Seagrass digestion by a notorious 'carnivore'

Area of Science:

• Marine Biology
• Animal Physiology
• Ecology

Background:

• Sharks are generally considered carnivores with digestive systems adapted for high-protein diets.
• Bonnethead sharks (Sphyrna tiburo) consume significant amounts of seagrass, but their ability to digest it was unknown.

Purpose of the Study:

• To investigate the digestive and assimilation capabilities of bonnethead sharks for seagrass nutrients.
• To determine if bonnethead sharks can derive energy from seagrass consumption.

Main Methods:

• Feeding captive sharks a 13C-labelled seagrass diet.
• Conducting digestibility analyses, digestive enzyme assays, and stable isotope analyses.
• Utilizing compound-specific stable isotope analysis to track carbon assimilation.

Main Results:

• Bonnethead sharks assimilated seagrass carbon with 50% digestibility of seagrass organic matter.
• Detection of cellulose-component-degrading enzyme activities in the hindguts of sharks.
• Evidence of seagrass nutrient assimilation in a species traditionally viewed as carnivorous.

Conclusions:

• Bonnethead sharks possess the physiological capacity to digest and assimilate seagrass.
• This digestive capability has significant ecological implications for omnivory and nutrient cycling in seagrass ecosystems.
• The findings challenge the strict carnivorous classification of some shark species.