The Culuccia Peninsula is located in north-western Sardinia, within the Santa Teresa di Gallura territory, facing the Conca Verde at the west and the Isola dei Gabbiani at the east (Fig. 1A-C). It is characterised mainly by rocky coasts and a few sandy beaches; relatively quiet weather conditions are met during the Summer, whereas strong currents and high-energy waves feature the Winter season. The western and eastern coasts of Culuccia are different. The western coast faces a narrow fjord, which hosts a small touristic harbour (Porto Pozzo, with a small Marina), minor stream mouths and a suspended aquaculture farm rearing the oyster Magallana gigas (Thunberg, 1793). The southernmost part of this fjiord encloses a small shallow water brackish lagoon, which is connected to internal salt marshes. The north-eastern side faces the open sea; the south-eastern coast forms, together with the Isola dei Gabbiani, a small bay where the Liscia River flows. Due to the different marine conditions and habitats of the shores around the Peninsula, 15 sampling sites (SS) regularly spaced were selected (Fig. 1). Habitat descriptions, geographic coordinates and depth range of the SS are provided in Table 1.

Specimens were collected either by direct hand or using an air-lift suction sampler during diving (SCUBA and free diving, depending on the depth) in benthic environments, on both rocky and soft substrates, particularly on/under rocks, rubble, amidst algae and in Posidonia oceanica L. (Delile) meadows. A total of 50 SCUBA dives (1 hour each) were performed by five divers down to 25 m depth, during about one-year monitoring (2022-2023). An ethical approach in this research was carried out by complying with the restrictions in term of collected specimen size, environmental survey of the collection sites, as well as complying with local, regional, national and international rules and regulations for access to biodiversity, sustainable use and benefit sharing (Convention on Biological Diversity and its Nagoya Protocol, national regulations). Species identification was initially made by each collector, but all specimens were then cross-checked by all authors who are experienced malacological taxonomists. Live animals were photographed in situ or in the Coluccia’s laboratory (Osservatorio Naturalistico) with the digital cameras Canon Powershot G11 and Olympus TG-6, equipped with housing for marine photographs. Specimens of particular interest were anaesthetised with 7.5% magnesium chloride (MgCl₂) and preserved in 95% ethanol as reference material and for future molecular studies. All sampled material was deposited in the Malacology Collection of the Department of Biology and Biotechnologies Charles Darwin, Sapienza University of Rome (Rome, Italy).

Sorting and identifications of the material was performed by using stereomicroscopes. Representative specimens were photographed by using a ZEISS Axio Zoom.V16 steremicroscope, equipped with a ZEISS AxioCam 503 camera. For Scanning Electron Microscopy (SEM) analyses, dried shells were mounted on aluminium stubs using self-adhesive carbon discs, sputtered with gold (Emitech k550 Sputter Coater) and analysed by a Zeiss Gemini 300 SEM at L.I.M.E. (Laboratorio Interdipartimentale di Microscopia Elettronica, Roma Tre University, Rome, Italy) laboratory.