Sharks are assumed to be carnivores, eating a high-protein diet of mammals, fish and other marine animals.  Most of the food animals eat is broken down by enzymes in their guts, often with the assistance of helpful bacteria.  Digestive enzymes are optimized for a certain type of diet, so the gut of a meat-eater may not be able to digest plant protein, and vice versa.  Plant cells have fibrous walls not present in animal cells, and require increased processing to be useful as a food source.

The bonnethead shark (Sphyrna tiburo), is an abundant bottom-dwelling shark frequently found in seagrass ecosystems.  Bonnethead sharks consume large amounts of seagrass; in some analyses seagrass constituted more than 60% of stomach contents.  It was not known if this seagrass is a food that can be digested and used by the shark, however, or simply incidental to hunting and eating within seagrass beds.  An elegant report by Leigh et al has shown that bonnethead sharks can in fact digest and utilize seagrass as a nutrient source, with up to 50% of ingested seagrass carbon incorporated into shark tissues.

The researchers grew seagrass in the laboratory that was labelled with a traceable form of carbon called 13C.  Captive bonnethead sharks were fed a diet of labelled seagrass, with a bit of added squid to ensure nutrition, over several weeks.  Blood and fecal material were collected weekly, and at the end of the study liver and digestive tract tissue were harvested.  All tissues were analyzed for incorporated 13C, indicating digestion and uptake of the labelled seagrass carbon.  Fecal samples were analyzed to compare the amount of seagrass remaining to the amount that had been fed.

Weight measurements showed that the sharks grew while on this diet, indicating that their nutritional needs were met on a nearly meat-free diet.  Carbon analysis showed that roughly 50% of the seagrass was digested and utilized, with 13C found in blood and tissue components.  This efficiency is comparable to that of some sea turtles, animals well known for subsisting largely on a seagrass diet.  Analysis of gut tissue identified enzymes that are capable of digesting plant tissue.  Interestingly, bonnethead sharks were already known to have flattened “molariform” teeth that could function to chew heavy grasses.  The bonnethead shark is therefore an omnivore.

Seagrass habitats provide essential ecosystem services, including oxygen production, water quality maintenance, food and habitat for many species, and shoreline erosion control.  They are now shown to also provide an important food source for bonnethead sharks.  Sharks may benefit the seagrass ecosystem as well, by dispersing nutrients across areas as they travel.

The article is:

Leigh SC, Papastamatiou YP & German DP. (2018) Seagrass digestion by a notorious ‘carnivore’. Proc R Soc B. 285:20181583. DOI:10.1098/rspb.2018.1583.