Context of the study
Invasive alien species, or invasive species, are animal or plant species that have been introduced by humans somewhere other than their area of origin. In some cases, these species may pose a threat to local biodiversity (predation, competition, transmission of pathogens) or have a negative impact on economic activities and human health.
In Europe, native crayfish are threatened by the introduction of exotic crayfish species. Belgian fauna includes only one endemic crayfish species, the European crayfish, Astacus astacus.
Exotic crayfish have been introduced for food and fishkeeping, predominantly from America. These species have a direct impact on our native populations. Indeed, American crayfish are healthy carriers of a fungus, Aphanomyces astaci, which is 100% lethal to our native crayfish. Populations of the European crayfish have therefore significantly declined over recent decades and the species is almost extinct in Wallonia.
Five species of exotic crayfish are of concern to the European Union and regarded as invasive. These species require management measures to control the size of their population and thus limit their impact. Three of these species are present in Wallonia and two others could soon become established (*):
- The American crayfish Orconectes limosus
- The Louisiana crayfish Procambarus clarkii
- The signal crayfish Pacifastacus leniusculus
- The virile crayfish Orconectes virilis (*)
- The marbled crayfish Procambarus fallax f. virginalis (*)
In order to limit their impact, we need to study the distribution of exotic and native crayfish populations in our ecosystems. These data are useful for setting up alert systems and appropriate management measures in case new species are introduced or species that are already present become more common.
Working with the Service public de Wallonie, we have therefore studied the distribution of invasive alien crayfish present in our rivers and water bodies by comparing traditional trapping and detection methods with environmental DNA.
Sampling and laboratory analysis
Samples were taken from several water bodies (ponds and small lakes), as well as various waterways (rivers and canals), using the environmental DNA method. This technique involves recovering the genetic traces released by crayfish into their environment by filtering the water through a capsule.
At the same time, the Service public de Wallonie put down traps to catch crayfish and so confirm the presence of the different species by identifying them on the basis of morphological characteristics: presence of a carpal spur, shape of the claws, colouring etc.
In the laboratory, the DNA contained in the filter capsules was extracted and then amplified thanks to quantitative PCR (qPCR) using specific primers for each of the crayfish species targeted by the study. This method, known as environmental DNA barcoding, is particularly sensitive when it comes to detecting rare or elusive species.
In order to guarantee the quality of our analyses and the reliability of our data, positive and negative controls were carried out to validate each step of the experimental process.
Results and conclusion
Invasive crayfish (American, Louisiana or signal) were captured in each aquatic environment sampled. In more than 30% of cases, two species were living alongside one another. However, the European crayfish was not observed during the study.
The water filtration process revealed the presence of each of the captured crayfish species, thus confirming the direct observations. The data are conclusive: the environmental DNA method makes it possible to quickly detect invasive crayfish populations.
Our objective is to use this method to continuously monitor the different species and benefit from a surveillance process and early-warning system.