There are over 500 species of shark, but the bonnethead shark (Sphyrna tiburo) is the only one found to be eating plants!

We all have this presumption that sharks (Clade Selachimorpha) are supposed to be strictly carnivores, with many of the shark species being apex predator that throne at the very top of the food web in marine (and some freshwater) ecosystems.

Despite being over 500 different species worldwide, anyone had yet to see a herbivorous (plant-eating) or omnivorous (both plant and meat-eating) shark - until this year, when a group of researchers witnessed that a bonnethead shark (Sphyrna tiburo) was feeding on seagrass while searching on top of the sediments for small crabs, and decided to investigate this further.

A bonnethead shark. As you can see, it's a type of hammerhead shark. Image is Public Domain.

The bonnethead shark is actually a rather small shark, with full-grown individuals being just under a meter long. Another unique feature about them is that males and females have a different head morphology, which has not been seen in any other shark species. This allows people to identify the sex of the shark very easily, which is nice when you are studying them.

Finding out about the omnivorous nature of the bonnethead shark

People have long noticed that the gut content of bonnethead sharks was seemingly very high, with records of over 60 % of the gut content being made up of seagrasses. This did not really strike out as anything special at first, seeing as they mainly feed on crabs, shrimps, and mollusks who all live in the seagrass ecosystem, meaning it was natural to assume that all this seagrass in the gut was a side effect of feeding in this ecosystem. If it really was a side effect, then the seagrass would be of no nutritional value to the shark, as no one expected sharks to be able to digest plant matter.

A group of scientists from University of California in Irvine, and Florida International University in Miami decided to check if this really was the case, and began an experiment that would eventually lead to the discovery of the very first known omnivorous shark.

The experiment

Finding out if the shark was actually gaining nutrients from the seagrass might sound simple in theory, but actually doing a research experiment to finding this out is more complex than it might sound like at first.

To figure it out, they assembled huge aquariums where they planted lots of seagrass that was harvested in the Florida Bay. Once it took root in the aquarium they added radioactive isotopes of sodium bicarbonate to the water, which would then be used by the plants to build mass.

A typical seagrass meadow that would be planted in the lab. Photo is Public Domain.

This radioactive isotope gave a unique chemical signature to the plants, and if actually digested and used by the shark, would eventually be traceable in the cells of the sharks.

The next part was obviously to get sharks, and the research group put a total of five sharks in these aquariums with the isotope-marked plants. During the next three weeks the sharks were fed squid, and as expected they also ate a lot of the seagrass in the tank.

During these three weeks, all the sharks gained weight, so now the only thing left was to look for the radioactive isotopes in the sharks to determine if the seagrass had actually been digested and used for building cells.

The results

The researchers sampled the blood cells and liver tissues from the sharks, and found that roughly half of all the carbon from the seagrass had been digested by the shark. This is actually a pretty high amount, which matches the amount digested by herbivorous, marine non-fish such as sea turtles.

The shark lack the teeth needed to grind down the seaweed, so instead they must rely on another mechanism to break down the seaweed into a usable form. The same article claims that this is due to very strong stomach acids that allows the cells to be degraded enough for the enzymes to begin to break it down.

Photo is Public Domain.

They researchers also found specialized cellulose-component-degrading enzyme activity in the hindgut of the shark, giving them yet another indication that the shark actually utilizes the seagrass it eats.

Could there be other omnivorous sharks?

I love research like this, where the researchers actually find indications that what we have believed for a very long time, to be wrong or only partially true. After learning that this particular shark is actually omnivorous, the next step is to find out if there are more of them. And there very well could be, because no one has really looked at this before, so we certainly don't know.

I would actually be a bit surprised if we don't find more of these now, since we finally know that it's a thing we should be looking at. There are definitely many other shark species that feed in the seagrass meadow ecosystem, and it's entirely possible that many of these are also able to utilize the eaten seagrass to an extent.

Sources

• "Seagrass digestion by a notorious ‘carnivore’" by Samantha C. Leigh et al. (2018).

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