New morphological and ecological insights into the deep-sea hydrozoan Rosalinda incrustans (Anthoathecata: Capitata) from the Mediterranean Sea

Rosalinda incrustans (Kramp, 1947) is a poorly known, deep-sea capitate hydrozoan whose biology, morphology, and ecological traits have remained largely obscure due to the rarity of collected material and its small, inconspicuous epibiontic habitus. Since its original description, the species has been collected only twice and recorded two additional times with Remotely Operated Vehicles (ROVs), invariably observed as an epibiont of the crab Anamathia rissoana (Roux, 1828). Here, we report a detailed morphological description of the species based on newly collected material, adding information from in situ pictures, as well as from ecological, structural, and histological perspectives. For the first time, the species is documented growing on substrates other than the crab, including polychaete tubes and scleractinian skeletons, revealing a broader host range and a non-obligate association with A. rissoana. These findings, supported by the analysis of an extensive ROV archive, allow a reevaluation of the species’ life habit, its symbiotic relationships, and its role as an early coloniser of deep-sea hard substrates. The combination of new host records, morphological plasticity of the hydrorhiza, and overlapping diagnostic traits also suggests a possible synonymy with the Atlantic species Rosalinda williami (Totton, 1949).