First Metataxonomic Characterisation of Gut Microbiota of Swordfish ( <scp> <i>Xiphias gladius</i> </scp> )

Swordfish (Xiphias gladius) is a large, migratory apex predator with a carnivorous diet, occupying a top position in the marine food chain. Although it is a valuable teleost pelagic fish with a significant commercial value, its gut microbiota has never been studied. The gut microbiota of 100 individuals was characterised by sequencing the V3-V4 region of the bacterial 16S rRNA gene. Gut microbiota findings were classified with consideration to diversity, taking into account their weight (10-20; 21-30; over 31 kg) and the FAO fishing areas in which they were caught (FAO 27, 34, 37.1.1 areas). Significant differences in the alpha diversity were observed among the weight categories for all metrics examined (except for the evenness index) and only by Shannon's index among the FAO fishing areas. Beta-diversity analysis revealed no significant differences. The phylum Pseudomonadota dominated the swordfish gut microbiota, followed by Fusobacteriota. Photobacterium was the most abundant genus across all weight categories and FAO fishing areas. Smaller fishes showed a less rich and diverse gut microbiota, dominated almost exclusively by Photobacterium. Conversely, Pseudoalteromonas, Psychrobacter, Psychrilyobacter, and Cetobacterium appeared to increase in abundance with fish weight. Although Photobacterium was dominant across the different FAO fishing areas, distinctive microbial community compositions were observed: Cetobacterium was more prevalent in FAO 27, while Pseudoalteromonas was more prevalent in the other areas. Unlike the gut microbiota of other marine fish species, Vibrio and Lactobacillus were largely absent. This study represents the first metataxonomic characterisation of the gut microbiota of swordfish using next-generation sequencing.