The Resource Acquisition Strategies and Digestive Physiology of Sharks
What an animal consumes and what an animal digests and assimilates for energetic demands are not always synonymous. Sharks, uniformly accepted as carnivores, have guts that are presumed to be well suited for a high protein diet. However, the bonnethead shark (Sphyrna tiburo), which is abundant in critical seagrass habitats, has been previously shown to consume copious amounts of seagrass (up to 62.1% of gut content mass), although it is unknown if they can digest and assimilate seagrass nutrients. To determine if bonnetheads digest seagrass nutrients, captive sharks were fed a 13C-labeled seagrass diet. Digestibility analyses, digestive enzyme assays and stable isotope analyses were used to determine the bonnethead shark’s capacity for digesting and assimilating seagrass material. Compound-specific stable isotope analysis showed that sharks assimilated seagrass carbon. Additionally, cellulose-component-degrading enzyme activities were detected in shark hindguts. I show that a coastal shark is digesting seagrass with at least moderate efficiency, which has ecological implications due to the stabilizing role of omnivory and nutrient transport within fragile seagrass ecosystems. Furthermore, 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 endogenously. The bonnethead shark represents an interesting opportunity to explore how intestinal microbes provide a mechanism for omnivory in a marine vertebrate. I use digestive enzyme assays, histological imaging, measurements of microbial fermentation, and 16S rDNA sequencing to identify processes by which the bonnethead shark can digest and assimilate plant material. Finally, I delve into the functional morphology of the spiral intestine in sharks since this unique structure appears to be important in terms of housing enteric microbes and slowing the transit rate of digesta. I use CT scanning technology to provide a new way of investigating the spiral intestine. In addition to being one of the most conclusive investigations of shark nutritional physiology to date, my results highlight the importance combining studies of structure and function in order to better understand the nutritional physiology of organisms.