A horseshoe whip snake has been filmed swimming in the waters of S’Estany des Peix in Formentera, raising fresh concerns about the spread of an invasive predator that has already caused severe ecological damage in Ibiza.
The snake was spotted on Monday, August 24, and the sighting was subsequently confirmed by island sources and the Balearic Government after video of the animal circulated. The specimen was later captured and removed.
A Snake That Should Not Be Established in Formentera
The species, Hemorrhois hippocrepis, is commonly known as the horseshoe whip snake. It is non-venomous and considered harmless to humans, although it may defend itself if threatened.
The concern is not primarily about people swimming at S’Estany des Peix.
It is about what the snake could mean for Formentera's native wildlife.
The species has already colonised almost 90% of Ibiza, where it has become a major predator of the island's endemic wall lizard, or sargantana.
Until now, the horseshoe whip snake had not established a permanent population across Formentera, although sightings and captures have increasingly raised concerns that it could eventually do so.
Sightings Have Been Increasing
The latest swimming sighting is not the first indication that the species is reaching Formentera.
Horseshoe whip snakes have been recorded around La Savina since 2023, with several specimens captured in the area. RadioIlla reports that seven were captured in 2025, while local authorities have increased monitoring around the port.
Researchers and officials suspect that snakes may be reaching Formentera by hitching rides on vehicles, boats or plant material transported from Ibiza, rather than swimming across the channel between the two islands. The distance between Ibiza and Formentera is considered too great for that to be the most likely explanation for regular movement between the main islands.
That makes the latest sighting particularly interesting.
It demonstrates that the species is capable of swimming in the waters around Formentera, even if human transport remains a more plausible explanation for how it reaches the island.
Horseshoe Whip Snakes Can Swim
The sighting also adds to growing scientific evidence that these snakes are capable swimmers.
A 2026 study published in the journal Ecology documented a horseshoe whip snake actively swimming from Ibiza to the nearby islet of Santa Eulària, around 430 metres from the island's eastern coast. Researchers subsequently identified 14 independent reports of horseshoe whip snakes swimming in open water around Ibiza between 2015 and 2025.
The research suggests that swimming is not simply an accidental event.
Instead, the snakes appear capable of deliberately entering the water and travelling between nearby land masses in search of new territory and prey.
Why Scientists Are Concerned About the Sargantana
The biggest ecological concern is the Pityusic wall lizard (Podarcis pityusensis), known locally as the sargantana.
The lizard is endemic to Ibiza, Formentera and surrounding islets and evolved without the horseshoe whip snake as a major predator.
Researchers have linked the snake's expansion across Ibiza with dramatic declines in native lizard populations. On some small islets, local populations have already disappeared.
A study published this year found that 10 islets have already lost their unique lizard populations, while the researchers documented the snakes reaching new islets by swimming.
The concern is that a similar invasion on Formentera could put its own lizard populations under serious pressure.
The Ibiza Invasion Started With Olive Trees
The horseshoe whip snake's presence in the Pityusic Islands dates back to the early 2000s, when the species was accidentally introduced to Ibiza, most likely hidden among imported ornamental olive trees from mainland Spain.
From there, the population expanded rapidly.
Scientists say the snake now occupies more than 90% of Ibiza, while its spread has been associated with the disappearance of native lizards from large parts of the island.
The snakes have few natural competitors on Ibiza and have access to abundant prey, allowing them to grow exceptionally large. Researchers have recorded individuals approaching two metres in length.
Formentera Now Faces a New Challenge
Formentera already has invasive snakes of its own, including the ladder snake (Zamenis scalaris), particularly around La Mola.
But conservation experts consider the arrival of the horseshoe whip snake especially concerning because of its strong preference for lizards and its potential impact on the island's endemic wildlife.
Authorities have therefore increased monitoring and trapping around areas where horseshoe whip snakes have been detected.
The latest sighting at S’Estany des Peix is unlikely to mean that the species has suddenly colonised all of Formentera. One confirmed swimming snake is not proof of an established population.
But it is another warning sign in an invasion that scientists are already watching closely.
For an island whose natural ecosystems evolved in relative isolation, the arrival of a highly effective new predator could have consequences far beyond the snake itself.
