).
The current broad geographical distribution
and high but localised abundance of the Killer
Shrimp in Britain suggests that it has potential
to occupy a wide area. Habitat favoured by the
species is found throughout Britain in the form of
embanked slow-flowing rivers, canals, reservoirs
and lakes with stony shores, artificial embank-
ments and populations of Zebra Mussels. Some
environmental parameters may restrict the species;
it has yet to be found in small, fast-flowing rivers
or waters with a low alkalinity (Bij de Vaate et al.
2002; Boets et al. 2010).
All three sites in Britain found to hold the
species in 2010 appear not to have been invaded
via tributaries (which are free from shrimps), but
all host recreational uses including sailing, water-
sports and fishing. It is therefore reasonable to
assume that other sites with suitable habitat and
water-based recreational activities could be at risk
of invasion, including more than 50 waterbodies
in England designated as Sites of Special Scientific
Interest (SSSIs). These areas are protected for the
conservation importance of their aquatic habitats
and species.
What next?
Since the initial discovery of the shrimp, Defra, the
Welsh Assembly Government, the Environment
Agency, Natural England, the Countryside Coun-
cil for Wales and experts within other organisa-
tions, institutions and NGOs have been working
hard together to review options and reduce the
risk of the Killer Shrimp spreading further. The
voluntary collaboration of recreational water-
users, as well as of organisations such as water
companies and managers of waterbodies, has been
key to the success of containment.
What can we do to stop the spread of
aquatic invasive species?
By following three simple steps when leaving the water,
we can help to stop the spread of aquatic invasive
species.
Check your equipment and clothing
for live organisms, particularly in
areas that are damp or hard to
inspect.
Clean and wash all equipment,
footwear and clothing thoroughly.
If you do come across any
organisms, leave them at the
waterbody where you found them.
Dry all equipment and clothing –
some species can live for many
days in moist conditions.
Make sure that you do not transfer
water elsewhere.
For more information visit www.direct.gov.uk/
checkcleandry.
Killer Shrimps in Britain: hype or horror?
412 British Wildlife August 2011
There is no known practicable means of success-
fully eradicating the shrimp (although options are
being evaluated), so the current emphasis has been
on containment, monitoring of sites likely to be
at risk of invasion, and raising awareness. Defra
and partners have launched the ‘Check, Clean,
Dry’ campaign (see box), aimed at water-users and
designed to encourage simple biosecurity practices
to reduce the risk of inadvertently spreading not
only Killer Shrimp, but a whole host of aquatic
invasive species. It is particularly important to
encourage this practice at waterbodies within
SSSIs.
Any investment put into developing control
agents, attempting eradication or controlling
spread will be worthwhile only if routes into the
country are also investigated and controlled. What
is more, a whole army of other aquatic invasive
species has colonised the River Rhine, and these
are just waiting for their passage across the chan-
nel (Leuven et al. 2009).
At the time of writing, the Environment Agency
has sampled over 3,000 locations, and so far had
not found new sites invaded by the Killer Shrimp,
but that does not mean that it is not out there
somewhere. You can help the monitoring effort by
looking out for the species yourself, but be sure
to check, clean and dry your waders afterwards!
More information and identification guides can be
found at: www.nonnativespecies.org/alerts/killer-
shrimp.
Acknowledgements
We are grateful for the efforts of all those involved
in the rapid response, from the environmental
agencies and NGOs to the site owners and water
users at the front line of our defence against spread.
Thanks also to Ross Holdgate, Lucy Wrapson
and Alastair Burn, and to various members of the
rapid response group for providing comments on
a draft of this article.
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Dr Genevieve Madgwick is a Freshwater
Ecologist at Natural England and until recently
provided specialist advice to the Killer Shrimp
Rapid Response. Dr David Aldridge heads the
Aquatic Ecology Group in Cambridge University’s
Department of Zoology. He sits on the Killer
Shrimp Scientific and Technical Advisory Group
(STAG) and conducted Defra’s Rapid Risk
Assessment for the species.
