Biodiversity of Marine Mollusk Assemblages from Two Contrasted Algal Habitats in the Mediterranean Sea (Tossa de Mar, Costa Brava, NE Spain)

The malacological faunae of two algal habitats with different lighting conditions—a sciophilous wall and a photophilous one—both located in Tossa de Mar (Costa Brava, Girona, NE Spain), have been studied to examine the differences in the malacological communities, their distribution over time, and the role played by micromollusks in these algal habitats. Every two months, for a year, samples were collected via scuba diving and scraping standardized surfaces (20 × 20 cm) of the algal communities. In the laboratory, the samples were washed and filtered using two metal sieves (2 mm for coarse fraction and 513 µm for fine fraction). Mollusks were separated, identified, and later analyzed statistically to assess biodiversity indices and sample grouping. A total of 7913 mollusk specimens from 148 species (112 Gastropoda; 32 Bivalvia and 4 Polyplacophora) were identified across all samples, with 1669 specimens (84 species) from the sciophilous algae and 6244 specimens (133 species) from the photophilous algae. Moreover, 10 species accounted for 76.27% of the total abundance, with each species represented by over 100 specimens in the total samples. The gastropod Bittium latreillii was the most abundant species, with 2106 specimens recorded. Regarding the two fractions, 337 specimens (69 species) were found in the coarse fraction, while 7576 specimens (111 species) were found in the fine fraction. Significant differences were obtained between samples with respect to time, habitat type, and size fraction. The Shannon–Wiener biodiversity index values indicated differences between the two communities over time, as well as variations in biodiversity, with the photophilous community exhibiting higher values. It was demonstrated that algal thalli provide a highly suitable substrate for juvenile forms and micromollusks. Our findings, combined with the literature and various online citations, establish Tossa de Mar as the town on the Catalan coast with the best-documented malacological fauna.