Morphology and Molecular Phylogeny of Endosymbiotic Ciliates (Peritrichia, Mobilida) of Marine Invertebrates with Descriptions of Two Novel Species <i>Urceolaria clepsydra</i> n. sp. and <i>Urceolaria bratalia</i> n. sp.

Mobilid ciliates are a morphologically distinct group of protists that form a wide range of symbiotic relationships with aquatic animals and includes three subgroups: Trichodinidae, Urceolariidae, and Polycyclidae. Trichodinids are best known for infecting fishes, whereas urceolariids infect diverse marine invertebrates. Polycyclidae was established for mobilid ciliates infecting sea cucumbers; however, molecular data have been unavailable for this group. In this study, we discovered and characterized two novel mobilid species, one infecting two species of sea cucumbers (Eupentacta quinquesemita and Cucumaria miniata) and one infecting brachiopods or lamp shells (Terebratalia transversa) collected from the Northeast Pacific Ocean. These new mobilid species were characterized at the morphological level using light microscopy (LM) and scanning electron microscopy (SEM). We also inferred the molecular phylogenetic positions of these species using small subunit (SSU) rDNA sequences. Based on combined morphological and molecular data, we demonstrate that the two new species belong to Urceolaria, U. clepsydra n. sp. and U. bratalia n. sp., and support synonymization of Polycycla with Urceolaria. By providing the first molecular data from new species of mobilids infecting sea cucumbers and brachiopods, we expand the host range and improve our knowledge of this diverse but poorly understood group of symbionts.