Invasive Alien Species in Northern Ireland

Dikerogammarus villosus, amphipod

Dikerogammarus villosus (Sowinsky, 1894)
Dikerogammarus villosus is a large amphipod crustacean, related to Gammarus pulex, and known throughout the world as the ‘killer shrimp’ (see PICTURE). Remarkably, it can survive not only in fresh water, but also in salty water up to about two-thirds the strength of sea water. It likes hard substrates, such as rocks, but can also be found in vegetation in rivers and lakes. Since the species has spread rapidly throughout Europe in just the last 10 years, it is predicted to reach Ireland soon.
D. villosus is very large for this type of animal, bigger than all our native species, males reaching 3cm in length, females slightly smaller. There are three distinct colour morphs of D. villosus – striped, spotted and melanic (see PICTURE). It also has huge ‘gnathopods’, a pair of appendages with hooks on the end, and its mouthparts are very well developed and strong. If you pick one up and hold it in your fist, you will feel it struggle and they can bite.
Country of origin
The ‘killer shrimp’ comes from the areas around the Black and Caspian Seas, known as the ‘Ponto-Caspian’ region (Pontus=Greek for Black Sea). For example, the Danube Delta region in Romania is the likely source of this species from where it invaded western Europe.
Current distribution
In recent years, D. villosus has been recorded in Germany, France, Italy, Switzerland, Belgium and Holland and new records of the species occur regularly.
Location in Ireland
As yet, this invader has not reached Ireland, but is likely to arrive. Since it likes to live in zebra mussel beds and these are now well established in Ireland, it is predicted that just a few D. villosus carrying young could thrive in these mussels beds and thus begin an invasion in Ireland.
Life cycle
D. villosus males carry females in ‘precopula’, that is, the males take hold of females and wait for them to moult, at which point copulation takes place. The pairs can be seen swimming around and are difficult to separate. Females are released after mating and carry their developing embryos in a special pouch (marsupium). Females can produce up to 200 young every two weeks, much more and faster than our native species.
Wildlife and habitat impacts
The ‘killer shrimp’ got its name from scientists who discovered the voracious appetite of this invader for all sorts of other invertebrates. Native shrimps, mayflies, worms and even adult insects are all consumed and sometimes just bitten and left mostly uneaten. This has a huge impact on the populations and communities of lakes, rivers and estuaries that D. villosus invades, driving native species locally extinct and reducing species diversity. This may ultimately affect fish and other wildlife through changes in their food resources. D. villosus is so effective at preying on other species it has even displaced previously successful invaders in Europe that came from North America (e.g. the shrimp Gammarus tigrinus).
Human impacts
Not demonstrated as yet.
Key vectors
The spread of D. villosus was greatly accelerated by canal construction between major European rivers, such as the canal between the Rivers Main and Danube. The species actively migrates upstream and hitches a ride on boats, perhaps also on birds, and possibly is moved around with fish stocks being introduced from one lake to another. Studies show the species could easily survive in the ballast water of large ships, thus the species could move all over Europe and even to N. America.
What you can do as an individual
The ‘killer shrimp’ is a striking animal in terms of its size and colours and makes our native (and other introduced species) look small and drab. Thus, if you find animals resembling those in the picture, please send some, alive if possible, to the person below for formal identification.
Advice to specific groups relevant to this species
These animals should certainly not be moved around if they reach Ireland. Thus, garden centres with ponds and fish, anglers, boat clubs, fish farms and any others dealing with the aquatic environment should be aware of this and take steps to:
  • be able to identify the animal and send to the author verification.
  • ensure it is not transported from one location to another.
  • try to eradicate if possible, for example, by filtering water.

    Management measures
    There are no current acknowledged methods to control or remove this species. Filtering of water may be useful under some circumstances, but this would require research.
    Management groups
    There is no group specifically for the management of this species. The author has many contacts in the scientific world who are interested in the biology of the species.
    Further information
    Predatory impact of the freshwater invader Dikerogammarus villosus (Crustacea: Amphipoda) Jaimie T.A. Dick, Dirk Platvoet, and David W. Kelly
    Text written by:
    Dr Jaimie Dick, Reader in Behaviour, Ecology & Environmental Biology, Queen's University Belfast