At the northern edge of the Adriatic: First records of the invasive Caulerpa cylindracea in the Gulf of Trieste

Abstract

The non-indigenous green alga Caulerpa cylindracea is among the most invasive macroalgae in the Mediterranean Sea and has progressively expanded throughout the Adriatic basin since its first record in 2000. This study reports the first records of C. cylindracea in the Gulf of Trieste, the northernmost part of the Adriatic Sea. The species was first recorded in September 2022 at San Bartolomeo (Italy), where only a few thalli were recorded within a Cymodocea nodosa meadow. In September 2023, two thalli were recorded at Lazaret, representing the first record of the species along the Slovenian coast. Targeted surveys conducted between 2023 and 2025 documented the current spatial extent of the species at both sites and revealed an additional population at the Rex site, between the towns of Koper and Izola. At San Bartolomeo, C. cylindracea occupied an estimated 14,497 m² in July 2023 and 12,331 m² in September 2024. At Lazaret, the species occupied an estimated 12,114 m² by September 2025, while the newly recorded population at the Rex site, between the towns of Koper and Izola, covered approximately 3,000 m². Because the surveyed areas were not monitored systematically before the initial detections, these estimates represent the observed spatial extent of the populations at the time of each survey rather than a quantified rate of spread. These findings confirm the establishment of C. cylindracea in the Gulf of Trieste, extend its known distribution to the northernmost part of the Adriatic Sea, and provide a baseline for future monitoring and assessment of its ecological impacts in this environmentally heterogeneous coastal basin.