International Academy of Ecology and Environmental Sciences Proceedings of the International Academy of Ecology and Environmental Sciences 2220-8860 11 3 2021 9 1 Diversity and distribution of meiofauna Mollusca in the southern Caspian Sea 137-149 EN M. Zarghami http://www.iaees.org/publications/journals/piaees/articles/2021-11(3)/diversity-and-distribution-of-meiofauna-Mollusca.pdf> In this study, in order to analyze Mollusca species and determining their relationship with the sediment factors, sediment samples were gathered during spring, summer, autumn and winter 2012 from 12 stations (ranging in depths 5, 10, 20 and 50 meters) in the Southern Caspian Sea from Behshahr to Ramsar (Mazandaran, Iran). The sediment factors (grain size, total organic matter and calcium carbonate concentration) were measured in laboratory. Two classes of Mollusca (Bivalvia and Gastropoda) were identified. Class Bivalvia had 3 species (Didacna protracta, Hypanis caspia, Abra ovata) which belong to 3 genera of 2 families and class Gastropoda had 4 species (Pergola sp., Anisus kolesnikovi, Abeskunus sphaerion, Ulskia ulskii) which belong to 4 genera of 3 families. Most dominant species of Bivalvia was Abra ovata (46.2 individuals/0.1m2) and that of Gastropoda was Abeskunus sphaerion (58.66 individuals/0.1m2). The mean of maximum and minimum density of Mollusca was observed in spring (29.67 individuals/0.1m2) and winter (0 individuals/0.1m2) respectively. The results of One Way ANOVA showed that density of Mollusca was different significantly with depth and season (P less than 0.05). Results of Pearson correlation showed that between density of Mollusca and salinity, %silt and clay, TOM and depth had been negative positive correlation and had been positive correlation with temperature.