Coinfection of slime feather duster worms (Annelida, ) by different gregarine apicomplexans () and astome ciliates reflects spatial niche partitioning and host specificity.

Individual organisms can host multiple species of parasites (or symbionts), and one species of parasite can infect different host species, creating complex interactions among multiple hosts and parasites. When multiple parasite species coexist in a host, they may compete or use strategies, such as spatial niche partitioning, to reduce competition. Here, we present a host–symbiont system with two species of (Apicomplexa, Gregarinida) and one species of astome ciliate co-infecting two different species of slime feather duster worms (Annelida, Sabellidae, ) living in neighbouring habitats. We examined the morphology of the endosymbionts with light and scanning electron microscopy (SEM) and inferred their phylogenetic interrelationships using small subunit (SSU) rDNA sequences. In the host ‘ sp. Quadra’, we found two distinct species of ; . cf. exclusively inhabited the foregut, and . n. sp. inhabited the mid to hindgut, reflecting spatial niche partitioning. n. sp. was also present in the host . , which harboured the astome ciliate n. gen. et sp. cf. and . n. gen. et sp. were absent in the other host species, indicating host specificity. This system offers an intriguing opportunity to explore diverse aspects of host–endosymbiont interactions and competition among endosymbionts.