They’re big, they’re slow, and they can live up to 130 years. Also, they’re polka-dotted!
Whale sharks may be confusingly named. They’re sharks (fish), not whales (mammals). But as the largest known species of fish—typically 9 meters long, with a record length at 18.8 meters (61 feet)—they have something of a whale-y vibe.
For it has to be said that they’re not typical sharks. Rhincodon typus, to use their scientific name, eat their ample fill by ram filter-feeding, vacuuming up gallons of water to strain out small prey before the water exits their gills. Typical foods include “copepods, arrow worms, fish eggs, crab larvae, coral spawn, and krill.” Most sharks, of course, slice and dice tuna and other fishes, and occasionally the rare human. Among the more than 500 species of shark, famous for their “pearly white” teeth, only two species besides the whale sharks are known to be filter-feeders.
It’s this kind of feeding that allows whale sharks to grow so big. It’s the same for other giant ocean vertebrates like the world’s biggest animal, the blue whale. “Ubiquitous and abundant” small prey animals like krill are a “key factor that allows these vertebrate filter-feeders to grow to immense sizes,” write M. G. Meekan, P. Virtue, L. Marcus, K. D. Clements, P. D. Nichols, and A. T. Revill.
On land, these authors write, herbivory “has been a key attribute of gigantism and critical to the evolution of the body size of all the largest animals.” But at sea, carnivorous “filter feeding offers access to a ubiquitous source of prey that is part of a very efficient (phytoplankton-nekton-filter feeder) food chain.” (Whale sharks do have thousands of tiny teeth, but these are evolutionary leftovers, no longer functional.)
A quick glance at various websites will tell you that whale sharks are carnivores, end of sentence. But using amino acid compound-specific stable isotope analysis (CSIA) and fatty acid analysis, Meekan et al. show that whale sharks have a “trophic level consistent with omnivory.” (“Trophic level” means one’s position in the food web.)
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Omnivory may perhaps be inevitable, given whale sharks’ essentially indiscriminate form of feeding. There’s a lot of stuff in the ocean, and marine debris tends to aggregate in the same places as does the whale sharks’ primary prey. The large-mouthed sharks end up hoovering in some of this natural debris.
So, whale sharks digest “at least some components of floating Sargassum” along with the animals associated with this macroalgae. Meekan et al. conclude, “Our results provide some of the first evidence that feeding on plant material may also be involve in the evolution of gigantism in marine environments.”
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Unfortunately, as they also note, indiscriminate filter-feeding means whale sharks are taking in the pollutants, particularly plastic, we humans pour into the ocean. Today, a lot of ocean debris is human-created: there are billions of pieces of plastic in the ocean, and every year more ends up there.
“Whale sharks can pass some plastics through the gut,” stress Meekan and co-authors, “but ingestion of this material in either micro or macro form via ram filer feeding may induce regurgitation or reduce gut capacity and slow digestion, potentially harming finely balance energetic budgets.”
These threats are on top of shark fin/meat consumers, bycatch hazards, human-made crises like 2010 Deepwater Horizon spill in the Gulf of Mexico, vessel strikes, and disruptive shark tourism (now a multi-million dollar industry). The “world’s largest omnivore” is listed as endangered on the IUCN Red List.
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