Australian marine scientists have discovered that the massive whale shark also eats plants, making it officially the largest omnivore on Earth. 

As the evolution of a very large body size requires a ubiquitous and abundant source of food, the consumption of plants could present an energetic challenge for these animals unless some components can be digested.

While previous studies have found seaweed in whale shark stomachs, a new study by a research team, led by University of Tokyo biologist Alex Wyatt, suggests they might in fact ingest such algae as a dietary staple.

Careful investigation of blood and tissue samples from over a dozen whale sharks suggests that they actually have a pretty omnivorous diet that includes plants and algae. 

The team found that whale shark tissues were rich in specific fatty acids and other compounds found in relatively high proportions in Sargassum—a type of brown seaweed that breaks off of the reef and floats at the water’s surface at Ningaloo. 

The findings were somewhat surprising as their tissue does not have a fatty acid or stable isotope signature of a krill-feeding animal. Though samples of collected whale poo showed that whale sharks were indeed eating krill, their tissues indicated that they did not metabolise much of it.

Instead, the fatty acid profiles in their tissues, faeces and potential prey items suggest that the floating macroalgae, Sargassum, and its associated epibionts are a significant source of food. An epibiont is an organism that lives on the surface of another living organism. An epibiont is, by definition, harmless to its host.