First report of a giant protist Tricranastrum trinibrachium Takahashi 1991 (syn. Myelastrum trinibrachium) from the Indian Ocean.

This article reports the first occurrence of the rhizarian species *Tricranastrum trinibrachium* Takahashi 1991 (synonym *Myelastrum trinibrachium*) in the Indian Ocean, specifically in plankton samples from the Arabian Sea. Previously known only from the Pacific Ocean, this large and fragile radiolarian, belonging to the class Polycystinea and order Spumellaria, was identified based on its distinctive morphology and size ranging from 747 to 1575 μm. The species’ siliceous skeleton dissolves before reaching the seafloor, explaining its absence in sediment samples, and its recently discovered mixotrophic lifestyle involving a haptophyte symbiont suggests a role in primary production and silica cycling in oligotrophic marine environments. This finding underscores the ecological richness of the Arabian Sea and highlights the need for targeted planktonic studies to better understand the diversity and ecological functions of large Rhizarians in ocean biogeochemistry.