Manta Rays of Mexico's Pacific
Copyright © 2002 Terry Maas, BlueWater Freedivers 

(Click on a photo for an enlargement)

What the giant manta rays do with humans is unique in this world. A totally wild animal, twice the mass of a horse, seeks out and revels in human physical contact.

"Here she comes," yells Sherry Shaffer as the big manta glides up from 60 feet below. The graceful, black winged giant looms ever larger to within a foot of Sherry’s out-stretched arms. Like Ariel and Pegasus, the two resume their underwater ballet—a dance they have rehearsed for hours.

This time, like a fighter plane, the Pacific manta ray descends with Sherry at a steep downward-angle while she holds her breath. Sherry’s hands tighten on the tails of the twin remora sucker fish—her finned reins affixed with superglue on the back of their accommodating host.

Suddenly, the descent angle increases and the jet-black ray transforms into a bright-white beacon as it flips over in an aerobatic outside loop 40-feet below. Sherry disappears under the bulk of the inverted ray then appears over-the-falls as her "gentle giant" ascends to re-deposit her safely at the surface.

Graceful Sherry plays with a large manta ray

Sherry has been swimming with the manta rays of San Benedicto for years. She recognizes several individuals by their unique belly markings. She and others have given them names—Ladyface,  Stringfellow, Loopy and Wizard. There is anecdotal evidence that these mantas recognize people as well. "More than once, Ladyface has picked me out of a group of divers and swam directly toward my waving hand," says Sherry. "I know they are intelligent, I can see it in their large inquisitive eye as it tracks me when I swim by."

Not all of the human interactions with manta rays are so joyful. A close friend recently filmed a commercial fishing boat not far from Sherry’s encounters. The video, punctuated with the racking sobs of her friend, depicts the butchering of a manta ray with rivulets of blood contrasting painfully against its black, hacked off wing as the fishermen hoisted it aboard.

Early superstitious fishermen called them Devilfish. Manta rays often feed at the surface on small zooplankton and krill funneling them into their mouths with specialized head fins—cephalic fins—unfurled below into twin scoops. Sometimes they accidentally swim into anchor ropes or into hard-hat diver’s air hoses. Since rays have no reverse gear, when caught they swim forward ever faster. Equally frightened fishermen or divers likened the fleeing rays to malignant beasts, with black wings sent from Hell to terrorize them. Fleeing mantas frequently lunge out of the water landing in a resounding slap—a maneuver that completes the Devilfish legend. The story goes that the rays would tow a skiff out to sea then crush the hapless crew by leaping on them.

Ladyface

As with most of the ocean’s animals, if we put our entire knowledge about them into a book, only blank pages would follow the preface. Much of what we know of the giant manta rays of Baja is a speculative amalgam of anecdotal behavior and physical science. For example, Sherry’s claim of intelligence is supported by the physical evidence that mantas have a larger brain than fish.

A closer look at the ray’s brain, however, reveals that the enlarged areas represent hearing, feeling and coordination. A more rational explanation for the enlargement is that these areas assist its host in finding and eating food.

Buddies, two eyes are better than one.

Mantas are not schooling fish. They are generally solitary or form loose aggregations of 3 to 6 animals. Sometimes they gather in larger groups of 30 to feed on a concentration of krill. Over and over, they summersault in tight loops, their white bellies flashing. Some suggest that the looping is a vortex generator—the hydrodynamic equivalent of a sheep dog herding millions of krill into a concentrated ball to be rammed into the gaping manta mouths on their next pass.

Krill are known to frighten into a compact immobile mass by a loud sound. Could this be why mantas leap and splash—to immobilize the krill? Behavioral scientists follow up on clues that nature provides. After a meal of krill, mantas defecate leaving huge bilious clouds in their wake, their color dependent on the ray’s food source. So dedicated is manta observer Captain Scott Sundby that he swims into the fecal clouds for clues about manta feeding habits. "I first note the color," he says "then I look for undigested skeletons of the manta’s last meal."

Starting in 1994 for and continuing for five years, Scott made over 130 dives-a-year with the mantas of San Benedicto. Besides studying their behavior, he records the belly patterns and chevron markings on the ray’s top to identify individuals. Scott identified one animal, named Stumpy for its missing cephalic fin. In 30 days Stumpy was sighted at each of the 3 islands of the archipelago, 30 to 60 miles apart.

Scott has observed mantas at 200 feet below the surface. Do they go deeper? Their anatomy suggests they can. Their large brain contains vascular retes—a series of parallel arteries and veins capable of mimicking a heat exchanger—the same set up found in other heat-conserving, deep sea diving animals. However, the retes might simply help keep the animal’s brain a steady temperature, transferring heat from the heat-absorbing wings while on the surface and from deeper within its body when submerged. Recently, Japanese researchers successfully attached pressure gauges to mantas, which recorded their depth excursions. During the day, they swam on the surface and down to 150 feet. During the night, however, they spent most of the time skimming the ocean floor at 600 feet while making only occasional trips to the surface.

