First record of the non-native seagrass Halophila stipulacea (Forsskål) Ascherson in Mallorca (Balearic Islands, Spain): Expanding its Western Mediterranean distribution

Halophila stipulacea is one of the first Lessepsian migrants to colonize the Mediterranean Sea (in 1894) after the opening of the Suez Canal. Since then, it has progressively expanded its distribution westward across the basin. Here, we report for the first time the presence of H. stipulacea in Palma Bay (Mallorca, Balearic Islands, Spain), representing the westernmost known occurrence of the species in the Mediterranean Sea. Initial surveys revealed that H. stipulacea patches (growing on sandy bottoms at ca.16 m depth) were not stable in the colonized area, disappearing in the first reported location (November 2023) and being found innearby areas a few months later (January 2024). Additionally, we collected samples at both sites to assess biomass and morphological traits of H. stipulacea in its early stages of colonization. Morphological data indicate that H. stipulacea in November 2023 exhibited larger leaf areas and longer rhizome internode length, producing smaller shoots in winter, when minimum registered temperatures were 14.5° C. We highlight the need for continued monitoring to assess the spread and possible ecological impacts of H. stipulacea on native species in the recently invaded areas.