Capturing the extensive diversity of marine anaerobic scuticociliates (Oligohymenophorea, Ciliophora) through cultivation.

Marine anoxic sediments are expansive ecosystems, effectively devoid of oxygen, where eukaryotic life is predominantly represented by protists. In this study, we surveyed a range of such habitats and uncovered novel diversity within ciliated protists from the subclass Scuticociliatia (class Oligohymenophorea). We establish three new genera of marine anaerobic scuticociliates within the family Anaerocyclidiidae that were previously detected exclusively through cultivation-independent environmental surveys. Our results show that marine Anaerocyclidiidae have a global distribution and occur frequently in anoxic sediments. Notably, all studied marine Anaerocyclidiidae host prokaryotic ectosymbionts of varying sizes and shapes, potentially representing distinct prokaryotic lineages. Through broad geographic sampling and the establishment of the largest culture collection of marine anaerobic scuticociliates to date, we investigated the diversity, morphology, behavior, and symbiotic associations within this significant ciliate lineage. These findings highlight the importance of cultivation approaches to uncover novel protistan taxa and their symbiotic relationships, expanding our understanding of biodiversity and ecosystem functioning in oxygen-depleted habitats. Supplementary Information: The online version contains supplementary material available at 10.1007/s42995-025-00350-5.