With over 100 animals cataloged, the population around San Benedicto is similar in size to the other two known Pacific manta ray populations, Hawaii and Yap. Certainly, with a maximum of 400 animals in each sub-population, commercial fishing for manta rays will not solve the world’s food problems. Similar to sharks, manta rays have a slow molecular clock—their mutation rates are low. When the Gulf of Panama closed 3.5 million years ago, the Gulf of Mexico mantas were separated from their Eastern Pacific relatives. Recent DNA studies by Texas A & M graduate student Tim Clark reveals only minor genetic variations among these sub-populations. His studies suggest that all the mantas of the world are of the same species.

Clarion angle fish leave the safety of their rock home to clean parasites from passing mantas.

Manta rays (Manta birostris) are the gentle giants of the ray family Mobulidae. The largest recorded manta was taken off the coast of India. It measured 22 feet tip-to-tip and weighed in excess of two tons. We think these animals live an excess of 30 years based on 3 observations. A baby manta in Hawaii, observed growing over the last 10 years has not yet reached sexual maturity. A rule-of-thumb for the manta family is that these animals live 3 times their age when they reach maturity. Supporting this evidence is the re-sighting of the same adult mantas at San Benedicto over 10 years. Since mantas bear only a single live pup, they fit into the pattern of a long-lived animal.

Next to the independent eyes, the dual cephalic fins provide mantas with a rich sensory tool for locating food. For streamlined cruising, these fins corkscrew into neat thin forward-projecting appendages. Unfurled, they arc below the mouth forming a scoop for feeding and maneuvering. They funnel small crustaceans and krill into their mouths where 120 to 130 comb-like teeth filter out dinner. Their long tails lack a stinging spine at the base like other rays. Clarion idle fish love to pick at the end of the tail. The tails might aid in balanced swimming, however, we have seen several mantas with missing or short tails that appear to function normally.

The sandpaper texture of the ray’s skin offers opportunities for other species. Captain Sunby once observed a shark dive-bomb a manta. At the last second, the shark turned and scraped its skin against the manta’s—perhaps to rid itself of parasites. Constant traveling companions with the mantas of Baja are the remoras also called disk fish. Generally found in mated pairs, these opportunistic hitchhikers secure their ride with tiny barbs on the top of their flat heads, which attach to the manta’s skin like Velcro. Additionally, the hydrodynamic shape of the remora’s head creates a force that helps it stick to its host. While they do not wander far, remoras will leave their manta long enough to grab a quick bite of passing food—small pelagic crabs.

Mantas gather at cleaning stations—shallow prominences on the reef that shelter cleaner fish. As mantas approach, small wrass and other cleaner fish pick off parasites and clean debris from their surface wounds. At San Benedicto, brilliant red clarion idol fish risk the shelter of the reef and cross the open water gap to waiting mantas. Since the remora fish tend to concentrate the manta ray’s parasites on their body, we often observe the clarion idols picking at the bodies and tails of the remoras as seen in the accompanying photo. (See accompanying photo.)

Mantas form a single-file train over the reef during mating. Twenty males may chase a single female. When she makes the final selection, she allows the male(s) to bite onto her long fin tip and slide beneath her for abdomen-to-abdomen copulation.

Because of the huge increase in interest, divers have flocked to see the mantas. Sherry no longer touches these magic carpets. A tour boat with 25 eager manta riders is simply too much pressure for even these most tempting and loveable of beasts. Still, the mantas of San Benedicto love human interaction. We’ve seen them, day after day, seek us out for rides and other interactions. The underbelly of the manta is especially sensitive. We used to swim under a manta and gently place a palm of our hand just forward of the vent and rub. Enraptured, the mantas simply stop swimming and sink, pushing us down with their slowly sinking mass.

Some mantas are hunted outright; others wind up as by-catch in nets. In Mexico, their dead value is about 40-cents a pound compared to the $10 to $20 that shark fins command (another sad story.) The slaughter of manta rays is senseless especially in light of their relative economic value. Andrea Tomba of the Cortez Club in La Paz, Mexico considers mantas a primary attraction drawing divers from around the world. He estimates that mantas contribute to an average daily income of $3,300-per-day for the 10 local diving tour operators. Sherry, Scott and anyone else who has ever swam with these gentle giants knows that the value of a manta experience cannot be calculated in dollars. For many of us, swimming with the majestic manta ray accounts for an experience of a lifetime.