Class Reptilia

From Sea Turtles to Crocodiles

Loggerhead Turtle / Upendra Kanda / Moment / Getty Images
Loggerhead Turtle. Upendra Kanda/Moment/Getty Images

Class Reptilia is the group of animals known as the reptiles. These are a diverse group of animals that are "cold-blooded" and have (or had) scales. They are vertebrates, which puts them in the same phylum as humans, dogs, cats, fish and many other animals. There are over 6,000 species of reptiles. They are also found in the sea, and referred to as marine reptiles. 

The Class Reptilia, or reptiles, traditionally included a diverse group of animals: turtles, snakes, lizards and crocodiles, alligators, and caimans. Many scientists believe that birds also belong in this class.

Characteristics of Reptiles

Animals in the Class Reptilia:

  • Are ectotherms (commonly called "cold-blooded"). These animals need to warm themselves up using external heat (e.g., the sun).
  • Mostly bear their young in eggs, which are protected by an amniotic membrane (so reptiles are referred to as "amniotes").
  • Have scales, or did have at some point in their evolutionary history.
  • Breathe using lungs rather than gills. Thus, reptiles in the ocean may be able to hold their breath to go underwater, but eventually need to go to the surface to breathe.
  • Have a three- or four-chambered heart.

Classifying Reptiles and Marine Reptiles

  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Reptilia

Marine reptiles are divided into several orders:

  1. Testudines: Turtles. Sea turtles are an example of turtles that live in the marine environment.
  2. Squamata: Snakes.  Marine examples are sea snakes.
  3. Sauria: Lizards. An example is the marine iguana. In some classification systems. lizards are included in the Order Squamata.
  4. Crocodylia: Crocodiles. A marine example is the saltwater crocodile.

The above list is from the World Register of Marine Species (WoRMS).

Habitat and Distribution

Reptiles live in a wide range of habitats. Although they can thrive in harsh habitats like the desert, they are not found in cooler areas like Antarctica, because they need to rely on external heat to keep warm.

Sea Turtles

Sea turtles are found in oceans worldwide. They nest on subtropical and tropical beaches. The leatherback turtle is the species that can go in cold waters, such as off Canada. These amazing reptiles have adaptations that allow them to live in colder water than other turtles, including the ability to shunt blood away from their flippers to keep their core body temperature warmer. However, if sea turtles are in cold waters too long (such as when juveniles don't migrate south quickly enough in the winter), they might become cold-stunned. 

Sea Snakes

Sea snakes include two groups: laticaudid sea snakes, which spend some time on land, and hydrophiid snakes, which live wholly at sea.  Sea snakes are all venomous, but they rarely bite humans.  They all live in the Pacific Ocean (Indo-Pacific and eastern tropical Pacific regions).

Marine Iguanas

The marine iguana, which lives in the Galapagos Islands, is the only marine lizard. These animals live on the shore and feed by diving in the water to eat algae.


In the U.S., the American crocodile often enters saltwater. These animals are found from southern Florida to northern South America and can be found on islands, where they swim or are pushed by hurricane activity. One crocodile, nicknamed Cletus, swam out to the Dry Tortugas (70 miles off Key West) in 2003. American crocodiles tend to be more timid than American alligators and the saltwater crocodiles, which are found in the Indo-Australian region from Asia to Australia.

Most reptiles give birth by laying eggs.  Some snakes and lizards can give birth to live young.  In the world of marine reptiles, sea turtles, iguanas and crocodiles lay eggs while most sea snakes give birth to live young, who are born underwater and must swim immediately to the surface to breathe.

Marine Reptiles

Reptiles that can live at least part of their lives in the marine environment include sea turtles, crocodiles and some lizards.

References and Further Information

  • Galapagos Conservancy. Iguanas and Lizards. Accessed October 30, 2015.
  • IUCN. 2010. Sea Snakes Fact Sheet. Accessed October 30, 2015.
  • Morrissey, J.F. and J.L. Sumich. 2012. Introduction to the Biology of Marine Life. Jones & Bartlett Learning. 466pp.