Noctiluca scintillans, a heterotrophic dinoflagellate, was previously thought to regulate its ammonium concentration by increasing food vacuole size through predation, enabling it to sink to deeper water layers and access diverse feeding niches. To investigate whether food vacuole size is the primary factor influencing N. scintillans' sinking behavior during predation, we conducted feeding experiments using in situ concentrations of tintinnid ciliates and varying prey concentrations of Pseudo-nitzschia pungens and Tetraselmis chui. Both food vacuole size and sinking behavior were measured during grazing on P. pungens and T. chui. The results reveal that N. scintillans consistently exhibits sinking behavior when predating on tintinnid ciliates, regardless of prey abundance. In contrast, although predation on high concentrations of T. chui can induce sinking, such high concentrations are rarely encountered in natural environments. This explains why the appearance of N. scintillans coincided with the decline of tintinnid ciliates in the Matsu Archipelago during field studies in 2020, but not in 2021, when tintinnid ciliates reappeared. These findings underscore the significant role tintinnid ciliates play in the sinking behavior of N. scintillans, rather than attributing it primarily to changes in food vacuole size. Over a two-year period, N. scintillans consumed tintinnid ciliates during their bloom, influencing their population dynamics, while showing no impact on aloricate ciliates, which it could not consume.

Noctiluca scintillans,Oligotrich,Ammonium,Food vacuole,Settling mechanism

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