1st course: Seagrass ****

Image: Hemprich F.G. & Ehrenberbg C.G. / Wikipedia

What’s the one dietary fact everybody knows about sharks? Correct, they eat human beings – as graphically shown in the creature feature film sensation of 1975, the movie Jaws (and its various good, bad, and indifferent sequels…). As so-called apex predators, sharks are famously considered to be carnivorous, but, that’s not necessarily so.

Samantha Leigh et al. have examined the bonnethead shark (Sphyrna tiburo) that eats seagrass.* It needs here to be pointed out that consumption of seagrass – along with cephalopods [animals such as squid and octopus] and crustaceans [animals such as crabs and shrimps] – by this shark species has previously been reported by Dana Bethea et al..

What Leigh et al. have added to that knowledge is evidence that the seagrass is being both digested and assimilated into the shark’s tissues. This is an important distinction and is a nod in the direction of the discipline of ecological energetics ** and recognises that just because something is taken into the mouth of an organism – i.e. it’s ‘eaten’ – doesn’t necessarily mean that it is ultimately digested, absorbed and contributes to growth of the consumer. Leigh et al’s study therefore not only adds information that has relevance to a better understanding of the operation of the ocean’s ecosystems, it also serves to underline how complex feeding webs can be – especially in assessing our understanding of the completeness, or otherwise, of the full range of feeding relationships that exist.

What the study doesn’t actually allow us to decide is whether this marine meadow-munching behaviour is a regular part of the shark’s dietary regime [in which case it may be vegetarian at best, or ‘mixivorous’ at ‘worst’] or whether it represents an attempt at self-medication for an unknown malady that seagrass is a shark-known treatment for. In any event, intake of vegetable matter happens in some sharks.*** Top tip for those considering going into the sea: Don’t wear your seagrass print bathing costume, no matter how sartorially-elegant you think it may be.


* Irritatingly – and another example of zoochauvinism/plant blindness? – Leigh et al. don’t specify the species (or species, plural) of ‘seagrass’ involved in their article (nor apparently in the Supplementary Material here and here). Given that there are 6172 (approx.) species of seagrass to choose from, we’d like to know what the sharks are eating.

** Although we won’t here go into the complexities of Consumption, Assimilation, and Production Efficiencies, these are important considerations for any studies related to feeding behaviour of animals.

*** The most notable ‘vegetarian shark’ is the whale shark (Rhynchodon typus), which lives on – off? – plankton. But, even so, plankton is a veritable smörgåsbord of both plant-like [phytoplankton] and animal [zooplankton] species. So, even this notable vegetarian isn’t exclusively a ‘plant’-eater…

**** This is the second in a 5-item series of ‘plants as food’ articles. The previous article is here. The next item can be found here.

Further reading

Leigh, S. C., Papastamatiou, Y. P., & German, D. P. (2018). Seagrass digestion by a notorious “carnivore.” Proceedings of the Royal Society B: Biological Sciences, 285(1886), 20181583. https://doi.org/10.1098/rspb.2018.1583

VanHook, A. M., & Patel, N. H. (2008). Crustaceans. Current Biology, 18(13), R547–R550. https://doi.org/10.1016/j.cub.2008.05.021

Bethea, D. M., Hale, L., Carlson, J. K., Cortés, E., Manire, C. A., & 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(5), 1009–1020. https://doi.org/10.1007/s00227-007-0728-7

Shurkin, J. (2014). News Feature: Animals that self-medicate: Proceedings of the National Academy of Sciences, 111(49), 17339–17341. https://doi.org/10.1073/pnas.1419966111