Marine Iguana
Credit: Wikipedia photo by David Adam Kess, CC BY-SA 3.0.

The marine iguanas of the Galapagos Islands are the only marine lizards on Earth.

Sea turtles, sea snakes, marine iguanas, saltwater crocodiles

Reptiles from three taxonomic orders – chelonians (turtles), squamates (lizards and snakes), and crocodilians (crocodiles and close relatives) – each have representatives adapted to living at least part-time in our planet’s oceans.[1] Out of about 12,000 species and subspecies of reptile, 100 are of the marine variety.[4]

Of the 100 species and subspecies of marine reptile, by far most of them are sea snakes.[2] The others are sea turtles, the marine iguana, and the saltwater crocodile.[1] Marine reptiles are less than one percent of all reptiles, and of all reptiles, sea snakes and sea turtles are the best adapted for living in a marine environment.[17]

From an evolutionary standpoint, marine reptiles are said to be those that have returned to the sea, since reptiles originated as land animals hundreds of millions of years ago, and their lineage can be traced back to the oceans, from where all life originated over three billion years ago.[3][17][3]

Sea turtles

Sea Turtle
Credit: Public domain, from NOAA.

Green sea turtle near the Hawaiian Islands.

There are seven sea turtle species, and they are found in all oceans except the polar regions.  Six of the species are in one taxonomic family, and one (the leatherback sea turtle) differs from the others enough to be considered a separate family.[5]

All sea turtle species are carnivorous when young, and all are omnivorous as adults except for green sea turtles, which transition to being fully herbivorous.  Each has a unique diet, and two species (hawksbill sea turtles and leatherback sea turtles) eat extremely venomous jellyfishes, to which they are immune.[5]

Sadly, two species are critically endangered, and another has large subpopulations that are critically endangered.  These are the hawksbill sea turtle, Kemp’s ridley sea turtle, and leatherback sea turtle.[5]

Loggerhead sea turtles are the largest hard-shell turtle species, surpassing the largest land tortoises.[6]  Leatherback sea turtles are the largest turtle species overall, and the third largest reptile on Earth (after saltwater crocodiles, discussed below, and Nile crocodiles of Africa).  Leatherback sea turtles get their name from their shells, which have soft leathery skin overlaying bony plates.[7]

Leatherback sea turtles have a shell (also called the carapace) that typically reaches 3.3 to 5.7 feet (1.0 to 1.75 feet) in length, although they can get up to 7.2 feet (2.2 meters) long.  Weight is typically 550 to 1,500 lbs (250 to 700 kg), although these turtles can reach up to 2,000 lbs (900 kg).[7]

Sea snakes

Sea Snake
Credit: Wikipedia photo by Craig D, CC BY-SA 2.0.

Sea snake in Indonesia.

Comprising about 80% of marine reptile species and subspecies,[17] sea snakes are part of a family of venomous snakes called elapids.[2]  Other elapid species include adders, cobras, mambas, taipans, kraits, and coral snakes.  Many of these are amongst the most venomous animals on Earth.[8][8]

Venomous snakes are the elapids, viperids (rattlesnakes, copperheads, vipers, etc), and some of the species from a family called the colubrids (venomous colubrids include boomslangs, tree snakes, and some others).  The vast majority of snakes on our planet are not venomous.[9]

Sea snakes mostly live in the Indian Ocean, Indonesia, the Western Pacific, and Australia, and some species live further into the Pacific Ocean in places such as New Caledonia, the Solomon Islands, and the Philippines.  One species, the yellow-bellied sea snake, is found in the Hawaiian Islands, and also exists along the western coast of the Americas, as far north as Baja California, Mexico.[2]

All have paddle-like tails for swimming, most are four to five feet (1.2 to 1.5 meters) in length, and one species can reach 10 feet (three meters).  Most species can partially breathe through their skin, making it possible for them to stay underwater longer.  All but one genus cannot move on land at all.  The one genus that goes onto land is the only to lay eggs, as all other sea snakes bear live young.[2]

None are found in the Red Sea, in the entire Atlantic Ocean, further north than Japan in the Pacific, or further south than Australia.  It’s believed that they stay out of the Red Sea because of higher salt content in this body of water.[2]

Sea snakes mostly eat fish, and some eat mollusks.  All are reluctant to bite humans, although some species are more aggressive.  More passive species are regularly handled by fishermen without incident.  Fatal bites from these highly venomous animals are rare, although sometimes occur.[2]

Galapagos footage and much more

The BBC Natural History Collection 2 (Life / Ganges / Wild China / Galapagos / Life in Cold Blood)
Amazon Price: $133.72 $43.68 Buy Now
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This is a very highly recommended DVD set with amazing wildlife footage, including the marine iguanas and other unique creatures of the Galapagos Islands.

