Whale Sharks – Now Officially The World’s Biggest Omnivore

By D.C. Demetre â€¢  Updated: 08/08/22 â€¢  6 min read

The whale shark (Rhincodon typus) is the world’s largest omnivore after marine scientists’ discovery that whale sharks eat plants. These massive animals are slow-moving, filter-feeding sharks and the largest known existing fish species. They live almost entirely off of plankton and small fishes.

But researchers analyzing biopsy samples from whale sharks at Western Australia’s Ningaloo Reef reef have found the animals were actually eating quite a lot of plant material.

“This causes us to rethink everything we thought we knew about what whale sharks eat. And, in fact, what they’re doing out in the open ocean,”

said Australian Institute of Marine Science biologist Dr. Mark Meekan.

Off The Food Chain

An omnivore is an animal that can eat and survive on both plant and animal matter. Other animals that are classified as omnivores include:

But the whale shark is by far the largest.

“On land, all the biggest animals have always been herbivores. In the sea, we always thought the animals that have gotten really big, like whales and whale sharks, were feeding one step up the food chain on shrimp-like animals and small fishes. Turns out that maybe the system of evolution on land and in the water isn’t that different after all,”

Dr. Meekan said.

How Big Is A Whale Shark?

A whale shark in captivity. Credit: Zac Wolf CC-BY

 

With whale sharks, males are believed to not grow as big as females. Evidence indicates that males on average reach a length of 26 to 30 ft, and females reach an average length of around 48 ft. There have been several reports of whale sharks around 59 ft in length.

It is hard to measure large whale sharks accurately, either on land or in the water. So most of these length figures are estimates and predictions from growth rates.

One research study focused on satellite tracking of whale sharks between 1994 and 1996. Out of 15 individual whale sharks tracked, two females were reported as measuring 15 m (49 ft) and 18 m (59 ft) respectively.

What Does A Whale Shark Eat?

Whale sharks eat by straining floating matter and food particles from water, typically passing the water over unique sieve-like structures, called filter pads, in their mouths.

They feed on copepods, krill, fish eggs, Christmas Island red crab larvae and small squid or fish. During mass spawning of fish and corals, they will feed on clouds of eggs. And now we can add seaweed to that menu.

Whale sharks need to eat a lot. A young whale shark, for example, is estimated to eat about 46 pounds of plankton daily.

Sargassum Snacks

To find out precisely what whale sharks were eating, the Australian researchers collected samples of possible food sources at the reef, from tiny plankton to large seaweed. They then compared the amino acids and fatty acids in the plankton and plant material to those in the whale sharks.

The whale shark tissue, Dr. Meekan said, contained compounds found in Sargassum, a type of brown seaweed common at Ningaloo, which breaks off the reef and floats at the surface.

“We think that over evolutionary time, whale sharks have evolved the ability to digest some of this Sargassum that’s going into their guts. So, the vision we have of whale sharks coming to Ningaloo just to feast on these little krill is only half the story. They’re actually out there eating a fair amount of algae too,”

he said.

“Something like a whale shark, which swims through the water with its mouth open, is going to ingest a lot of different things, But you don’t know how much of that has been used by the animal and how much just goes straight out the other end. Whereas stable isotopes, because they’re actually incorporated into the body, are a much better reflection of what the animals are actually utilizing to grow,”

said organic biogeochemist Dr. Andy Revill, who analyzed the whale shark tissue using compound-specific stable isotope analysis.

Why Is The Whale Shark Endangered?

Whale sharks are listed as endangered by the International Union for Conservation of Nature. The main reasons are the impact of fisheries, losses to being caught unintentionally in nets targeting species and being struck by ocean-going vessels. Their long lifespan and late maturation make them more vulnerable to these threats than shorter-lived species.

As if that wasn’t enough, hundreds of whale sharks are said to be illegally killed yearly in China for their fins, skins, and oil. In 2003, the species was added to the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) to regulate the international trade of live specimens and their parts.

A 2021 study reported whale sharks’ ability to re-grow a partially amputated first dorsal fin. As well, their unique spot markings were also seen forming over previously injured spots, which suggests that these markings are an essential feature for this species and persist even after being damaged.

Such healing capabilities indicate that whale sharks could be resilient to impacts from humans. Still, there may be many other less recognizable impacts of injuries to these animals, such as reduced fitness, foraging capacity and altered behaviours, so injuries need to be prevented where possible.

Watchful Eyes Needed

Variation within healing rates was observed, with lacerations, typical of propeller injuries, taking longer to heal than other kinds of wounds, highlighting the need for further research to determine the influence of environmental and more nuanced individual factors on injury healing.

Meticulous management of whale shark gathering sites, which occur seasonally in several coastal regions worldwide, is essential to ensure the sharks are protected while spending time in areas of high human activity.

If sharks are encountered with injuries in these locations, research such as this can help local teams estimate how old the damage is and assess where and how it might have been inflicted based on knowledge of whale shark movements and the tendency to return to the exact locations.

Sources:

  1. Colman, J. G. (1997). A review of the biology and ecology of the whale shark. Journal of Fish Biology. 51 (6): 1219–1234.
  2. Eckert, Scott A.; Stewart, Brent S. (1 February 2001). Telemetry and Satellite Tracking of Whale Sharks, Rhincodon Typus, in the Sea of Cortez, Mexico, and the North Pacific Ocean. Environmental Biology of Fishes. 60 (1): 299–308.
  3. Freya Womersley, James Hancock, Cameron T Perry, David Rowat. Wound-healing capabilities of whale sharks (Rhincodon typus) and implications for conservation management. Conservation Physiology, 2021; 9 (1) DOI: 10.1093/conphys/coaa120
  4. Hilton/Greenpeace, Paul (5 February 2014). Hundreds of sharks killed in China. ABC News
  5. M. G. Meekan et al, The world’s largest omnivore is a fish, Ecology (2022). DOI: 10.1002/ecy.3818
  6. Philip J. Motta, et al. Feeding anatomy, filter-feeding rate, and diet of whale sharks Rhincodon typus during surface ram filter feeding off the Yucatan Peninsula, Mexico. Zoology, Volume 113, Issue 4, 2010, Pages 199-212, ISSN 0944-2006,