Do sibling hosts harbor sibling parasites? The case of Aggregata sp. (Apicomplexa: Aggregatidae) infecting Octopus bimaculoides Pickford and McConnaughey, 1949 in the northwest Mexican Pacific

The genus Aggregata comprises cosmopolitan Apicomplexa parasites that infect cephalopods and show high host specificity. Their heteroxenous life cycle requires two hosts: a crustacean (merogony) and a cephalopod (gamogony/sporogony). Poynton et al. (1992) reported Aggregata millerorum in Octopus bimaculoides from California (USA) and described the smoothness of the sporocyst surface. In this study, four of eight O. bimaculoides collected in Ensenada, Baja California, harbored Aggregata sp. showing bifurcate tubular projections on sporocysts similar to those of Aggregata polibraxiona, which was described in Octopus bimaculatus from the Gulf of California. Thus, Aggregata sp. (present study) and A. polibraxiona (from the Gulf of California) may be the same species. We employed morphological, histological, and molecular approaches to describe and identify Aggregata sp. inhabiting O. bimaculoides from Ensenada. The sporocyst surfaces of Aggregata sp. (present study) and A. polibraxiona shared tubular bifurcated projections but differed in the sizes of the sporocysts (10–18 vs 12–24 µm) and sporozoites (11–18 vs 16–26 µm). Phylogenetic analyses of partial 18S rDNA sequences placed both Aggregata species infecting O. bimaculatus and O. bimaculoides in the same clade, indicating a lack of phylogenetic informativeness. Additional 18S rDNA data are needed to clarify whether O. bimaculoides from Ensenada harbor A. polibraxiona, are co-infected by multiple Aggregata species, or host a novel species. This work highlights the importance of integrative taxonomy in studies of Aggregata species and the need to develop novel molecular markers (e.g., ITS and COX) to enhance species-level classification.