Abstract
We investigated the community structure, spatial distribution, and molecular phylogeny of peritrich ciliates living as epibionts on the shells of Gundlachia lutzi . Seven peritrich species were registered, including Epistylis portoalegrensis , Epistylis sp., Opercularia articulata , Opercularia nutans , Opercularia sp., Platycola decumbens , and Thuricola sp. We delimited specific shell regions (e.g. anterior, posterior, A, B, C, and D) to determine attachment sites, calculating prevalence, mean intensity, mean abundance, and the discrepancy index for each epibiont species. A great proportion of basibionts presented epibionts in the anterior region, with P. decumbens and O. articulata being the most prevalent. The greatest values of mean intensity were presented by O. nutans , E. portoalegrensis and O. articulata . The lower abundance of peritrichs in region B, compared to regions A, C, and D, suggests that intense mechanical disturbance in this area may be a limiting factor. These results confirm that the G. lutzi shell is a complex substrate with areas of varied suitability for epibionts. Furthermore, we obtained four new 18S-rDNA sequences for epibiont peritrichs, two of which corresponded to species lacking prior molecular data. The molecular analysis corroborated the morphological identification of the two species, E. portoalegrensis and P. decumbens . It also revealed indications of a possible relationship between epibiosis and the evolution of the Operculariidae family, which opens an exciting avenue for future research on the role of host association as a key evolutionary driver and its potential influence on the diversification of the Sessilida.