Gammarus pulex



Conservation status

Not Assessed

Legal status

Listed as a schedule 9 species under Articles 15 & 15A of the Wildlife Order (Northern Ireland) 1985 (Article 15A not yet enacted).

Native status


First reported in the wild



Invasive species - risk of Medium Impact

Irish status


Introduction pathways - 1

Release in Nature

Introduction pathways subclass - 1

Landscape/flora/fauna improvement

Invasive score


NAPRA Ireland risk assessed


Species Biology


Freshwater shrimp, generally less than 20mm long. Identification to species difficult.


Gammarus pulex displaces Gammarus duebeni celticus (native freshwater amphipod) in river systems though the exact pattern of the displacement is unclear, with pure populations of G d. celticus occurring in some upstream areas and may be affected by subsequent introductions of Ponto-Caspian invaders (Dick et al., 1993; Dick, 2008; MacNeil & Platvoet 2005; McLoughlin & Reynolds, 2001). Juvenile trout in areas invaded by G. pulex consumed higher invertebrate biomass than fish in the other reaches though invaded areas had a lower native benthic invertebrate abundance, biomass, species richness and species diversity (Kelly & Dick, 2005). While other mechanisms may be at play, the displacement of G. d. celticus by G. pulex is based on intra-guild predation (Dick, 2008), while Dikerogammarus villosus (another invasive amphipod) displaces G. pulex by the same method (MacNeil & Platvoet, 2005), suggesting freshwater communities may be affected by G. pulex differently based on the species composition of that community.


Inland surface waters


As with a number of Amphipod species engage in pre-copulatory mate guarding, where the male guards a female by carrying her beneath him waiting for her to moult to allow mating. Some of the factors affecting pre-copulatory mate guarding and more technical descriptions are dealt with for G. pulex (Franceschi et al., 2010) and for G. duebeni celticus (Dunn et al., 2008). Females produce between 1-43 eggs (average 16) (Hynes, 1955).

Pathway and vector description

Deliberately introduced in Ballinderry river, Co Derry, and possibly other sites, in 1958 to enhance fish feeding (Strange & Glass, 1979). Has since expanded its range in Northern Ireland by natural means and is still expanding. In 1988 G. pulex was found in the river Brittas in Co Wicklow (Keatinge, 1989) and in 2009 in Co Limerick, suggesting overland, human mediated transport of this species within Ireland, though the mechanism is unclear.

Mechanism of impact

Competition, Predation

Broad environment


Habitat description

Freshwater species, found in rivers, lakes and streams.

Species group


Native region


Similar species

Gammarus tigrinus


World distribution(GBIF)

Irish distribution

Established - Widespread, more common in Northern Ireland (Dick et al., 1990, Dick et al., 2015) and localised in the Republic but expanding its range (McLoughlin & Reynolds, 2001). Distribution may be an artefact of recording effort.

Native distribution

Native to Europe however the exact distribution is difficult to ascertain, as it has been introduced by man into a number of different areas (Dick, 2008).

Temporal change

Date of first record category


Fifty year date category


Records submitted to Data Centre in 2021

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How can you help

Report any sightings to the National Biodiversity Data Centre.



Minchin, D, (2007). A checklist of alien and cryptogenic aquatic species in Ireland. Aquatic Invasions, 2(4), 341-366. Strange, C. D., & Glass, G. B. (1979). The distribution of Freshwater gammarids in Northern Ireland. In Proceedings of the Royal Irish Academy. Section B: Biological, Geological, and Chemical Science 79:145-153. Dick, J. T., Elwood, R. W., & Irvine, D. E. (1990). . The Irish Naturalists' Journal, 313-316. McLoughlin, N & Reynolds, J. (2001). Further records fro the introduced species Gammarus pulex (Crustacea:Amphipoda) from the Republic of Ireland. Irish Naturalist's Journal 26(12):460-463. Kelly, D. W., & Dick, J. T. (2005). Introduction of the non-indigenous amphipod Gammarus pulex alters population dynamics and diet of juvenile trout Salmo trutta. Freshwater Biology, 50(1), 127-140. Franceschi, N., LemaƮtre, J. F., Cezilly, F., & Bollache, L. (2010). Size-assortative pairing in Gammarus pulex (Crustacea: Amphipoda): a test of the prudent choice hypothesis. Animal Behaviour, 79(4), 911-916. Dunn, A. M., Dick, J. T., & Hatcher, M. J. (2008). The less amorous Gammarus: predation risk affects mating decisions in Gammarus duebeni (Amphipoda). Animal Behaviour, 76(4), 1289-1295. Hynes, H. B. N. (1955). The reproductive cycle of some British Freshwater Gammaridae. The Journal of Animal Ecology, 352-387. Dick, J. T. (2008). Role of behaviour in biological invasions and species distributions; lessons from interactions between the invasive Gammarus pulex and the native G. duebeni (Crustacea: Amphipoda). Contributions to Zoology, 77(2), 91-98. Dick, J. T., Montgomery, I., & Elwood, R. W. (1993). Replacement of the indigenous amphipod Gammarus duebeni celticus by the introduced G. pulex: differential cannibalism and mutual predation. Journal of Animal Ecology, 79-88. MacNeil, C., & Platvoet, D. (2005). The predatory impact of the Freshwater invader Dikerogammarus villosus on native Gammarus pulex (Crustacea: Amphipoda); influences of differential microdistribution and food resources. Journal of Zoology, 267(1), 31-38.

Invasive Alien Species in Northern Ireland