Seagrass digestion by a notorious ‘carnivore’

Abstract

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 (), 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 C-labelled 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 (13.6 ± 6.77‰C mean ± s.d. for all sharks and all amino acid types analysed) with 50 ± 2% digestibility of seagrass organic matter. Additionally, cellulose-component-degrading enzyme activities were detected in shark hindguts. We 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.

Enhancing insights into foraging specialization in the world's largest fish using a multi‐tissue, multi‐isotope approach

“… and Leigh et al. ) but could also reflect the uncertainty in, or an overestimation of, Δ 15 N Glu‐Phe . Regardless, the low δ 15 N (“oceanic”) individuals appeared to consistently depend on prey supported by nitrogen fixation (e.g., Raes et al.…”

“…9d,e). Evidence of TPs averaging markedly less than 3 (i.e., 2.4 AE 1) could suggest significant ingestion, incidental or otherwise, of phytoplankton or algal debris (e.g., herbivory; see Rohner et al [2013] and Leigh et al [2018]) but could also reflect the uncertainty in, or an overestimation of, D 15 N Glu-Phe . Regardless, the low d 15 N ("oceanic") individuals appeared to consistently depend on prey supported by nitrogen fixation (e.g., Raes et al 2015) over months to years prior to sampling, perhaps in the central Pacific gyre where mid-water particle d 15 N values are at their minimum of close to À2& (Somes et al 2010).…”

Physiological genomics of dietary adaptation in a marine herbivorous fish

“…Thus, Bile Salt Activated Lipase (BSAL; Murray et al 2003) represents another important digestive enzyme for herbivorous fishes because lipids (especially essential fatty acids) are essential to survival. The importance of BSAL in herbivores has been validated in prickleback fishes (German et al 2004;German et al 2015), as well as in Danio rerio fed a high-fiber, low-lipid diet analogous to an herbivorous diet in the laboratory, which elicited elevated lipase activities in this species (Leigh et al 2018). Thus, it appears that C. violaceus, the algal-consuming Xiphister taxa, and other herbivorous species (German et al 2004;Leigh et al 2018) invest in lipase expression to ensure lipid digestion from their algal diet, consistent with what is known as the Nutrient Balancing Hypothesis, or that animals can invest in the synthesis of digestive enzymes to acquire limiting nutrients (Clissold et al 2010), in this case, lipids.…”

Peer Review #2 of "13C values of glycolytic amino acids as indicators of carbohydrate utilization in carnivorous fish (v0.1)"

“…Compound specific stable isotope analysis (CISA) of proteinogenic amino acids (AA) is an emerging tool for dietary reconstruction of finfishes. While stable carbon isotope analysis of AA is predominantly applied for tracing biosynthetic origins of amino acids (Larsen et al 2009;Larsen et al 2013;O'Brien et al 2002;Scott et al 2006), a number of recent studies show that the method has the potential to characterize a consumer's intake of lipids and carbohydrates (Leigh et al 2018;Newsome et al 2011;Newsome et al 2014;Whiteman et al 2018). For example, CSIA has been applied to characterize assimilation and digestion of diets with varying macromolecular composition in e.g.…”

“…For example, CSIA has been applied to characterize assimilation and digestion of diets with varying macromolecular composition in e.g. tilapia and sharks, which in turn can be used to understand a species' nutritional requirements and ability to use resources in its environment (Leigh et al 2018;Newsome et al 2011;Whiteman et al 2018). The origins of AA building blocks are highly diverse.…”