Pisaniello, AlessandroHandley, Kim MWhite, W LindseyAngert, Esther RClements, Kendall DSteven, Blaire2025-12-102025-12-102025-10-20Microbiology Spectrum, ISSN: 2165-0497 (Print); 2165-0497 (Online), American Society for Microbiology, 13(12), e0231725-. doi: 10.1128/spectrum.02317-252165-04972165-0497http://hdl.handle.net/10292/20383Gut microbiota can be shaped by host-related (e.g., species, diet) and environmental (e.g., habitat, geography) factors. Gut microbiota of the herbivorous fish family Kyphosidae vary between individuals and gut sections and can be influenced by diet and geography. Temperate Kyphosus sydneyanus are abundant on rocky reefs of northeastern New Zealand, where adults mainly consume Phaeophyceae, whereas juveniles typically feed on Rhodophyta and Chlorophyceae. We compared the gut microbiota of K. sydneyanus adults and juveniles to investigate the relative effects of ontogeny versus diet on bacterial community composition. We used 16S rRNA gene amplicon sequencing and ddPCR quantification to determine the following: (i) ontogenetic variation in gut microbiota of K. sydneyanus, (ii) shared microbial taxa, and (iii) processes influencing bacterial community assembly, given the lack of vertical transmission. To further explore dietary effects, we also compared these data to the gut microbiota of adult K. sectatrix, which are rare tropical vagrants to northeastern New Zealand. Juvenile K. sydneyanus exhibited greater dietary similarity to K. sectatrix adults than conspecific adults. Overall, K. sydneyanus gut microbiota diversity increased with fish size, and K. sydneyanus adults had a more diverse gut microbiota than K. sectatrix adults. Estimated absolute abundances and community structure varied considerably across individuals. K. sydneyanus juveniles had relative abundances more similar to those from adult K. sectatrix than adult conspecifics. However, a comparison of amplicon sequence variants (ASVs) showed that juveniles of K. sydneyanus shared more ASVs with conspecific adults. This suggests that historical contingency and selection are important drivers of community assembly.Copyright © 2025 Pisaniello et al. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license.community assemblyddPCRgut microbiotaherbivoryontogenetic variation3107 Microbiology31 Biological Sciences3103 EcologyNutritionPediatricMicrobiome2.2 Factors relating to the physical environmentInfection14 Life Below WaterJournal ArticleOpenAccess10.1128/spectrum.02317-25