Sea snake

Sea snakes are highly specialized elapid snakes that inhabit marine environments. They are among the most completely aquatic air-breathing vertebrates on Earth, a status they share with cetaceans like whales and dolphins.

These reptiles are divided into two subfamilies: Hydrophiinae and Laticaudinae. The Hydrophiinae group includes many species that live entirely at sea, as well as some terrestrial snakes in Australasia. The Laticaudinae subfamily consists only of sea kraits, or Laticauda. While most sea snakes are venomous, the genus Emydocephalus is an exception, as it feeds almost exclusively on fish eggs.

To survive in the ocean, sea snakes have developed several remarkable physical adaptations. Most species possess a paddle-like tail that significantly improves their swimming ability. Many also have laterally compressed bodies, which gives them an eel-like appearance.

Because they lack gills, they must surface regularly to breathe air. To assist with this, their lungs have become very large and extend nearly the entire length of their bodies. These large lungs help store air for deep dives and may also aid in buoyancy. Interestingly, some species like the black-and-yellow sea snake, Hydrophis platurus, can satisfy about 30% of their oxygen requirements through their skin.

Living in saltwater presents a major challenge: salt intake. When sea snakes swallow seawater while feeding, they ingest much more salt than land animals. To manage this, they use posterior sublingual glands located near the tongue sheath. These glands allow them to expel excess salt from their bodies through tongue action.

Their nostrils also feature specialized spongy tissue that acts as a valve to exclude water. This allows them to breathe even when their heads are only partially submerged.

There is a clear distinction between the two subfamilies regarding how they move on land. Most sea snakes have reduced ventral scales, which are the scales on their bellies. This loss of large scales makes them virtually helpless and unable to move effectively on land. However, the sea kraits of the Laticaudinae subfamily have retained enlarged ventral scales. These scales provide the necessary grip for them to spend much of their time on land.

Because of these scales, sea kraits are considered more primitive than the fully aquatic species.

Sensory perception in the ocean is very different from life on land. Vision, hearing, and chemoreception (using the tongue to smell) are often distorted by water. Because light is dim and chemicals are diluted, sea snakes have developed unique ways to sense their world. Some species, such as Lapemis curtus, have small sensory organs on their heads to detect the movement of fish. The olive sea snake, Aipysurus laevis, even has photoreceptors in its tail skin. This allows the snake to detect light and ensure its tail is hidden inside coral holes during the day.

Sea snakes are found primarily in the warm tropical waters of the Indian Ocean and the western Pacific Ocean. They do not occur in the Atlantic Ocean, partly due to cold currents. One species, Pelamis platurus, has a geographic range wider than almost any other reptile except for certain sea turtles. It stretches from the east coast of Africa all the way to the western coast of the Americas. While many species prefer shallow coastal waters and mangroves, others are pelagic, meaning they live in the open ocean.

Behavioral patterns vary widely among these reptiles. Most sea snakes are generally mild-tempered and reluctant to bite, though some species like Aipysurus laevis are more aggressive. In the water, they are efficient hunters, but on land, their movements become erratic and awkward. They cannot coil and strike like land snakes, so they instead strike wildly at anything that moves. Despite their venom, many local fishermen handle them without concern, often tossing them back into the sea by hand.