Microbial Resource Regimes Shape Early Survival, Growth, and Reproductive Morphology in a Phally Polymorphic Terrestrial Gastropod.

Microorganisms are fundamental components of terrestrial ecosystems, acting not only as symbionts or pathogens but also as key nutritional resources for invertebrates. Despite their ubiquity, their role in shaping early-life survival and phenotypic variation in terrestrial animals remains poorly understood. Here, we investigated how microbial resource regimes act as early-life ecological filters influencing survival, growth, and reproductive morphology in the phally polymorphic wetland microsnail Vertigo antivertigo. Using eggs and early juveniles, life stages characterized by high natural mortality, we conducted a series of laboratory experiments manipulating access to the species' fecal- and litter-associated bacteria, soil and litter fungi, and the cyanobacterium Limnothrix sp. Survival emerged as the most consistent response variable across treatments. Fungal supplementation combined with plant litter supported growth and survival to sexual maturity, whereas bacterial resources alone did not compensate for the absence of intact litter-associated microorganisms. Exposure to Limnothrix sp. resulted in severe growth suppression and complete juvenile mortality, irrespective of bacterial supplementation. Disrupted microbial conditions were also associated with shifts in reproductive morphology, including reduced or absent male copulatory organs. Our findings demonstrate that microbial resource regimes can function as strong early-life ecological filters, shaping survival, growth and reproductive morphology.