In addition to frequently being more competitive than European crayfish, the alien crayfish of North American origin are also latent carriers of a further invasive alien species — Aphanomyces astaci, the causative agent of crayfish plague.
Aphanomyces astaci is a fungus-like oomycete (Chromalveolata) that parasitizes the cuticle of crayfish (and only crayfish!). Since 1859, it has repeatedly been introduced into Europe with infected North American crayfish, which are the natural hosts of this parasite.
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Spread of crayfish plague in Europe (click to enlarge and to show additional information).
An infection with A. astaci is fatal for European crayfish and leads to devastating mass mortalities1). In fact, crayfish plague is one of the most devastating wildlife diseases known, and A. astaci is listed among the “100 of the world’s worst invasive alien species”.
The immune defense system of North American crayfish species is generally able to cope with A. astaci, leading to stable parasite–host equilibrium upon infection. Consequently, North American crayfish are carriers of A. astaci, and act as “deadly” vectors in that they transmit the parasite to previously uninfected native crayfish.
The “calamitous alliance” of North American crayfish and the crayfish plague agent has been among the leading causes of the dramatic native crayfish declines during the last 140 years and still constitutes an ever-increasing threat to native crayfish.
Female Stone crayfish killed by crayfish plague
The invasive North American Calico crayfish (Orconectes immunis) was recently confirmed as a carrier of A. astaci
Alderman, D.J. (1996) Geographical spread of bacterial and fungal diseases of crustaceans. Revue Scientifique Et Technique – Office International Des Epizooties 15: 603–632
Diéguez-Uribeondo, J. (Ed.) (2006) Pathogens, parasites and ectocommensals. In: Souty-Grosset C, Holdich DM. Noël PY, Reynolds JD, Haffner P (eds), Atlas of Crayfish in Europe. Muséum national d'Histoire naturelle, Paris, (Patrimoines naturels, 64), pp 133–149.
Evans, L.H., Edgerton, B.F. (2002) Pathogens, Parasites and Commensals. In: Holdich, D. M. (ed.): Biology of Freshwater Crayfish. Blackwell Scientific Press, Oxford pp 377–438.
1)However, see Jussila et al. [2011: Latent crayfish plague (Aphanomyces astaci) infection in a robust wild noble crayfish (Astacus astacus) population. Aquaculture 321: 17–20], Schrimpf et al. [2012: Crayfish plague pathogen detected in the Danube Delta – a potential threat to freshwater biodiversity in southeastern Europe. Aquatic Invasions7: 503–510], and Kokko et al. [2012: Recovering Turkish narrow-clawed crayfish (Astacus leptodactylus) populations carry Aphanomyces astaci. Knowl Managt Aquatic Ecosyst 404: 1–7] for remarkable exceptions from this general view.