Download Marine Iguana

Marine Iguanas Older Than Their Islands
Jeff Mitton
Natural Selections (Appeared in the Boulder Camera, December 11, 2009)
The Galapagos Islands ride on the Nazca plate, a tectonic plate drifting toward
Ecuador at the rate of one and a half inches per year. But the Galapagos Islands will
never reach South America, for waves erode the islands until they slip below the surface.
The oldest island, Española, is four million years old and is on the southeastern edge of
the archipelago. As Nazca slides to the south and east it passes over a sedentary hotspot
that spawns volcanoes and builds new islands. Fernandina, on the western edge of the
archipelago, is clearly the youngest island because lava flows earlier this year
demonstrated that it is still growing.
Six hundred miles of open ocean prevent most terrestrial species from reaching
these tiny specks of land. However, the rare colonists, liberated from competing species,
are free to occupy novel environments and to consume novel foods. This ecological
novelty frequently promotes evolution of endemic species, or species found nowhere
else.
Long ago, iguanas colonized the Galapagos and began to adapt to the local
environment. Today, the original colonist is gone, but four endemic species remain, three
land iguanas and the only marine iguana in the world.
The marine iguana, Amblyrhynchus cristatus, inhabits the black lava shores that
fringe each of the Galapagos Islands. At first glance, their habitat is barren and
inhospitable, for the lava shores have no fresh water and are virtually devoid of plants.
The iguanas are forced to shuttle between temperature extremes; the mid day tropical sun
heats black and jagged rock to lethal temperatures above 120 degrees Fahrenheit, while
the ocean is typically in the low 60s, too cold for sustained activity.
From the perspective of an obligate herbivore, this barren landscape is
transformed when the receding tide reveals an intertidal zone carpeted by a variety of
green and red algae. Marine iguanas rely solely on marine algae but they are not
restricted to grazing in the intertidal. They are capable swimmers and can stay underwater
for over 30 minutes and venture more than 40 feet below the surface.
Marine iguanas have evolved numerous adaptations that suit them to their harsh
environments. Their black color absorbs solar heat quickly to warm them after a plunge
in chilly water. Their heads and snouts are foreshortened to allow them to graze on very
short algae. They have developed powerful, flattened tails for swimming. Excess salt
imbibed while eating under water is extracted by salt glands, dribbled into nasal passages
and expelled with explosive sneezes. Large hooked claws and strong legs allow the
iguanas to hold onto rocks and graze when waves are high and surges are strong.
Genetic data indicate that the marine iguana diverged from the land iguanas about
10 million years ago. But how is that possible, when Española, the oldest island, emerged
from the sea only 4 million years ago?
The same genetic data revealed a pattern of variation in marine iguanas that can
only be explained by slow but ongoing migration from old, shrinking islands in the east
to young, growing islands in the west. The marine iguanas are riding on islands drifting
to the east as the waves wear them down. Before older islands subside, the iguanas must
migrate to younger, higher islands to the west.
An old marine iguana is twelve years old, but the lineage of marine iguanas is
older than the islands that they live on.
Land iguana, Galapagos Islands
Marine iguana, Galapagos Islands