It has always been widely accepted that sharks are "carnivores". This means that their gut is adapted to favour a high protein diet, and they do not require other food groups in order to get all the vitamins and minerals their body needs. However, recently scientists have discovered that not all sharks are solely carnivores! Meet the bonnethead shark (Sphyrna tiburo), the first ever species of shark which we have learned is "omnivorous". So what plants do these sharks eat? Why do they eat them? And why is this significant?
Hammer Time
Bonnetheads are a type of small hammerhead - reaching a maximum size of 1.5 m - which can be found in coastal seagrass meadows, especially off the coasts of the USA. They also differ from their hammerhead cousins thanks to the shape of their "cephalofoil". Their hammer is relatively short, and gently curved and rounded, in comparison to the relatively straight-fronted hammers seen on other hammerheads. Bonnetheads primarily eat mollusks, crustaceans and cephalopods, including shrimps, crabs and lobsters (Bethea et al, 2007).
However, scientists have now learned that bonnetheads also ingest an enormous amount of seagrass; a flowering plant (including 60 different species) found in shallow marine environments. At first it was thought bonnetheads might accidentally consume plant material whilst they were foraging or that they may consume seagrass to protect their stomach lining from the spiny shells of their blue crab prey. Recent research suggests this is not the case (Bethea et al, 2007).
A Varied Diet
For an animal to be considered an omnivore, they must not only consume both plant and animal food sources, but must assimilate nutrients from both sources into their bodies. To asses whether the bonnethead is a true omnivore, scientists used a technique known as "stable isotope analysis".
This method analyses how animal tissues assimilate alternative forms of elements (usually carbon and nitrogen) with different molecular weights, in order to determine what and where an animal has had a meal. They discovered that bonnetheads do indeed assimilate plant materials into their tissues as they grow. They also showed that seagrass was an important component of their diet at all different life stages; from newborn to mature adult. This makes them a true omnivore! (Bethea et al, 2007; Leigh et al, 2008).
In fact, is seems bonnetheads are capable of digesting and assimilating nutrients from plants with relative efficiency. This is possible because, as well as having fat-degrading enzymes (called lipase) and prtotein-degrading enzymes (called aminopeptidase and trypsin) which other shark species also posses, the bonnethead also has carbohydrate-degrading enzymes (called amylase and maltase) in their gut, which allows them to digest fibrous plant material (Leigh et al, 2008).
Food for Thought
It is very important to understand what and where animals consume food because this allows us to understand the role that a specific organism plays in the ecosystem as a whole (Leigh et al, 2008).
Seagrass habitats are critical to health of ocean environments for many reasons. Firstly, seagrasses are an important "carbon sink", meaning they sequester carbon as they grow, which could have critical implications for climate change. Secondly, seagrasses filter the water around them, which is important for water quality. They also create a critical habitat for thousands of fish species, especially in their juvenile life stages, so they play a vital role in recruitment of new individuals to adult populations of endangered species (Leigh et al, 2008).
Bonnetheads are abundant in seagrass habitats and therefore play an important role in coastal community structure. As these sharks consume seagrasses, it will be important to understand how bonnetheads impact upon seagrass ecosystems. It is likely bonnetheads stabilise food webs and pay a role in "nutrient translocation" (displacing nutrients from the site they are ingested, eg a seagrass habitats, to a different location, eg. deeper oceanic waters). Therefore, their protection may be critical for ensuring the health of seagrass ecosystems in the future (Leigh et al, 2008).
References
Bethea DM, Hale L, Carlson JK, Corte ́s E, Manire CA & Gelsleichter J (2007). Geographic and ontogenetic variation in the diet and daily ration of the bonnethead shark, Sphyrna tiburo, from the eastern Gulf of Mexico. Marine Biology, 152, 1009–1020. Access online.
Leigh SC, Papastamatiou YP & German DP (2018). Seagrass digestion by a notorious ‘carnivore’. Proceedings of the The Royal Society Biological Sciences, 285:1886. Access online.
By Sophie A. Maycock for SharkSpeak
