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Brown macroalgae belonging to the genus Cystoseira (Fucales: Sargassaceae) are canopy-forming organisms the recent decline of which at a basin and local scale has been widely documented, which urgently calls for research to fill knowledge gaps and support new and effective measures for protection. We, hereby, characterised the molluscan assemblages associated with three Cystoseira taxa (C. amentacea, C. compressa and C. crinita) from Ischia Island (Italy, Tyrrhenian Sea), and tested whether different congeneric taxa may syntopically support a different biota. In particular, these assemblages were compared among three Cystoseira species, between two times of sampling (June–July 2015 and June–July 2016), and among six sites in terms of multivariate structure (identity and relative abundances of constituting taxa combined, and presence–absence composition), as well as for synthetic measures of diversity, including the total richness of taxa, the exponential Shannon index and the reciprocal Simpson index. In total, 24736 molluscan individuals were collected, overall belonging to 52 taxa. The majority of the identified species included micrograzers and filter feeders, which is in agreement with similar previous studies. The composition of associated molluscan assemblages, which was mainly represented by juvenile individuals, differed among the three Cystoseira species, suggesting that even congeneric taxa do not support an analogous benthic fauna. The present findings have shed light on the molluscan biota associated with Cystoseira taxa in the Gulf of Naples and strengthened the importance of such habitat-forming macroalgae in structuring the local infralittoral invertebrate biodiversity and as a nursery for species-specific associated molluscs.