Don't Wake the Sharks

Don't Wake the Sharks

The underwater caverns off Isla Mujeres attract a dangerous species of shark—though nobody knows exactly why. Stranger still, once the sharks swim into the caves they enter a state of relaxed nonaggression seen nowhere else. Naturalists have two explanations, both involving the composition of the water inside the caves—it contains more oxygen, more carbon dioxide, and less salt. According to the first theory, the decreased salinity causes the parasites that plague sharks to loosen their grip, allowing the remora fish (the sharks' personal vacuum cleaner) to eat the parasites more easily. Perhaps the sharks relax in order to facilitate the cleaning, or maybe their deep state of relaxation is a side effect of having been scrubbed clean.

Another theory is that the caves' combination of fresh- and saltwater may produce euphoria, similar to the effect scuba divers experience on extremely deep dives. Whatever the sharks experience while "sleeping" in the caves, they pay a heavy price for it: a swimming shark breathes automatically and without effort (water is forced through the gills as the shark swims), but a stationary shark must laboriously pump water to continue breathing. If you dive in the Cave of the Sleeping Sharks, be cautious: many are reef sharks, the species responsible for the largest number of attacks on humans. Dive with a reliable guide and be on your best diving behavior.