Marine Iguana

Published: 2021-10-01 12:45:07
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Category: Animals, Charles Darwin

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The marine iguana, scientifically known as Amblyrhynchus cristatus, is the only lizard in the world that takes to the sea.  Markedly different in appearance, size, and physical attributes, than its land-living cousin.  As one of the many varieties of iguana that Darwin observed in the Galapagos Islands, the marine iguana demonstrates an evolutionary response to a particular set of environmental challenges and has offered marine and land biologists alike the opportunity to study one of the true cross-over species.
The discovery of this iguana variant was part, clearly, of the evidence that led so many scientists to immediately see the logic and accuracy of what Darwin had “discovered”.   Understanding the marine iguana’s habitat, diet, behavioral patterns, survival techniques, and outlook is the purpose of this examination. It is intended to gain a broad understanding of how the marine iguana lives, breeds, and survives.
First discovered by Charles Darwin, aboard the Beagle, while touring the Galapagos Islands,  the marine iguana, or Amblyrhynchus cristatus became one of the most remarkable finds of that journey. The marine iguana is distinguished from other iguanas by “their short, blunt snouts and slightly laterally compressed tail that efficiently moves this lizard along the surface  or beneath the water,” (Amblyrhynchus cristatus, Marine Iguana - Retrieved Monday, April 9, 2007, from

The marine iguana is of a similar length to the full-grown land iguanas (reaching two to three feet from nose to tail) and can weigh up to eight pounds.  The color of the marine iguana is quite closely matched to the black volcanic rocks in which it lives.  While there are no natural predators of the marine iguana on land, thus making the evolutionary argument for the coloring of the creature to be necessary for camouflage, there are very specific benefits – primarily being that of heat absorption.  In the water, however, predators abound.
The marine iguana, living both on land and in the ocean, and being a cold-blooded creature, requires an adjustment time when going from hot to cold and heat-absorbing coloring, such as the dark black dominant color on the marine iguana helps to reduce that shift in temperatures and helps to return the iguana to normal speed of functioning faster.  The marine iguana, like all reptiles, does not have the ability to thermoregulate which also accounts for the dark coloring as a necessary boost in the absorption of heat from the sun.
This ability is absolutely critical to the marine iguana’s pursuit of food.  All reptiles will physically slow down to a near halt the colder they get.  Therefore, in order to prevent an absolute cessation of movement during a dive, the marine iguana must raise its body temperature to combat the nearly 10degree-Celsius loss that comes from an average dive. In fact, heat is a very significant part of the marine iguana’s life. They must warm themselves to dive, but they also must maintain a consistent temperature or risk getting too hot. (Rothman, p1). The marine iguana’s adaptations also include a nasal gland that excretes the excess salt taken in while in the ocean.  All of these factors, and more, contribute to making the marine iguana a most fascinating creature.
The habitat of the marine iguana is not just limited to the Galapagos Islands but that is the only place it is naturally found (there are many marine iguana exhibits in zoos.  The Galapagos Islands are generally characterized by a variety of  both high and low and scrub or cling-vegetation.  Each island is unique in its combination of vegetation and animal life.  The marine iguana does not appear on all of the Galapagos islands, but, as has been previously stated, it is only found there which shows that it, as a unique creature, came to be what it is as a result of living in its particular habitat.  Interestingly, there are also variations in average size depending upon which island you find the marine iguana upon.
Those found on Isabela and Ferdinandina are the largest and the smallest are found on the island of Genovesa.   How the marine iguana found its way to the Galapagos islands (and the marine iguana is the only iguana species on the islands) is unknown.  But, the prevailing theories center on the idea that the iguanas crossed on a land-bridge that sank long ago, or that they were transported from the mainland of Argentina or elsewhere in South America.
Regardless, because of their distance from their origins, their unique environment that other iguanas had not been exposed to, and the relative lack of traditional iguana food (which is, actually, just about anything) but on these islands, the iguanas were either too slow to catch prey, or the natural vegetation was simply not nutritious enough for their needs. So, faced with this, the iguanas adapted to their environment and found that algae, one of the worlds most nutrient-rich foods, was a better and more consistent source of food than any other.
There are thousands of insect varieties that other lizard species feed upon on the islands, but for the marine iguana, it is the algae growing on the rocks under the ocean surface that provides their food. Getting the algae does not require a great deal of hunting or foraging.  As algae is exceptionally abundant.  Because of this, the marine iguana has an average dive depth of up to 15 meters (with most only needing to be in the 1.5-5 meter range) and can remain under water for three to five minutes (with a notable few observed dives of up to 30 min), (
The day of the marine iguana is spent doing predictable reptile behavior: sunning to absorb heat in order to have a more successful dive for food, diving for food, and, reproducing.  The reproduction cycle for the marine iguana begins in December and goes through March.  The nesting season follows immediately after breeding, which takes place in the January to April time frame.    Breeding begins when females hit three to five-years of age and when the males are within the 6 to 8 year range.
As is the case with only a few reptiles, the male marine iguanas have nothing to do with the guarding of the eggs.  But, the females will do so for up to a week (Rubenstein & Wikelski, “Seasonal Changes in Food Quality: A proximate cue for reproductive timing in marine iguanas”, 3013).  After that period, they leave the eggs on their own to incubate under two to four feet of sand.   Hatchlings weigh an average of 55 grams and are geared for survival from the moment they emerge from the shell.
The challenges to the survival of the marine iguana are many, but they are no more so than those facing every other species of plant and animal on the islands or, indeed, anywhere else in the world.  Pollution, climate change, environmental shifts (such as El Nino), and human encroachment all play a part in determining the long-term survival of the marine iguana.  Fortunately, though, for this species at least there is relatively little industrialization or human colonization of the Galapagos.
This results in much lower amounts of land and ecosystem loss.  But, as is the case for any creature that can’t make its own food, the marine iguana’s challenge is to eat enough to survive each day.  As long as ocean pollution does not find a way to kill off the algae, the marine iguana will continue to thrive in the Galapagos.
The marine iguana is, indeed, a unique and special animal.  Referred to derogatorily as being “ugly”, the marine iguana is uniquely suited (adapted) to its environment through a series of evolutionary shifts that took it away from the land iguana to one that can  swim under water, expel salt through a nasal gland, and gets nearly all of its food underwater.  These creatures eat primarily algae found on the rocks and reefs below the ocean surface and, in order to make these dives to get that food, must raise their core temperature to as to have quick reflexes even as their body temperature is going down.
Found on many of the Galapagos islands, the marine iguana even shows variations in body size from island to island.  Breeding takes place in the summer months (of the Southern Hemisphere) and nesting follows shortly.  The marine iguana provides a look into the deep biological past (one can see the dinosaurs in the background) for its appearance which is designed to assist with the absorption of heat.  While the marine iguana is not currently under environmental threat, but can be greatly affected by a host of events both locally and globally.
Amblyrhynchus cristatus, Marine Iguana - Online. Internet. Avail. Info acc 8 March, 2007.
Rothman, Robert. “Marine Iguana”. Online. Internet. Avail: Access: 8 March, 2007.
Rubenstein, Dustin R. and Wikelksi, Martin. Seasonal changes in food quality: a proximate cue for reproductive timing in marine iguanas. Ecology 84.11 (Nov 2003): p3013.

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