Gut microbial diversity and digestive function of an omnivorous shark

The intestinal microbiome of vertebrates has been shown to play a crucial role in their digestive capabilities. This is particularly true for omnivores and herbivores that rely on enteric microbes to digest components of plant material that are indigestible by host-derived enzymes. While studies of microbe-host interactions are becoming more frequent in terrestrial systems, studies of this type are still limited in marine systems, particularly for higher trophic level organisms. Although sharks are largely carnivorous, the bonnethead shark (Sphyrna tiburo) has been identified as an omnivore, given that it assimilates seagrass material in addition to proteinaceous prey items such as crustaceans. The mechanisms by which bonnetheads digest seagrass, including microbial digestion, are still unknown. We use digestive enzyme assays, histological imaging, measurements of microbial fermentation, and 16S rDNA sequencing to explore potential processes by which the bonnethead shark may digest and assimilate plant material. We found evidence of microbial fermentation (as evident by moderate short-chain-fatty-acid concentrations) as well as evidence of greater epithelial surface area in their spiral intestine compared to other gut regions. We identified specific orders of microbes that make up the majority of the bonnethead shark gut microbiome (Vibrionales, Clostridiales, Pseudomonadales, Mycoplasmatales, Rhizobiales, and others), some of which are known, in other organisms, to be involved in the production of enzymes responsible for the breakdown of chitin (found in crustacean shells) and components of cellulose (found in seagrass). Our results highlight that an organism from a stereotypical “carnivorous” group is capable of breaking down seagrass, including potential for some fiber degradation, as well as advances our knowledge of gut microbe community structure in sharks.