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Tropical gar (Atractosteus tropicus) is freshwater and estuarine fish, inhabiting the Earth since the Mesozoic era and undergoing limited physiological variation ever since. Besides its recognized cultural and scientific relevance, the species has seen remarkable growth in its economic impact due to pisciculture. In this study, we present the first report of the whole taxonomic composition of microbial communities in gut contents in juveniles and adults of A. tropicus, by sex and origin (wild and cultivated). For this study, 508 genera were identified, with the most and least abundant being Cetobacterium and Paludibacter, respectively. Fusobacteria, Proteobacteria, Firmicutes, and Bacteroidetes phyla are the core gut microbiome of A. tropicus juvenile and adult by sex and origin. Deinococcus-Thermus phylum sequence was only identified in wild-type males. In the phylogenetic trees reconstruction Lactococcus lactis strains CAU929 and CAU6600, Cp6 and CAU9951, Cetobacterium strain H69, Aeromonas hydrophila strain P5 and WR-5-3-2, Aeromonas sobria strain CP DC28 and Aeromonas hydrophila were identified, some of them with probiotic potential within the three dominant phyla in core gut microbiome in A. tropicus adults, especially in wild-type organisms. Myroides genus was recognized in microbiota gut of the cultivated juvenile A. tropicus. Nevertheless, Alpha diversity indicated that the highest gut microbiota abundance and richness is found in cultivated juvenile and wild-type adult A. tropicus female, rather than adult wild-type males and the least gut microbiota abundance and richness is found in a cultivated adult of A. tropicus for both sexes., The authors thank the Mexican National Council of Science and Technology for the postgraduate studies thesis scholarship and the mixed scholarship granted to carry out the research internship at the University of Valencia (Spain). This research received external funding from the Mexican National Council of Science and Technology "Strengthening of the Master's Degree in Environmental Sciences for its Permanence in the National Register of Quality Graduates of CONACYT" Reg. No. TAB-2014-C29-245836 and the Project "Estudio de la fisiología digestiva en larvas y juveniles de pejelagarto (Atractosteus tropicus) con base en técnicas histológicas, bioquímicas y moleculares" Reg. No. CB-2016-01-282765.

Tropical gar (Atractosteus tropicus), is freshwater and estuarine fish that has inhabited the Earth since the Mesozoic era, undergoing limited physiological variation ever since. This omnivorous fish is endemic to southern Mexico and part of Central America. Besides its recognized cultural and scientific relevance, the species has seen remarkable growth in its economic impact due to pisciculture. Previous studies have highlighted the role of microbial communities in fish, particularly those in the gut microbiome, in maintaining their host homeostasis or disease. In this study, we present the first report of the whole taxonomic composition of microbial communities in gut contents of adults’A. tropicus, by sex (female/male) and origin (wild/cultivated). Using culture-independent techniques, we extracted metagenomic DNA that was used for high throughput 16S rDNA profiling by amplifying the V4 – V5 hypervariable regions of the bacterial gene. A total of 364,735 total paired-end reads were obtained on an Illumina MiSeq sequencing platform, belonging to 508 identified genera, with the most and least abundant areCetobacterium, Edwardsiella, Serratia, Clostridium sensu stricto, PaludibacterandCampylobacter, Snodgrassella, Albirhodobacter, Lentilitoribacter, respectively. We detected that, by sex and origin, Proteobacteria, Fusobacteria, Firmicutes and Bacteroidetes phyla are the core gut microbiome of the adults’A. tropicus. We discover the Deinococcus-Thermus phylum sequence, wildtype males only, with extremophile capacity in another freshwater fish. We also identified the speciesLactococcus lactisstrains CAU929 and CAU6600, Cp6 and CAU9951,Cetobacteriumstrain H69,Aeromonas hydrophilastrains P5 and WR-5-3-2,Aeromonas sobriastrain CP DC28 andAeromonas hydrophilawith probiotic potential in aquaculture within the three dominant phyla, especially in wild-type organisms.