Marine iguanas

Marine Iguanas
Credit: Wikipedia photo by Kjersti Holmang, CC BY-SA 3.0.

Marine iguanas on Fernandina Island, Galapagos Islands.

The only lizard in the world that can live and forage in the sea is native to the Galapagos Islands near Ecuador, South America.  They can dive more than 30 feet (nine meters) deep as they forage for seaweed and algae.[10]

Charles Darwin famously encountered them in 1835 and was so repulsed by their appearance, that he dubbed them “imps of darkness.”  Most of these lizards are entirely black, although slight variations occur.  The largest males reach about 5.6 feet (1.7 meters) in length including their tails, and females reach up to 3.3 feet (one meter).  They cling to rocks with powerful claws while waves crash over them.[10]

They interestingly can grow longer when food is plentiful, and shrink in size when there’s less food.  They don’t walk well on land, but swim very gracefully.  Eggs are buried inland under sand, and the young have to make their way to the sea on their own.[10]

Saltwater crocodiles

Saltwater Crocodile
Credit: Wikipedia photo by Toursim NT.

Saltwater crocodile on a beach near Darwin, Northern Territory, Australia.

The largest reptile on Earth typically reaches 14 to 17 feet in length (4.3 to 5.2 meters) for males, although some individuals surpass 20 feet (6 meters).  Weight range for males is normally 900 to 2,200 lbs (400 to 1,000 kg), although some exceed 4,000 lbs (1,800 kg).  Females are considerably smaller, typically around 10 feet (3 meters) in length, with the largest measuring 14 feet (4.3 meters).[11]

Their mouths and heads are much larger than other crocodile species.  Just the head of a large male can weigh over 400 lbs (180 kg).[11]

They are native to India, Sri Lanka, Southeastern Asia, the Philippines, Indonesia, New Guinea, the Solomon Islands, Palau, Vanuatu, and Northern Australia.  Occasionally they will make their way to other island locations such as Fiji, Taiwan, islands that are part of Japan, and New Caledonia.  They used to be found much further into the Indian Ocean, with some known to have once existed in Seychelles.[11]

Most crocodiles can enter ocean water temporarily, although none do so nearly to the extent of saltwater crocodiles.  They are typically found along coasts, within estuaries, and in rivers close to the ocean.  They eat almost any animal, including large animals such as buffaloes, sea turtles, sharks, dugongs, and more.  They are the most dangerous crocodilian species to human beings.[11]

Saltwater crocodiles have the strongest bite of any animal alive, and no others really come very close.  This includes great white sharks, alligators, and other crocodile species.[11]

Honorable mentions

Some other reptiles found in the world's oceans

Diamondback Terrapin
Credit: Public domain photo.

This is a diamondback terrapin, which is the most able to tolerate saltwater of all freshwater turtle species.[12]

Some snakes are sometimes found in saltwater environments other than sea snakes.  These include three aquatic species native to Australia and Indonesia, called file snakes,[13] some species of water snake in another taxonomic family native to Asia and Australia,[15] and there are a few others known to inhabit coasts or to hunt amongst mangrove forests.[18] 

American crocodiles are the one other crocodilian species, besides saltwater crocodiles, which regularly swims around, lives, and thrives in the oceans.  They live in the state of Florida in the USA, and also in the Caribbean Sea, Mexico, most of Central America, and the northern part of South America.[15]

There are some water turtle species that are able to swim in saltwater.  Three terrapin species in Southeast Asia regularly live amongst mangroves, and in estuaries.[17]  This is also the case with the pig-nosed turtle of New Guinea.[16]  The turtle that does the best in saltwater which is not a sea turtle is the diamondback terrapin of the Atlantic and Gulf Coasts of the USA.[12]