Evidence for spatial niche partitioning in the ectocommensal Symbion americanus (Cycliophora) on its lobster host, Homarus americanus (Arthropoda, Malacostraca)

Symbion americanus is a microscopic marine invertebrate in the phylum Cycliophora that lives as an ectocommensal on the mouthparts of the American lobster, Homarus americanus. Previous phylogeographic work on S. americanus identified three lineages corresponding to one described and two potential new species, along with evidence of sympatry. But these studies did not explore whether individuals of S. americanus from different genetic lineages segregate onto different host mouthparts. The present study examines the population structure and microhabitat of 196 individuals of S. americanus from lobsters from five North American localities (from Newfoundland, Canada, to Boston, MA) collected between June and September 2019. Specimens were sequenced at two mitochondrial DNA (mtDNA) markers, a 487 bp fragment of cytochrome c oxidase subunit I (COI) and a 481 bp fragment of 16S rRNA. Phylogenetic analyses recover three distinct lineages of Symbion americanus, corroborating previous studies. Population genetic analyses of individuals belonging to the C and G lineages show clear population structure at the level of host mouthparts. Microhabitat data suggest the segregation of different genetic lineages in S. americanus onto different host mouthparts, perhaps indicating the role of spatial niche partitioning in the incipient speciation of S. americanus.