Islas Revillagigedos (good luck with pronunciation, “ray-vi-ya-he-hay-dos”) are a group of four volcanic islands, in the Pacific Ocean, the nearest of which lies about 300 miles west of the west coast of Mexico and about 250 miles south and slightly west of Cabo San Lucas. For whatever reason, perhaps ease of pronunciation, divers refer to this group of islands simply as Socorro, notwithstanding that the diving around Socorro is not comparable to that around the others. These islands are only possible to visit by liveaboard boat, and the passage from Cabo generally lasts between 20 and 30 hours, often through rough seas. But the difficult logistics are well worth it, for divers seeking close encounters with plentiful and very large marine wildlife. The islands are sometimes referred to as Mexico’s “little Galapagos.”
The two islands closest to the mainland are Socorro, the largest of the four, and San Benedicto. The most popular place for divers is off San Benedicto Island, where a submerged rock, maybe a quarter mile in circumference, rises to within 16 feet of the surface. It is here that divers encounter what they come to see . . . Giant Manta Rays. Roca Partida “Broken Rock” is the smallest of the Revillagigedo Islands and lies about 100 miles further west of the first two. It is hardly an island, but rather a volcanic rock about 50 feet wide and about 800 feet long. It is covered with guano, which adds to the drama of the two “horns” that rise from each end. It lies in deep water and is truly unsheltered, and the combination of often rough water and usually swift current makes for advanced diving. Quite a bit further out into the Pacific lies the fourth island, Clarión, but its remoteness makes it an infrequent dive destination.
Giant Manta Rays. The Manta Genus is comprised of two species of rays: Reef Manta Rays and Oceanic Manta Rays. The Oceanic Mantas are the largest rays in the world. Wikipedia claims that the largest recorded such Manta was more than 25 feet across, with a weight of about 2,400 pounds. Interestingly, Manta Rays have the largest brain-to-body ratio of all members of the subclass of cartilaginous fish that includes sharks, rays, and skates. This ratio is so high that it approaches the ratio found in mammals.
Mantas, just as their cousins sharks, skates, and other rays, do not have bones. Instead, their body structure is composed entirely of cartilage. They are primarily filter feeders; and, to help funnel plankton and other small creatures into their mouths when they filter feed, they unfurl their large cephalic fins, on each side of their mouths. Although they are closely related to the stingray family, they do not possess a stinger. Thus, except for their large size, they are not dangerous to humans.
Their behavioral patterns also make them quite interesting. First, Humpbacks are well know to whale-watchers for their “breaching,” and they can leap almost totally out of the water. While they are not the only specie to do this, they seem to be the most enthusiastic. Scientists are not certain why they engage in this behavior. Many believe they do it to rid themselves of parasites. Others seem to believe they may do it out of sheer joie de vivre. Second, they appear to be the only specie that “sings.” These magical songs involve complex sequences of moans, howls, cries, and other noises and can go on for hours at a time and be heard as far away as 20 miles. These songs are not just random noises. Instead, the songs have a distinct beginning and ending, as well as a definable progression. As a result, their songs can be repeated, and scientists can record and recognize particular songs. Moreover, we have learned that whales tend to sing the same songs within a large area. For example, all North Atlantic Humpbacks will sing the same songs, while Humpbacks in the North Pacific will sing different songs; and scientists have discovered that the songs in the repertoire of the whales in an area will change slowly over time and not repeat. Interestingly, whales have no vocal cords and, instead, generate their songs by forcing air through their nasal cavities.
Third, whales are highly intelligent and social animals, and Humpbacks have developed an extraordinary and unique way of coordinated hunting, known as “bubble net feeding.” For this technique, a group of Humpbacks will locate a school of fish, swim as a group under the school, commence swimming in a circle below the fish, and begin exhaling bubbles as they swim. This creates a “bubble ring” that, as it rises, forms a curtain that surrounds the school. This circular “curtain” can start as large as 100 feet in diameter and grow increasingly smaller, as the fish are driven together. Scientists have observed with video cameras that this activity is so closely coordinated between the whales that some of them blow bubbles, others dive below and drive fish toward the surface, and still others herd into the “bubble net” by vocalizing. At the right moment, in a sudden and carefully coordinated action, the hunting whales will swim swiftly upward, in the center of the ring, with their mouths wide open. They continue until they reach the surface and have taken in thousands of fish.