Challenging the paradigm: the Asian fish tapeworm (Schyzocotyle acheilognathi, Yamaguti 1934) lacks an intrinsic symbiotic bacterial community.

Schyzocotyle acheilognathi is an invasive generalist cestode with a high capacity for adaptation to multiple hosts and freshwater environments. Recent reports suggest that this parasite possesses an intrinsic symbiotic microbiota distinct from that of its fish hosts, and its presence induces gut dysbiosis in the host. In this study, we reassessed these ideas. For this, we collected naturally parasitized fish specimens from different locations in Mexico, encompassing different host species, including Cyprinus carpio, Pseudoxiphophorus bimaculatus, Tlaloc hildebrandi, and Vieja hartwegi. We also tested whether this parasite induces a dysbiotic process in the gut bacterial community of Tlaloc hildebrandi. Parasites were identified based on morphological and molecular criteria, and their bacterial communities were characterized using metataxonomy. Our results revealed that S. acheilognathi does not harbor a consistent microbial community among the different host species surveyed. We also did not detect any dysbiotic effect on the gut microbiota of Tlaloc hildebrandi. These findings contradict previous data and provide evidence of the loose relationship between this parasite and bacteria, which we propose could be a part of its successful generalist strategy. The results presented herein offer a novel perspective on the quest for understanding the microbial ecology in generalist cestodes of freshwater fish.