Department of Ecology and Evolutionary Biology, University of California-Irvine, Irvine, CA 92697, USA
Marine Sciences Program, Department of Biological Science, Florida International University, 3000 NE 151st Street, Miami, FL 33181, USA.
Proc Biol Sci. 2018 Sep 5;285(1886):20181583. doi: 10.1098/rspb.2018.1583.
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.
动物的摄食和消化吸收用于满足能量需求并不总是完全一致的。鲨鱼通常被认为是肉食动物，其肠道被认为非常适合高蛋白饮食。然而，丰下巴天竺鲨（）在关键的海草生境中大量存在，先前的研究表明它们会大量摄食海草（高达肠道内容物质量的 62.1%），尽管尚不清楚它们是否能够消化和吸收海草中的营养物质。为了确定丰下巴天竺鲨是否能消化海草中的营养物质，研究人员给圈养的鲨鱼喂食了一种带有 C 标记的海草饮食。采用消化率分析、消化酶测定和稳定同位素分析来确定丰下巴天竺鲨消化和吸收海草物质的能力。化合物特异性稳定同位素分析表明，鲨鱼同化了海草中的碳（所有鲨鱼和所有分析的氨基酸类型的平均值为 13.6 ± 6.77‰ C 均值 ± s.d.），对海草有机物的消化率为 50 ± 2%。此外，在鲨鱼的后肠中检测到了纤维素成分降解酶的活性。我们的研究表明，一种沿海鲨鱼能够以至少中等的效率消化海草，这由于杂食性在脆弱的海草生态系统中具有稳定作用和营养物质运输的功能，因此具有生态意义。
