Temporal shifts in diversity and functional traits of free-living nematode associated to Cystoseira

This study presents the first comprehensive, year-long assessment of free-living marine nematode assemblages inhabiting Cystoseira-dominated rocky substrates along the Rimel coast (Bizerte, Tunisia). Monthly sampling was conducted from March 2021 to February 2022 to explore both the taxonomic composition and functional trait dynamics of nematodes in response to temporal changes of the environmental conditions. A total of 50 species belonging to 38 genera and 18 families were identified, with Chromadoridae and Linhomoeidae being the most species-rich families. Despite the persistence of a core nematode assemblage, notable temporal changes were observed in both species-rich families and trait composition, particularly during spring (March-May) and summer (June-September) months. Multivariate analyses (nMDS, cluster analysis, and SIMPER) revealed distinct seasonal groupings, with brief periods of ecological turnover detected in April-May and June-July corresponding to short temporal windows during which nematode assemblages undergo rapid compositional and functional change. Functional traits, including feeding groups, tail and amphidial shapes, adult size, and c-p life-history strategies, exhibited significant temporal reorganization, corresponding to seasonally shifts, emergence, or decline of specific traits over time. Opportunistic traits (e.g., cp-2, non-selective deposit (1B) feeders, elongated tails, pocket and spiral amphids) were dominant during spring (April-May) and early autumn (September). In contrast, long-lived, stress-sensitive groups (cp-5) were not present. Although abiotic variables such as temperature, salinity, and oxygen displayed a strong temporal variability, no direct linear correlations with nematode diversity were observed, suggesting that assemblages' dynamics are shaped by other and complex factors such as macroalgae structure, food sources availability, and colonization processes.