The Malta Independent 29 September 2022, Thursday
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Alien crayfish invade Malta’s valleys and watercourses

Wednesday, 19 September 2018, 07:49 Last update: about 5 years ago

A recently published study in the Journal of Crustacean Biology has revealed the extent of the invasion of freshwater valley systems in the Maltese Islands by a number of introduced crayfish species. The study, conducted over 2016 and 2017 and captained by Prof. Alan Deidun from the Department of Geosciences of the University of Malta, involved colleagues from Malta (Arnold Sciberras, Justin Formosa) and Sicily (Bruno Zava, Gianni Insacco and Maria Corsini-Foka) and also Prof. Keith Crandall from the George Washington University in the US.

The study has painstakingly documented the occurrence of at least five non-native (alien) crayfish species within four valleys in Malta (Fiddien/WiedQlejgha, Bahrija, Ghajn Zejtuna and Wied l-Isperanza) and one valley in Gozo (Wiedil-Lunzjata), as well as within a number of freshwater pools, ponds and reservoirs including Ta’ Sarraflu and Ghajnil-Papri in Gozo and Pembroke in Malta.

Narrow-clawed crayfish (photo top) and Red swamp crayfish (above)

The five alien crayfish species are all popular within the pet industry or are reared commercially for human consumption and include the red swamp crayfish (Procambarusclarkii), the marmorkrebs (Procambarusvirginalis), the Galician or narrow-clawed crayfish (Pontastacusleptodactylus), the signal crayfish (Pacifastacusleniusculus) and the Australian redclaw (Cheraxquadricarinatus). A sixth alien species, this time the bamboo shrimp (Atyopsismoluccensis), was also documented within the study from a reservoir in Pembroke, whilst individuals of the common yabby (Cherax destructor) were documented from the island of Sicily for the first time. Most of the exotic species are native to freshwater habitats in the US, with the exception of the Australian red claw and the yabby, native to Australia, and the bamboo shrimp, native to south-east Asia.


The invasive nature of some of the alien crayfish species documented in this study is such that some of the same species (the red swamp crayfish, the marmorkrebsand the signal crayfish) have been included, as of August 2017, in the Invasive Alien Species (IAS) list slated for direct intervention by Regulation 1141/2016 of the European Union.

According to Prof. Deidun, the scale of the invasion of Maltese freshwater ecosystems, already susceptible to a clutch of other anthropogenic impacts, including diversion of water for irrigation purposes and pollution through released of pesticides and fertilisers, is cause for concern, especially considering that some of the invaded valleys, notably the ones at Bahrija and at Xlendi (Wiedil-Lunzjata) are amongst some of the last haunts for the endangered freshwater crab and painted frog.

Bamboo shrimp

The fact that the non-indigenous crayfish species were also recorded from man-made reservoirs and ponds is evidence, according to Deidun, that the species were intentionally released by irresponsible individuals, unaware of the ecological disaster such releases could unleash.

Australian redclaw

Some of the alien crayfish species, in fact, can withstand prolonged periods of draught, and exhibit prolific rates of reproduction, besides being opportunists when it comes to feeding. Deidun thus urged aquarists and the public in general to desist from releasing such exotic species into the wild and encouraged those encountering such species to report their occurrence to his research team on [email protected]. The Environment and Resources Authority (ERA) has also been alerted of the findings of the study so as to consider future management options.

The full text of the paper can be accessed here:

Signal crayfish


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