From Mud to Meat: Comparative Metabarcoding Reveals Two Different Evolutionary Paths to Carnivory in a Group of Meiofaunal Annelids

Evolutionary transitions to carnivory represent profound shifts in feeding mode that are often accompanied by widespread changes in organismal function, behaviour and ecology. Such transitions have evolved numerous times among animals, and predator-prey interactions have been major drivers of animal evolution. Despite the ecological and evolutionary importance of carnivory, the evolutionary steps leading to this feeding mode are poorly understood. Although relatively rare, lineages that have recently adopted predatory lifestyles are particularly valuable for understanding the evolution of carnivory. The annelid genus Chaetogaster, composed of small freshwater oligochaetes, is unusual in having recently evolved carnivory not just once but twice, making it an excellent model to infer evolutionary steps from detritivory to carnivory. We performed a gut-content analysis of eight Chaetogaster species and a detritivorous outgroup, using 18S rDNA metabarcoding complemented by visual gut content assessment to infer diets. We found that species within the lineages presumed to be carnivorous had large fractions of animal metabarcoding reads, as predicted. Their closest relatives, however, differed in dietary profiles. We infer that the closest relatives of one carnivorous lineage, which are generalist predators, primarily feed on ciliates, while the closest relatives of the second carnivorous lineage, which are mollusc symbionts, are detritivores. Our data suggest that carnivory evolved two ways in Chaetogaster, with one transition mediated by ciliate feeding and a second mediated by symbiosis. Overall, this study suggests that carnivory can evolve from noncarnivorous ancestors through distinct evolutionary pathways, even among closely related lineages.