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 An overhead view of a marine iguana shows its unusual body proportions, blunt snout, a paddle-like tail and large claws—all important adaptations for a marine life. The iguana's quick, continuous motion was interrupted upon his arrival at the beach. He immediately became clumsy and slow, dragging his plump body with relatively puny legs. Landing on a sun-warmed rock, he prostrated himself. While feeding in the ocean, the ectothermal marine iguana's temperature drops an average 18 to 22 degrees Fahrenheit, from its optimal body temperature of 95 to 99 degrees Fahrenheit (Thornton, 1971). Upon returning from the ocean, they must immediately warm themselves to restore their body temperature. The iguanas typically make one daily visit to the ocean for a meal. Being smaller, the females usually remain within 20 meters of the shoreline while grazing. Although the larger males swim out farther and dive deeper than the females, they are rarely seen beyond 400 meters from shore. This is probably due to the fact that their main food is intertidal algae, which is not found further from shore. Drinking water would not pose any problem, however. Due to the lack of fresh water on the Galápagos, the marine iguanas evolved a salt gland enabling them to consume salt water (Schmidt-Nieken and Fange, 1958, in Thornton, 1971). This specialized gland concentrates the excess salt into a salty secretion, which is ejected through the nostrils like a sneeze. It is doubtful that this swimming reptile could survive the cold and fatigue required to swim between islands. Marine iguanas typically limit their time in the ocean to less than one hour, and most of that time is spent stationary and feeding. To swim the 40 kilometers to Española from the nearest island, San Cristobal, would require at least 12 hours if the iguana were able to swim continuously. The time and effort required to complete the journey could become even greater because of the fact that any iguanas swimming toward Española would also be fighting the ocean current, which flows from southeast to northwest. Even though venustissimus could drift on floating vegetation from Española to other islands, using the current and the wind to help it, it would be virtually impossible for marine iguanas elsewhere to fight the current to swim to Española. This is probably the most important factor in ensuring that the marine iguanas on Española remain isolated from those on other islands despite their excellent swimming ability. Geologic history and the Nazca plate: Española has been dated by potassium-argon decay and was found to be about 3.3 million years old, making it the oldest island in the current Galápagos. The other islands string out to the northwest of Española, gradually growing younger. San Cristobal, the closest island to Española, is 40 kilometers to the north and is about 2.4 million years old. Fernandina Island is the youngest island, at only 700,000 years old, and has a tall, sharp volcano that is active, like several on nearby Isabela. |
 The Nazca plate moves eastward over a stationary "hot spot" in the Pacific Ocean. This produces the string of volcanic islands known as the Galápagos. (Click to enlarge) Recent improvements in the understanding of plate tectonics have revealed that the Galápagos Islands are the result of the Nazca plate moving eastward over a stationary "hot spot," producing a string of volcanic islands. As the plate moves southeastward at seven centimeters per year, the volcanoes become extinct, are carried toward South America, and eventually erode to the point of becoming submerged. To the east of Española, below the surface of the ocean, are remnants of former Galápagos islands, now seamounts estimated to be between 8 million and 10 million years old. Interestingly, scientists have recently discovered by genetic analysis that the marine iguana diverged from the land iguana about 8 million years ago (Kunstaetter, 2003). This means that the marine iguana evolved on one of the now submerged islands of the Galápagos before any of the current islands even existed, and it was probably on Española before it was on any other island in the present archipelago. Using the information on plate tectonics and the genetic age of the marine iguana species, it can be inferred that the marine iguana has repeatedly emigrated from eroding older islands to younger ones. Conclusion: Based on my study and observations, I have concluded that there are two main factors causing the divergence of the Española subspecies. First of all, the unusual cliffs on the southern shore of the island influenced some different behavioral patterns and changes in appearance to occur in its population of marine iguanas. Secondly, two complementary conditions allowed venustissimus to remain isolated on Española: the ocean current flow to the northwest, and the Nazca plate's movement to the southeast. If either the currents or the tectonic plate moved in a different direction, marine iguanas on other islands may have migrated toward Española, mixing up its gene pool. The venustissimus subspecies of marine iguana that has emerged on Española is clearly the result of a combination of unusual factors, and it occupies a special place in Earth's complex mixture of species. The sun was beginning to set as my group began to put on life jackets and climb onto the sturdy white panga that had brought us. My expedition on Española Island was coming to a close, and I had learned much about the marine iguana. But this new knowledge had succeeded in raising many new questions to explore. Will the races of the marine iguana ever become separate species, or will the island "leapfrog" migrations cause too much genetic mixing? How much longer will Española last before it, too, becomes a seamount? And will the marine iguana ever drift out of the archipelago to the mainland and establish itself somewhere else as successfully as it has in the Galápagos? |
