Gut Microbiome‐Metabolome Reconfiguration Associates With Phenotypic Plasticity of <i>Daphnia</i> Under Predation Risk

ABSTRACT Predation is an important selective pressure shaping phenotypic plasticity in aquatic organisms. As the key mediator between environmental changes and host physiology, gut microbiota and their metabolism play crucial roles in regulating host fitness. Although a few studies have extended the effects of predation risk on gut microbial composition, relatively little is known about whether and how the gut metabolite profiles are reshaped and linked to host defensive responses. This study integrated phenotypic assays, 16S rDNA sequencing and metabolomic analysis, systematically revealing the coordinated shifts in gut microbes and metabolites of Daphnia magna under fish kairomone exposure, which may be associated with D. magna 's morphological and reproductive defences. Particularly, the enrichment of the indicator taxa Selenomonadaceae and Sporichthyaceae was negatively correlated with dAMP and adenine in the purine metabolism pathway, suggesting restricted nucleotide synthesis and ATP production. The resulting energy deficit may activate AMPK while inhibiting mTOR signalling, reallocating energy from somatic growth to reproductive investment. Moreover, Selenomonadaceae enrichment was linked to reduced PGD 2 in the neuroactive ligand‐receptor interaction pathway, potentially weakening Gs‐cAMP‐PKA signalling, suppressing cell proliferation and leading to a smaller body size of D. magna . These coordinated associations suggest a potential mediating role for gut microbe–metabolite interactions in the growth‐survival trade‐off of Daphnia under predation risk, which requires further experimental validation. These findings expand our understanding of host ecological adaptation from a gut microbial functional perspective.