About the Whale Sharks You’ll Encounter around Isla Mujeres

The Whale Shark — Rhincodon typus

The whale shark, Rhincodon typus, is the largest fish in the sea and the largest shark on Earth — though, despite its name, it isn’t a whale at all. It’s a shark: a gentle, slow-moving filter feeder that drifts through the warm tropical oceans of the world, usually alone.

Its back is a deep blue-grey, scattered with a constellation of pale spots and stripes that run the length of its body — and like a fingerprint, no two patterns are exactly the same. Researchers use those markings to identify individual sharks year after year. Its underside is white.

With a mouth that can stretch nearly five feet wide, lined with some 3,000 tiny teeth, a whale shark can look intimidating at first. It’s anything but. These are remarkably docile animals that pose no real threat to people — divers and snorkelers have even described them circling back, seemingly curious, as if to take a closer look at us.

Why they gather here

Whale sharks are usually loners, only rarely seen together. That’s what makes what happens off Isla Mujeres each summer so extraordinary. For a few months a year, hundreds of them gather in the deep blue water offshore — the largest known gathering of whale sharks anywhere on the planet. Local fishermen named the spot the Afuera, “the outside,” for how far out it lies.

They come for a feast. While whale sharks in other parts of the world strain plankton and krill from the water, the ones who gather here are after something far richer: fish eggs. Every summer, around the full moons of July and August, a small tuna called the little tunny — known locally as bonito — spawns offshore in staggering numbers. Their tiny, buoyant eggs rise toward the surface, where the currents gather them into dense, almost milky slicks of caviar. The whale sharks cruise straight through, mouths agape, slurping up the bounty. A single shark can pass more than 1,500 gallons of water through its gills in an hour.

It’s such a rich meal that the sharks aren’t the only ones who show up — manta rays, sailfish, and even marlin come to share in the spawning. And the sharks themselves travel remarkable distances to be here: tagging studies have followed individuals from these waters all the way to West Africa, Brazil, and the U.S. coast.

How they eat

A whale shark feeds by opening its enormous mouth and drawing in a great gulp of water, then closing it and pushing the water back out through its gills. A spongy mesh lining the gills traps everything but the water — the eggs, the plankton, the tiny creatures — and the shark swallows what remains. Its teeth play no part in the process at all. Unlike many filter feeders that simply strain whatever drifts in as they swim, the whale shark feeds actively — which is why you’ll often see one hanging almost vertically in the water, bobbing up and down as it works a patch of eggs.

A long, slow life

Whale sharks are thought to live a very long time — somewhere between 60 and 150 years — reaching maturity only around the age of 30. They grow to match: the largest reliably recorded have measured around 40 to 46 feet and weighed up to 15 tons, though the average animal you’ll meet here is closer to 25 feet. For all that size, they are gentle, unhurried swimmers, moving at about three miles per hour by sweeping their whole body slowly from side to side.

To share the water with one — to be a guest in its world for even a few minutes — is something you never forget.

Want to dig deeper?

The gathering off Isla Mujeres isn’t just spectacular — it’s a genuine scientific landmark. In 2009, researchers documented up to 420 whale sharks feeding here in a single aerial survey, the largest aggregation ever recorded, and confirmed through DNA analysis that they had come for the eggs of spawning little tunny.

• Smithsonian Institution — “Scientists Discover the Largest Assembly of Whale Sharks Ever Recorded”
• The underlying study — de la Parra Venegas et al. (2011), PLoS ONE