Basking in the glorious sunshine, the Marine iguana, Amblyrhynchus cristatus, lies on the volcanic shores of the majestic Galapagos Islands, warming itself on the black rocks. Unlike any other lizard, this cold-blooded enigma lives on land but feeds solely from the ocean, surviving on a herbivorous diet. Once venturing into the water, the Marine iguana must swiftly return to land, hurtling itself out of the ocean and onto the baking rock. Here, it once again warms its scaly skin before making the same pilgrimage for food the following day, risking the freezing temperatures of the water around it. Thought to have arrived here many millions of years ago, the land iguana drifted to each of the Galapagos Islands, evolving into the sea going marine iguana, forced to find their food in the sea. Found in congregations of thousands of individuals on the seashore, this eclectic looking individual is a unique organism, with striking appearance and an even more interesting physiology.

The Marine Iguana, with sharp hooked claws and a blunt snout. Source
The Marine Iguana, with sharp hooked claws and a blunt snout. Source

Adapt or die

The marine iguana, distinguished from its land based relatives by their short, blunt snouts and flattened tail, is not the most beautiful creature found on the sea shore. Throughout most the year, this marine lizard is black, perfect for absorbing heat from the sun as it lies on the beach, with its scaly skin designed to reflect the UV rays. Reaching up to 1.3 meters in length, the male A. cristatus is over double the size of its female counterpart, dominating the shoreline and the neighbouring seas. Not used to finding food beneath the ocean surface, the marine iguana has developed a sharp, curved claw with which is battles the turbulent ocean current to remain stable whilst searching for its next meal. Whilst foraging, the salt in the seawater collects in the air breathing vertebrate’s nostrils, becoming increasingly uncomfortable, forcing the A. cristatus to return to land. Once a safe distance from the water, the marine iguana uses its large nostrils to eject excess salt, firing it into the air, via a specialised gland connected to their nose.

The Marine Iguana ejected salt from glands in its nostrils. Source
The Marine Iguana ejecting salt from glands in its nostrils. Source

Vegetarians at heart

Feeding almost solely on a diet of marine algae, they are adapted physiologically to find their food in the ocean, using their blunt snout to get as close as they can to the substrata, ripping off the algae. Whilst females and juveniles tend to forage primarily in the shallow intertidal, male marine iguanas venture much further out to sea, sometimes diving for up to 30 minutes, using their flattened tail to swim efficiently. Having been known to occasionally consume the odd grasshopper or crustacean, these ocean-going lizards still have some resemblance of their omnivorous ancestors. However, on the remote Galapagos Islands, the most abundant food source definitely comes from the oceans and the marine iguana has learnt to exploit that resource, with very little competition for it.

Marine Iguana (Amblyrynchus cristatus) feeding on algae underwater, Galapagos islands
Marine Iguana (Amblyrynchus cristatus) feeding on algae underwater, Galapagos islands. Source

The battle to stay warm

Despite its sea-going habits, the marine iguana is physiologically very similar to the land iguana, bearing no such adaptations to enable to stay warm in the cold seas. In order for this ectoderm to control its body temperature, it relies on behavioural adaptations to exploit heat from its surrounding environment. This species of lizard has developed a mechanism for maintaining its body temperature using behavioural thermoregulation, which allows the marine iguana to stay alive during its daily dives. When the iguana becomes submerged in 22oC water, its heart rate rapidly slows down, reaching a minimum within 10 minutes of being underwater. By controlling its heart rate, A. cristatus lowers its body temperature sufficiently, to reduce the overall amount of heat lost and therefore prolong its diving time. The bigger the organism, the more heat it can hold onto, limiting the smallest individuals to very short dives in shallow water or even just picking on certain vegetation available on land. In order for the marine iguana to digest its food efficiently, its body temperature must be high enough, around 37o, which it will reach once basking once again in the sunshine on the shore.

The mating game

Breeding from December through to march, the male marine iguana morphs into bright red and green during the mating season, attracting the attention of the surrounding females. Sometimes known as the Christmas tree lizard, these vibrant colours allow individuals to stand out from the crowd, persuading a prospective partner with its enticing new appearance. When approaching a female, the male will bob its head continuously, signally his interest and gauging her response. If allowed to approach, the male will circle the female until he can get close enough to latch onto her back, using his blunt teeth to hold onto her scaly skin. A battle ensues, as the male iguana tries to force his tail underneath the female, revealing her cloaca and allowing copulation. Up to four weeks following successful mating, the female will part from her colony and roam miles and miles of sandy beaches, competing with others to find the perfect spot to bury her eggs. Laying up to six eggs at a time, the female may spend 6 hours digging a burrow, which she will continue to guard for 16 days, before returning to the sea shore with the rest of the marine iguanas. Upon hatching, the juvenile marine iguana, using their innate instincts, hurry to the sea, in search of its first meal and to join the safety of the colony.

The marine iguana is a unique organism, different from its ancestors and adapted to exploit its environment. Finding its food in the ocean, the sea-going lizard has learnt to control its body temperature using its heart rate, maximising its ability to dive for food and enjoy the riches of the sea. Found on almost every Galapagos Island, each colony is slightly different, each specialised to their surroundings and with increasing tourism and interest in the Islands, one can hope that these interesting lizards remain protected.

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