An unexpected guest has appeared in the depths of the Adriatic: the Monrovia surgeonfish (Acanthurus monroviae). It is a species native to the tropical waters of West Africa, already known to scientists for its ability to expand outside its original habitat.
The first documented sighting dates back to 2024 along the Croatian coast, near Kostrena, where a specimen was observed feeding on algae a few meters deep. Since then the reports have multiplied, suggesting no longer an isolated episode but a possible process of stable settlement in the Adriatic basin.
The “blades” on the tail for defense
The name “surgeon fish” is not accidental. In fact, the animal has two sharp bony structures on the sides of the tail, called caudal spines, which resemble small blades or scalpels. These natural tools are not used for attack, but for defense: the fish uses them when it feels threatened, with rapid movements of its tail.
While they can cause injury, the Monrovia surgeonfish is not aggressive and does not pose a danger but must be observed from a distance. From an aesthetic point of view, it has an oval body compressed laterally, with yellowish-brown tones and thin bluish lines. The characteristic yellow spots on the sides make the area where the defensive spines are located even more evident.
A decades-long journey in the Mediterranean
The entry of this species into the Mediterranean is not recent. Scholars believe it crossed the Strait of Gibraltar decades ago, with the first reports dating back to the 1980s along the Spanish coast. From there the spread was slow but constant: first in the western Mediterranean, then eastwards until reaching Israel in the 1990s.
Today its presence in the Adriatic represents the last step of a progressive expansion. The fish, a herbivore, feeds mainly on algae, which it scrapes from the rocks with a small specialized mouth. A behavior that makes it competitive with some native species.
The signal of the tropicalization of the sea
Experts do not consider this sighting an isolated case, but part of a larger phenomenon: the tropicalization of the Mediterranean. The increase in sea temperatures is progressively transforming the ecosystem, making it increasingly similar to that of warm seas. In this context, species from tropical regions manage not only to survive, but also to reproduce and stabilize. The surgeonfish thus joins a growing list that includes other alien species that are now increasingly frequent in our seas.
A slowly shifting balance
The risk is not linked to the danger of the individual animal, but to the overall change of the ecosystem. The presence of Acanthurus monroviae is a biological indicator: it tells of a sea that changes temperature, composition and food dynamics. For researchers, monitoring these species is crucial to understanding the future of the Adriatic. For fishermen and swimmers, however, the rule remains simple: observe without interfering. The sea is changing and it is doing so before our eyes.
You might also be interested in:
