This study focuses on the taxonomy of epiphytic diatoms in the area of invasive macroalgae from the genus Caulerpa. Caulerpa species are characterized by the presence of secondary metabolites, such as caulerpenyne (CYN), the main function of which is a chemical defense mechanism against herbivores and epiphytes. Epiphytic diatoms were studied on fronds of Caulerpa taxifolia (“killer seaweed”) and, for comparison, on autochthonous macroalgae Padina sp. and Halimeda tuna at the eastern Adriatic Sea coast (Island of Hvar) in the summer and autumn of 2010. The qualitative analysis was performed with the use of light and scanning electron microscopy. The Shannon–Wiener Diversity Index determined for Caulerpa taxifolia showed a wide range of values (3.11–4.88), with a maximum in August and a minimum in October. While the number of taxa on Caulerpa taxifolia fronds increased from June (41) to August (88), it declined in autumn due to the high relative abundance of Cocconeis caulerpacola, which is a diatom typical for Caulerpa. On the other hand, the largest number of taxa on Padina sp. was observed in September (82). The detailed composition of epiphytic diatoms and seasonal dynamics in the area affected by the invasive macroalga Caulerpa taxifolia have been determined for the first time.
