Chapter 7: Marine Reptiles

The Galapagos Marine Iguana, image from Arkive.org.

Reptiles in the Ocean

In many ways, the ocean is an odd place for a reptile to live.  Reptiles (like mammals) all have backbones and all breath air.  Reptiles (like birds) lay eggs on land. Also, almost all reptiles are ectotherms (cold blooded.) Despite these disadvantages, several species of reptile thrive in the ocean and play a valuable role in its food webs.  This weeks text will teach you about each of these species and their struggle to survive the challenges of the ocean and the challenges of a planet dominated by human beings.

Marine Iguanas

At one time, swimming reptiles dominated the sea much as dinosaurs did the land. Today there are only a few dozen species of reptiles that still thrive in the sea.  Certainly, one of the more interesting is the Galapagos Marine Iguana. The marine iguanas of the Galapagos are the only species of lizard that will spend much of its time in the ocean.  The males, which are larger than the females, may grow to a length of 4 feet or more (almost half of which is tail).

They appear ghoulish to some.  Charles Darwin said of the marine iguanas, “The black lava rocks on the beach are frequented by large, disgusting clumsy lizards. They are as black as the porous rocks over which they crawl & seek their prey from the sea. I call them ‘imps of darkness’.”

As for feeding, they are completely herbivorous and feed exclusively on algae growing on rocks near the shore. When they feed, they can remain submerged for up to an hour, although shorter dives are much more common. The water in the Galapagos is often quite cold, and being cold-blooded, iguanas spend much of their time restoring body heat by sunning themselves on the dark colored lava rocks along the shore.

Baby leatherback sea turtles, image from Arkive.org

Sea Turtles

Sea turtles differ from their land counterparts by having flattened shells and flippers in the place of legs. Like most reptiles, they are cold-blooded; their body changes temperature with the surrounding environment.  Despite having been in the ocean for over fifty million years, sea turtles are only partially adapted to life in the ocean.  They have lungs and regularly need to come to the surface to breath air. Their shells are heavy and they are slow swimmers.

There are eight species of sea turtle. Leatherback sea turtles (Dermochelys coriacea) are the largest and most distinctive. Leatherback sea turtles have a leathery shell, and are the only turtle that does not have its spine fused to its shell.

The others are the green turtle (Chelonia mydas),  the Eastern Pacific green turtle (Chelonia agassizii), the loggerhead turtle (Caretta caretta), the Kemp’s ridley turtle (Lepidochelys kempii), the olive ridley turtle (Lepidochelys olivacea), the hawksbill turtle (seen above, Eretmochelys imbricata), and the flatback turtle (Natator depressus).  Click on any of the links for video of these turtles.

A green sea turtle, image from Arkive.org.

Once sea turtles reach reproductive maturity (which may take 20 years), females will return to land every 2 or 3 years to lay eggs in the sand.  Reproductively, mature males will return every year to attempt to mate. This means that over a lifetime a sea turtle may cover 100,000 miles.  We are still not not exactly sure how they navigate, but we do know that, much like birds, they are able to orient themselves to the Earth’s magnetic field lines.  In other words, they can feel north, south, east, and west. Using this ability along with visual and current clues, each female sea turtle will return to the very same beach where she was born to lay eggs.  There, she will dig a hole in the sand, above the high tide line, and deposit her eggs.  When she is done, she will head back out to sea never to see her offspring again.  Males will also return to these beaches, seeking opportunities to mate with females that have finished laying eggs.

Depending on the species, sea turtles may be carnivorous (eating jellyfish, tunicates, sea sponges, or arthropods like shrimp), herbivorous (eating algae or sea grass), or omnivorous. The jaw structure of many species is adapted specifically for their diet.  Click here to read about the diets of individual sea turtles.

Sea Turtle Conservation

Every species of sea turtle is considered endangered.  Turtles are long-lived and reach reproductive maturity late in life.  Sea turtles are slow swimmers and vulnerable to accidental catch by long-lines and fishing nets.  However, what threatens sea turtles more than anything else is their reproductive habits.  Many beaches have seen significant real estate developments, and only recently has the harvesting of sea turtle eggs for food become illegal in most parts of the world.  .

A beaked sea snake, image from Arkive.org.

Sea Snakes

There is not very much written about sea snakes.  What is known is that there are a 62 species that inhabit the Indian and Pacific Oceans.  They are highly adapted to life in the ocean.  Their bodies are compressed laterally, which makes them much better swimmers but makes it impossible for many of them to move on land.   Unlike eels (which are fish) they lack gills and must periodically come to the surface to breathe air.   Most sea snakes feed on fish that inhabit warm coral reefs.  Most sea snakes are highly venomous (including the yellow-bellied sea snake to the right), but rarely aggressive towards humans. They use their venom to help subdue their prey. Unlike sea turtles, sea snakes do not lay eggs on land, but rather give birth to live young in the water. Click to read more about sea snakes.

Salt Water Crocodiles

The largest crocodile on Earth, and the only one considered truly marine, is the saltwater or estuarine crocodile (commonly called Salties). Salties can reach up to 7 meters long and weighing 2,200 pounds (1,000 kilograms.) Salties inhabit the estuaries and bays of Eastern india, Southeast Asia, and Northern Australia but have been found far out to sea.  They are excellent swimmers and voracious predators.

A young saltwater crocodile, image from Arkive.org.

Questions to Research:

  1. The marine iguana of the Galapagos is pretty unique.  Go learn about it and tell me why it’s so unique among all of the lizards of the earth.
  2. Artificial lighting along sea turtle nesting beaches is a huge problem for baby sea turtles.  What kinds of things can local residents do to help nesting sea turtles?
  3. Go track a sea turtle.  Pick any of the active turtles.  Give me a name, and tell me where they have been traveling.
  4. Describe what have researchers learned about how sea turtles navigation.
  5. Watch some newly hatched baby turtles leave their nest.  How do they know where to go, and what have researchers learned about where they go for their first few years.
  6. Not all sea turtles are truly cold blooded.  Describe what it means when we call leatherback sea turtles Gigantotherms.
  7. Take a dive with a leatherback sea turtle.  What can be learned by attaching cameras to sea turtles? What do they hope to do with this knowledge?
  8. Take the turtle quiz from the Sea Turtle Conservancy.  Write one true statement for each answer.
  9. Sea snakes are not eels and eels are not sea snakes: sea snakes are reptiles while eels are fish.  Even though they may look alike, they are very different.  Explain three differences you find between the following examples: yellow-bellied sea snake and green moray eel.
  10. Adapting to Life at Sea:  Take a look at this article from Natural History Magazine.  Explain the connection between sea snakes diet, the way in which they eliminate nitrogenous waste, and their need for freshwater. Then explain why this makes life (in at least one way) difficult for them in the ocean.