An IMEDEA study detects the spread of an exotic alga across Formentera’s seagrass meadows

An IMEDEA (CSIC-UIB) study confirms that Batophora occidentalis, first detected on the island in 2020, has spread throughout s’Estany des Peix.

The exotic green alga Batophora occidentalis, first detected in the Mediterranean in Formentera in May 2020, has now spread throughout s’Estany des Peix and has begun to colonise Posidonia meadows. This is the main conclusion of a study by the Mediterranean Institute for Advanced Studies (IMEDEA, CSIC-UIB), which warns that this could be a critical moment to try to prevent its spread to other areas of the Balearic coastline.

The results, published in the scientific journal PLOS One, show that the alga has reached considerable abundance in just four years. Surveys conducted by researchers in 2024 recorded the presence of the alga at 100% of the sites studied within this Formentera lagoon. In fact, by 2024 the alga covered, on average, around 30% of the Cymodocea nodosa meadows studied and 26% of the Caulerpa prolifera meadows, becoming the most abundant macrophyte in both cases.

One of the study’s main findings is the ability of Batophora occidentalis to grow directly on the leaves and rhizomes of Posidonia oceanica. This is the first time this behaviour has been documented for the species on Posidonia. At s’Estany des Peix, researchers observed that the alga could cover nearly 25% of the surface of colonised leaves. In the most extreme cases, the attached alga weighed up to five times as much as the leaf itself.

Andrea Antón, principal investigator of the study, explains the risks this behaviour poses to Posidonia: “The weight could reduce the amount of light received by the plant and increase the risk of leaf breakage. In addition, during fieldwork, heavily colonised leaves were observed to be bent, folded or partially broken.” However, Antón stresses that further studies are still needed to determine the long-term consequences of this interaction for Posidonia meadows.

The study also provides evidence of a development that is of particular concern to researchers. During the survey, they found a specimen of Batophora occidentalis growing on a Posidonia leaf in a nearby meadow outside s’Estany des Peix, in the open sea. For now, this is an isolated detection, but it indicates that the alga may be beginning to spread beyond the boundaries of the lagoon. “Any eventual expansion,” Antón adds, “would be particularly significant for the Pityusic Islands, which are home to some of the largest and best-preserved Posidonia meadows in the Mediterranean.”

The research points to boats as a highly likely pathway for the alga to spread beyond Formentera, as Batophora occidentalis can attach not only to natural substrates but also to artificial structures. Scientists found it on 11 of the 63 boats they examined at the s’Estany des Peix marina, as well as on port infrastructure.

This finding is significant because recreational boats can transport organisms attached to their hulls between different ports and anchoring areas. The study therefore considers maritime traffic a potential secondary pathway for the species to spread to other parts of the Mediterranean.

Boating is also among the hypotheses proposed to explain how Batophora occidentalis initially reached Formentera. Previous studies have identified boats as a possible introduction pathway, particularly yachts that spend part of the year in the Caribbean before sailing to the Mediterranean, although the precise origin of this population has not been established.

Scientists believe the situation requires swift action to prevent further expansion. They point out that completely eliminating the alga from s’Estany des Peix is not feasible because of its high abundance. The study estimates an average density of around 5,400 Batophora occidentalis stems per square metre in a lagoon covering approximately two square kilometres.

Measures proposed in the study include strengthening monitoring in lagoons and ports across the Balearic Islands, removing the alga from the hulls of boats in s’Estany des Peix, and assessing its removal at strategic locations near the entrance to the lagoon.

The authors also call for increased research into basic aspects of the species’ biology needed to design management measures, such as its reproductive cycle and seasonal growth patterns, which remain unknown.

The expansion of Batophora occidentalis is taking place in a Mediterranean that is particularly vulnerable to biological invasions. More than 1,000 exotic species have been recorded in its waters, making it the most heavily invaded marine region on the planet.