Case Study 1 – Marbled Crayfish

Case Study 1: Marbled crayfish

(Procambarus virginalis)

Description

The marbled crayfish is a high-risk freshwater invader and has been listed as an Invasive Alien Species (IAS) of Union Concern since 2016.

This species is unique because it is parthenogenic, meaning every individual is a female that clones itself to create genetically identical offspring. No males have ever been found to exist. Because a single female can produce hundreds of clones, just one individual introduced into a new environment can rapidly establish a massive population.

Originally created artificially by aquarium hobbyists in Germany in the late 1990s, it has since escaped into the wild to become a major ecological threat.

Habitat

This species is a super-coloniser capable of inhabiting various freshwater environments including rivers, lakes, and wetlands. It is currently invading a major biodiversity hotspot in the Netherlands (the Haterse Vennen near Nijmegen), a landscape characterised by numerous shallow, mesotrophic fens. The area is home to many vulnerable native amphibians such as the common spadefoot, the great crested newt, the moor frog, and the natterjack toad. Recent discoveries of new marbled crayfish populations have also been recorded in Belgium. However, in each of these cases, population control and eradication may still be feasible through coordinated efforts.

Impacts

Biodiversity: Marbled crayfish outcompete native species for food and shelter due to their aggressive behaviour, fast growth rates, and very large clutches.
Native Amphibians: They are highly detrimental to native amphibian communities—such as the great crested newt, moor frog, common spadefoot, and natterjack toad—by consuming their eggs and larvae.
Disease: They are carriers of the crayfish plague, a fungus that is lethal to native European crayfish species.

Prevention and Control Actions

The GuardIAS project focuses on early detection and rapid response, as eradication is still considered feasible in newly discovered and established populations through coordinated efforts.

eDNA Monitoring: Researchers use environmental DNA (eDNA) to detect this hidden threat. This method is highly sensitive—capable of detecting the equivalent of one small crayfish in an Olympic-sized swimming pool—making it more effective and less labor-intensive than traditional netting in thick vegetation. Preliminary findings in the marbled crayfish metapopulation in the Netherlands indicate that the number of sites colonised by this crayfish, as well as their eDNA concentrations, has increased over the years. As such, marbled crayfish populations become more abundant and may be spreading further in the area, potentially invading more breeding grounds from vulnerable native species.
Impact Assessment: In the Netherlands, eDNA concentrations of marbled crayfish as well as native amphibians are being quantified in 30 fens and ponds for a third consecutive year to track the impact on protected amphibian populations and prioritize conservation. While increased observed eDNA concentrations suggest that marbled crayfish are increasing in abundance, those of great crested newt and natterjack toad are declining over the same time interval. Parallel catches show that offspring of native species are numerically reduced over the years, as in some sites, only a handful of larvae could be captured anymore.
Evaluation of eradication success: Quantitative eDNA analyses are used to measure the success of eradication efforts by analysing infected waterbodies before and after management interventions. Preliminary evidence suggests that physical removal using fyke nets already noticeably decreased marbled crayfish population sizes, even after a 1 week treatment.
Public Responsibility: A key prevention message is the importance of not releasing invasive species into the environment, as prevention is significantly more cost-effective than attempted eradication after an introduction.

Useful links

Meet the case study team

Research Institute for Nature and Forest (INBO): Teun Everts and Rein Brys

Case study contacts:

Teun Everts, teun.everts@inbo.be
Rein Brys, rein.brys@inbo.be

Anastasija Zaiko

“This project is not just about managing invasive species today—it’s about developing a proactive approach that anticipates future ecological challenges, ensuring robust ecosystems and sustainable marine environments for generations to come”

Sergej Olenin

“For over 35 years, since I first began studying non-indigenous species, I have dreamed of tools that could reveal the ‘Big Picture’ of the transfer, spread, and impacts of invasive aquatic species. Thanks to advances in new technologies, the GuardIAS project now offers an outstanding opportunity to make that vision a reality.”

There are 7 Work Packages (WP) focusing on different stages of invasive alien species ‘invasion’ in the GuardIAS project. Each Work Package is a distinct project area, but all are interlinked through GuardIAS as a whole.