Researchers at IMEDEA detect the presence of an invasive tropical marine plant in the Balearic Islan

The finding represents the westernmost record of the species in the Mediterranean and the first in Spain.

Researchers from the Mediterranean Institute for Advanced Studies (IMEDEA, a joint center of the CSIC and the University of the Balearic Islands), together with scientists from the Blanes Center for Advanced Studies (CEAB-CSIC) and the Balearic Center for Applied Biology, have detected for the first time in the Balearic Islands the presence of the invasive tropical marine plant Halophila stipulacea.

The discovery, made in the Bay of Palma, constitutes the westernmost record of this species known so far in the Mediterranean and the first documented in Spain. The study, led by IMEDEA researchers, highlights the progressive advance of biological invasions in a context of sea warming. The results were recently published in the journal Mediterranean Marine Science.

The first observation was made in October 2023, about three kilometers from the port of Palma, following an alert from the Balearic Center for Applied Biology. The team subsequently carried out dives and underwater surveys that confirmed the presence of small established patches on sandy seabeds.

“This record confirms that the invasion of Halophila in the Mediterranean continues to move westward,” explains IMEDEA researcher Andrés Arona, lead author of the study. “It is also a clear sign of the ‘tropicalization’ of the Mediterranean: if this species is managing to establish itself here, it is because environmental conditions are changing.”

Halophila stipulacea is a marine seagrass native to the Red Sea, the Persian Gulf, and the Indian Ocean. It reached the Mediterranean after the opening of the Suez Canal more than 150 years ago and is considered one of the first “Lessepsian” species, that is, species that colonized the Mediterranean from the Red Sea through this artificial connection.

Although its expansion in the western Mediterranean has been slow, in other regions such as the Caribbean its behavior has been much more aggressive.

“In the Caribbean, in less than 20 years, it has colonized large areas and displaced native species,” says Arona. “In the eastern Mediterranean it is already much more established; the fact that it has now reached and established itself in the Balearic Islands is another step in that expansion.”

IMEDEA researcher Fiona Tomàs stresses that the finding should not be seen as an isolated event: “We are becoming ‘tropicalized.’ The Mediterranean is warming and becoming increasingly favorable to tropical species. For example, other tropical species of algae or fish are already established in the eastern Mediterranean, and some are now also found along the Balearic coasts.”

The team considers it likely that the species may previously have reached the Balearic Islands through maritime transport, but had not found suitable conditions to survive.

“It is possible that it arrived earlier and failed to establish itself,” notes Tomàs. “Now temperatures are higher, and that may be facilitating its settlement.”

In summer, the plant can grow rapidly at water temperatures close to 30 degrees Celsius, values that have already been recorded recently in the Balearic Sea.

The proximity of the finding to a port strengthens the hypothesis that maritime transport, especially through anchors, hulls, or ballast water, is one of the main pathways of dispersal.

The species’ possible ecological impact will depend on its future expansion and on the type of habitat it colonizes.

In degraded sandy-bottom areas, its presence could increase structural complexity and attract new species, but it could also displace soft-bottom species. In the Balearic Islands, a similar phenomenon has already been observed with the invasive tropical alga Halimeda incrassata, which has displaced the “pedaç” (Bothus podas), a flatfish of importance to local fisheries.

Likewise, if it were to displace native seagrasses such as posidonia (Posidonia oceanica) or cymodocea (Cymodocea nodosa), the impact could be even greater.

“Posidonia is like a sequoia; Halophila is much smaller,” explains Tomàs. “It does not create such complex structures or store carbon on the same scale. A shift in species dominance can profoundly alter the ecosystem.”

In the Caribbean, it has been observed that in some areas colonized by Halophila, biodiversity declines compared with native meadows. In the eastern Mediterranean there is also evidence of displacement of local species.

The team highlights that detection at an early stage is key in order to assess its evolution and design monitoring strategies.

“The earlier we detect these species, the better able we will be to understand how they spread and what effects they generate,” says Arona. The researcher also points to the role of citizen science and platforms such as Observadores del Mar in the early identification of new invasions.

The team is currently carrying out monitoring efforts to assess the real extent of the expansion, its interaction with other species, and possible consumption by herbivorous fish and other organisms.

Beyond this specific case, the researchers believe this finding is a sign of a broader process.

“The fact that this species is here and establishing itself is no coincidence,” Arona concludes. “It is a consequence of the warming of the Mediterranean and increasing maritime connectivity. If our sea continues to resemble the Caribbean more and more, we will see more and more tropical species establishing themselves along our coasts.”