Origin of Invasive Tropical Algae in the Mediterranean Revealed

Researchers have identified the origin of Caulerpa taxifolia, a tropical green alga that invaded the Mediterranean Sea and covered 4,630 hectares by 1997. According to reporting from Sciencepost, the invasive species persisted in the region despite previous scientific assumptions that the alga would not survive Mediterranean winters.

The alga, often referred to as “killer algae,” is native to tropical waters. Its introduction to the Mediterranean triggered a biological invasion that displaced native seagrasses and altered the marine ecosystem. For years, the exact source and the mechanism that allowed it to withstand colder temperatures remained subjects of scientific inquiry.

The discovery of the alga’s origin follows a period of rapid expansion. By the late 1990s, the plant had established a massive footprint in the sea, creating dense monocultures that prevented other species from growing. This growth occurred despite the Mediterranean’s temperature fluctuations, which were expected to kill off tropical strains.

The specific strain of Caulerpa taxifolia found in the Mediterranean differs from the wild tropical variety. Sciencepost indicates that the invasive population is a clone of a specific strain that had been subjected to laboratory conditions, which likely increased its resilience to cold water. This genetic uniformity explains why the alga was able to spread so aggressively from a single point of origin.

The alga’s presence in the region led to significant ecological shifts. It produces caulerpenyne, a toxin that makes it unpalatable to most Mediterranean fish and invertebrates. Because few local species can eat the alga, it faces little natural predation, allowing it to dominate the seabed and suffocate native Posidonia oceanica meadows.

The 1997 measurement of 4,630 hectares served as a benchmark for the scale of the invasion. This rapid colonization demonstrated the vulnerability of the Mediterranean basin to non-native species, particularly those introduced through human activity such as shipping or the aquarium trade.

Current understanding of the Caulerpa taxifolia invasion highlights the risks associated with the release of laboratory-bred organisms into the wild. The ability of this specific clone to survive the winter transformed a tropical plant into a permanent and destructive fixture of the Mediterranean marine environment.