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The green alga Caulerpa cylindracea Sonder (Chlorophyta; Bryopsidales) is one of the most invasive alien macroalgae in the Mediterranean Sea, where it is also spreading on rhodolith beds, an important biogenic assemblage typical of deep substrates. Despite the importance of rhodoliths, data on the competitive interactions with C. cylindracea are still scarce. To deepen the knowledge on the topic, C. cylindracea occurrence on the rhodolith bed of Capo Carbonara Marine Protected Area (Italy) was explored. Quantitative analyses of videoframes obtained from Remote Operated Vehicle records in three different MPA sites, Is Piscadeddus, Santa Caterina, and Serpentara, allow for estimates of both the cover of rhodoliths (considering the main morphotypes) and of C. cylindracea, as well as their competition. All sites showed a well-developed rhodolith bed, although some differences were highlighted in their composition in terms of morphotype, shape, and dimension of rhodoliths, as well as in the C. cylindracea cover. In particular, Santa Caterina appeared to be the site with the highest mean total cover of rhodoliths (68%), and of C. cylindracea (25%). The obtained results suggest that different competitive interactions occur between C. cylindracea and rhodolith beds, in relation to the morphostructural composition of the latter and in response to environmental conditions that affect rhodolith bed composition.

Horticulture is a major pathway of introduction of aquatic plants. Among traded aquatic plants, we found two species belonging to the genus Lagarosiphon Harv. (Hydrocharitaceae), native to South and Central Africa, L. major (Ridl.) Moss and L. cordofanus Casp. L. major is the main representative of the genus, having already been introduced via horticulture sale beyond its native range, and often becoming invasive. In contrast, L. cordofanus is a lesser-known congener that could be potentially sold as an alternative to L. major. It is relatively understudied, and has yet to be recorded in the wild outside its native range. Many factors can promote the invasiveness of an alien plant; among them, increased nutrient availability often facilitates opportunistic alien species such as L. major. In a horizon-scanning perspective, a manipulative experiment under controlled conditions was performed to test the response of L. cordofanus to different trophy levels using L. major as the tolerant alternative species. According to our results, the naturalization of L. cordofanus in temperate shallow waters does not seem likely, especially if considered in comparison to L. major.