Marine biologists and coastal authorities are issuing a cautionary alert following the arrival of the Monrovia surgeonfish (Acanthurus monroviae) in the Adriatic Sea. Known colloquially as the “fish with blades” due to the sharp, scalpel-like spines located near its tail, the species poses a physical risk to swimmers, divers, and fishers who may inadvertently come into contact with it.
The presence of this species in the region is no longer considered a random occurrence. According to recent research, the fish has established a stable presence in the Adriatic, having migrated from the wider Mediterranean and crossed the Strait of Otranto. This movement is part of a broader ecological shift termed “meridionalization,” where species typically found in warmer, southern waters move northward as sea temperatures rise.
Experts warn that the danger is not limited to live encounters. Since the Monrovia surgeonfish is an edible species, those handling the fish after it has been caught are urged to exercise extreme caution. The “blades” remain sharp and capable of causing deep lacerations even after the animal is dead.
The Discovery and Spread in the Adriatic
The first formal documentation of the species in the northern reaches of the sea occurred in the summer of 2024. A study conducted by Croatian researchers Jakov Dulčić, Pero Ugarković, Jure Miočić-Stošić, and Pero Tutman, published via Reabic, titled “First report of the Monrovia surgeonfish in the Adriatic Sea,” detailed the initial sighting.
The first specimen was observed on June 2, 2024, near the coastal town of Kostrena, Croatia. The fish was spotted grazing on algae over a rocky seabed at a depth of between 3 and 5 meters. Following this initial discovery, subsequent sightings were reported, eventually leading to coverage by Croatian public television as the species became more visible to the public.
Crucially, researchers have ruled out the possibility that this is a localized colony resulting from the release of aquarium pets. Instead, the evidence suggests a natural migration pattern, indicating that the species is capable of adapting to the specific environmental conditions of the Adriatic basin.
Physical Characteristics and Identification
Identifying the Acanthurus monroviae is essential for those spending time in the water. The fish is characterized by an elliptical, compressed body covered in compact scales. It typically features a small mouth with thick lips and a crescent-shaped caudal fin.
The most distinctive markers are two large, oval yellow spots that surround the “blades”—the sharp spines located on either side of the tail base. These specimens can grow up to 48 centimeters in length and weigh as much as half a kilogram.
| Feature | Description |
|---|---|
| Maximum Length | Up to 48 cm |
| Maximum Weight | Approximately 0.5 kg |
| Key Marker | Two oval yellow spots near the tail |
| Defense Mechanism | Two sharp, blade-like spines (lamettes) |
| Primary Diet | Algae (grazing on rocky bottoms) |
Understanding the Risk: “Do Not Touch”
Professor Cataldo Pierri, a specialist in Marine Zoology at the University of Bari, has emphasized that while the fish is not inherently aggressive, its defense mechanism is highly effective. The “blades” are used as a deterrent; with a sudden, whip-like flick of the tail, the fish can inflict deep cuts on anyone attempting to touch or grab it.
“If you see it in the water, do not try to pet it because it can cause deep wounds,” Pierri noted, stressing that the general rule of not disturbing wild species should always be applied. He further explained that while some surgeonfish species are highly territorial and may actively attack, the Monrovia surgeonfish is generally less confident and less aggressive than its relatives, reacting primarily when it feels threatened.
The risk is particularly high for snorkelers and divers who may be tempted to interact with the fish due to its striking appearance. Because the fish often travels in schools, a single encounter could potentially involve multiple animals reacting defensively.
Ecological Impact and the “Meridionalization” of the Sea
The arrival of the Monrovia surgeonfish is more than just a safety concern; it is a biological indicator. The phenomenon of “meridionalization” describes the northward shift of southern species as the Mediterranean warms. This process alters the composition of local marine ecosystems and can lead to new competitions for resources.
In the case of the Acanthurus monroviae, Professor Pierri suggests that it is unlikely to cause massive disruption to the local food chain. He notes that it is not expected to enter into direct, aggressive competition for food with most native species, with the possible exception of the Salema sarpa (Sarpa salpa), which shares a similar diet of algae.
This shift mirrors other recent arrivals in the Mediterranean, where invasive or migrating species are increasingly rewriting the biological map of the region. The ability of the surgeonfish to cross the Strait of Otranto and establish a stable population in the northern Adriatic suggests that the environmental barriers that once kept these species in the south are diminishing.
For those interested in the broader context of Mediterranean biodiversity and the impact of climate change on marine life, resources such as the International Union for Conservation of Nature (IUCN) provide comprehensive data on species migration and invasive threats.
As the species becomes more common, local authorities and marine biologists will continue to monitor its population density and its interaction with indigenous flora and fauna. The next phase of observation will likely focus on whether the species successfully spawns in the Adriatic, which would confirm a permanent and self-sustaining colony.
This article provides information for general awareness and safety. If you sustain a cut from a marine organism, please seek professional medical attention to prevent infection.
We invite our readers to share their sightings or experiences with migrating species in the Adriatic in the comments below. Please share this report to support keep fellow divers and swimmers safe.
