The incident took place at Pirita beach, a popular spot close to the city center, while the board's toxins information center reported the individual, a 32-year-old woman, experienced acute pain, whose source they identified after spotting a "spiky, globular animal" under the water.

Severe swelling presented almost immediately, in the vicinity of the puncture point, the board said.

"Sea urchin spines can break off easily and cause severe pain and reddening of the skin if they penetrate the tissues. As a first aid measure, the affected area of skin should be washed immediately on the beach with salty and warm sea water. In cases where pain lasts for days, or of nausea or vomiting, the individual must present at an emergency room," the board stated.

In most cases, pain and swelling are the main effects.

While sea urchins, of which there are close to a thousand known species, can be found in virtually all waters, often at great depth, they have not been an issue in Estonia. However, warmer water varieties having spread as far north as Denmark as of 2018 meant it was only a matter of time before they reached Estonian waters also, the board said.

"Together with Health Board environmental specialist, we are still investigating the possible locations of sea urchins on our beaches, while in the meantime, we ask the public to remain attentive to sea urchins on the beach, in addition to blue-green algae," the board went on, referring to cyanobacteria blooms which are a regular occurrence on some Estonian beaches in summer – and also spotted on Pirita beach this week.

If members of the public spot what appears to be sea urchin(s) (Estonian: "Merisiilikud") on Estonian beaches, they should notify the board's toxins information center on 16 662 (telephone, operators speak English).

The effects of some sea urchin stings in warmer waters can be viewed here (viewer discretion advised).

Expert: Unlikely to be a sea urchin, Baltic Sea not sufficiently saline

However, according to one expert, the cause of the woman's injuries at Pirita beach was unlikely to be a live sea urchin, simply because the seawater in the Baltic does not have nearly a sufficient level of salinity to support the animals.

Given the proximity of the beach to Tallinn harbor, one possible explanation is that the woman had stepped on the remains of seafood cast overboard from a cruise ship, marine biologist Jonne Kotta told ERR on Friday.

Seemingly exotic finds in the vicinity of Tallinn have been reported before, Kotta said. "These concern mostly edible species, which have either by accident or by design been thrown overboard, only for the current brings them to the shoreline. Then members of the public enquire if they have found a species alien to Estonia.

Some varieties of sea urchins are edible, while the remains of other seafood such as crab could also be the explanation, Kotta added.

Another possibility is that a sea urchin had arrived in Tallinn attached to the hull of a vessel, then fallen off (or as part of a catch, in the case of a fishing vessel that had come from more saline seas). In any case, the animal would have been long dead by the time it reached Tallinn, though its spines could obviously still puncture the skin if trodden on, and potentially deliver a toxin.

Other possibilities are that the culprit was a plant or seashell unfamiliar to the victim – who reportedly did not take a photo of the find and had initially thought she had stepped on a piece of glass – or that someone had emptied the contents of a home aquarium into the bay.

There is no reason to panic about the invasion of alien species, Jonne Kotta added.

He said: "If there was such a species of sea urchin that had entered and could survive in the Baltic Sea, most likely we would have already heard about it, for instance from Poland or Sweden."

This article was updated to include expert opinion.

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