Louisiana crayfish reached Spain in 1973 and 1974. Now the world's most widespread freshwater crayfish, it harms farms and is too costly to wipe out.

Published on: Oct 4, 2026, 09:47:43 IST

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Some of the biggest environmental problems can often start with the smallest plans. The red swamp crayfish is a good example. This North American species was introduced to Spain from Louisiana in the 1970s, and more than 50 years later, it remains a huge headache for Europe. In a 2020 study in Knowledge and Management of Aquatic Ecosystems, Francisco Oficialdegui and colleagues traced the species' complicated path across the continent. Today it is the most widely distributed freshwater crayfish in the world.

Two introductions, one big problem

The study explains that the crayfish first came to Europe in 1973. That batch was released in the Guadiana basin in the province of Badajoz in Spain. In 1974, a second, separate introduction took place in the rice fields of the lower Guadalquivir, in Seville province. Both sets were from Louisiana. For years, it was assumed that the crayfish of Europe originated from these two groups, aided by many uncontrolled shifts from place to place. But the authors found a more complex story: the genetic evidence points to at least one more, entirely separate introduction route. Their DNA work found a link between crayfish from Lake Naivasha, Kenya, and populations in Western Europe, as well as a second, unrelated link between a population in Malta and crayfish found mainly in Asia, pointing toward the pet trade, rather than the original Louisiana stock, as that population's likely source. The authors say this shows the invasion of Europe is more complex than often assumed.

Why people passed it around

This species is native to the southern United States and northeastern parts of Mexico. As the study points out, the species has been purposefully introduced by people to be bred as food or sold as an aquarium pet. It has been introduced to at least 40 countries, on every continent except Australia and Antarctica. The authors call it one of the world’s most destructive ecological invaders.

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What it does to nature

In 2016, Catherine Souty-Grosset and colleagues published a review on the impact of the crayfish on European waters in Limnologica. The crayfish carries the crayfish plague, a disease responsible for the large-scale disappearance of native crayfish species. The authors cite its fast life cycle, strong ability to spread, burrowing, and dense populations as reasons for the damage. They also describe the crayfish as a keystone species, meaning it has a large impact on the habitat it lives in. The review notes that the species has been found even in caves. At the time of writing, it had been recorded in 16 European territories.

Trouble for farms and water systems

As a farm pest, the crayfish feeds on young rice plants in Mediterranean regions. In southern and central Europe, the authors describe the species as a damaging digger of drainage systems. In north-western Europe, it is viewed as a threat to the restoration of water bodies. The review analyzes the impact of this species on both ecosystem and human well-being.

Why it’s so difficult to remove

Souty-Grosset and team note that the red swamp crayfish meets the EU's criteria for a species that is so widespread that eradicating it entirely is no longer cost-effective. This is a narrower claim than it might sound: it's not that the crayfish's damage is small, but that after 50 years of uncontrolled spread across the country, physically removing every last individual from every rice paddy, canal, and waterway would cost far more than even its substantial ongoing damage. The review cites Article 4(3) of EU Regulation No 1143/2014. That doesn't mean nothing can be done. It's just that a full cleanup would cost more than the benefits it would deliver.

A lesson that still matters

The crayfish was introduced into Spain in the hope that it would help farming, not cause a disaster. However, some 50 years later, it is still in Spain. This example highlights how one decision can often have consequences for decades to come. The Oficialdegui study adds a warning. Learning how a certain species gets introduced into a country can help people understand, foresee, and control an invasion. Introductions may sometimes not be well documented, thus making the history hard to trace. Once a species is released, its path can be tough to follow. Before moving any living thing to a new home, it is worth thinking twice.

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