Rhithropanopeus harrisii, the estuarine mud crab, is native to the northwestern Atlantic, ranging from Canada to northern Brazil. The crab was discovered in Pärnu Bay off the coast of Estonia in 2011.

Redik Eschbaum, a research fellow in ichthyology and fisheries science at the University of Tartu, said that the crab must have arrived in the Baltic Sea with ship's ballast water and has since established itself here. "The seabed at Pärnu Bay is fully coated with those crabs. There are so many of them," the researcher said.

Jonne Kotta, a marine biologist at the University of Tartu, said that crabs are now successfully reproducing and their population is rapidly growing as a result of the recent warm summers. When they were first discovered in Parnu Bay, there were only a few crablets on the seafloor, but a year later there were many more in the same location. "This summer we have evaluated the population density of crabs in Pärnu Bay again and found a number of places with over 100 crabs per square metre. This is a massive bulk," the researcher said.

"Last year, we have only found a small juvenile in Matsalu Bay area. Based on the recent discovery of an adult specimen, it appears that the crab has successfully established itself also in Matsalu Bay, and there is a risk now that its population will reach the same size as in Pärnu Bay," Kotta added.

Kotta does not think it possible that the crabs were transported there from a different country. It most likely came from Pärnu Bay, because baby crabs can attach themselves to seaweeds and float with them throughout the sea, as, for example, to Matsalu Bay, Kotta said. "All it takes is a story weather to remove the algae from the rocks, and wherever the algae floats, the crab follows," said the marine expert. If the conditions are favorable, the crab will most likely settle in its new environment.

Matsalu Bay, Kotta said, is a very suitable habitat for them, as the river that flows into the bay is rich in nutrients and the bay is shallow enough, which helps to retain summer heat.

According to Kotta, the conditions in Estonia are ideal for these crabs.
Not only Pärnu and Matsalu Bays, but also Haapsalu and Narva Bays are suitable for them. Despite being slightly colder, the latter is fed in by the Narva River, which provides the crab with an abundance of food.

Mud crabs do not attack humans and thus do not pose a direct threat to us. Nonetheless, its spread in coastal seas could have serious environmental consequences. Kotta said that the crabs in Pärnu Bay ate almost all of the seabed-dwelling shellfish, and because the mussels in the bay provided a self-cleaning habitat, acting as natural filters, the water quality has deteriorated significantly. "Water treatment facilities must now clean water much more effectively to achieve the same results," Kotta said. And as the natural balance shifts, the Gulf of Parnu may become more vulnerable to flooding, she added.

As crabs are also feeding on eggs and small fish, they can have a negative impact on fish stocks. "They consume everything they can get their claws upon," Kotta joked about the crab's feeding habits.

Kotta and Eschbaum both say that the mud crab is here to stay and is unlikely to disappear. "We'll have to wait and see what this expansion brings," Kotta added, "but there's no reason to anticipate anything particularly positive."

Eschbaum said that mud crab is not really edible even though it is not poisonous. "It's a 2-euro coin size, which is so little - you can't really eat it."

Rhithropanopeus harrisii, the estuarine mud crab, can grow to be two centimeters wide. It has olive-brown or greenish-brown claws with white tips that are unequally sized and shaped.

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