Spatio-temporal patterns of holoplanktonic mollusc assemblages and indicator species of hydrodynamic conditions in the northwestern South China Sea

Holoplanktonic molluscs are a group of overlooked but ecologically important zooplankton in marine ecosystems that play a crucial role in the marine food web and carbon cycle. However, changes in the marine environment can lead to an increase in the abundance of pelagic molluscs, causing ecological disasters. To understand the environmental factors that structure holoplanktonic mollusc assemblages, their diversity and abundance were analyzed in the northwestern South China Sea (NWSCS). A total of 39 holoplanktonic molluscs were identified, including 24 pteropod and 15 heteropod species. Significant seasonal and regional differences were observed in species diversity and abundance. Species richness was higher in offshore waters than in nearshore waters during summer and winter, whereas species abundance was significantly higher in summer than in winter. High species abundance was mainly concentrated in waters influenced by cyclonic eddies and coastal upwelling during summer. Temperature, salinity and chlorophyll a are important factors for structuring holoplanktonic mollusc assemblages. Creseis acicula was an effective indicator species in nearshore waters in both seasons, whereas Limacina bulimoides and Heliconoides inflatus were the best indicators in offshore waters in summer and winter, respectively. These findings can form a baseline for understanding the distribution of holoplanktonic molluscs in the marginal sea of the northwestern Pacific Ocean and provide a solid foundation for monitoring zooplankton in a changing ocean and for sustainable ecosystem management. • Diversity information for holoplanktonic molluscs was supplemented. • Differences in seasonal and regional distributions of holoplanktonic molluscs were observed. • Holoplanktonic mollusc assemblages were strongly influenced by ocean physical processes dynamics. • Some species were effective indicators of local hydrodynamic conditions.