While the idea of snakes in the ocean may seem unusual, a fascinating group of reptiles has made the marine world their home. These specialized serpents have adapted to saltwater environments, spending much, if not all, of their lives beneath the waves.
Meet the Ocean’s Serpents
The snakes inhabiting marine environments primarily belong to two groups: true sea snakes (subfamily Hydrophiinae) and marine kraits (subfamily Laticaudinae). True sea snakes are extensively adapted to aquatic life and typically cannot move effectively on land. They often exhibit a laterally compressed body, resembling an eel, and possess a paddle-like tail that aids propulsion through water. Their scales are usually small and non-overlapping, which helps to streamline their movement in water.
Marine kraits, in contrast, are semi-aquatic, able to navigate both water and land. While they also have a paddle-shaped tail, their bodies are more cylindrical, and they retain enlarged belly scales similar to their terrestrial ancestors. These larger scales enable them to crawl and climb effectively on land, a necessity as they return to shore for specific activities. There are approximately 69 recognized species of sea snake.
How They Thrive in Saltwater
Marine snakes have developed remarkable adaptations to survive in the ocean’s saline environment. A key adaptation is their specialized sublingual salt glands, located beneath their tongues, which allow them to excrete excess salt ingested from seawater or their diet. These glands secrete a highly concentrated fluid, effectively removing salt from their bodies as they flick their tongues.
Breathing underwater presents a significant challenge for these air-breathing reptiles. Sea snakes have evolved unique respiratory systems, including a single, elongated lung that extends almost their entire body length. This aids both oxygen storage for prolonged dives and buoyancy control. Many species can also absorb oxygen directly through their skin, a process known as cutaneous respiration, which can account for a substantial portion of their oxygen needs while submerged. Some species can remain underwater for several hours.
Their physical form is also highly specialized for aquatic locomotion. The paddle-like tail generates powerful thrust for efficient swimming. Their streamlined bodies and reduced ventral scales in many species further decrease drag, enabling agile movement through water. When hunting, most sea snakes are ambush predators, using their potent venom to immobilize fish and eels before consuming them whole.
Their Marine Homes
Sea snakes are primarily found in the warm, tropical, and subtropical waters of the Indian and western Pacific Oceans. Their distribution extends from the coasts of Africa eastward across the Pacific, with high diversity around Australia and Southeast Asia. The yellow-bellied sea snake (Hydrophis platurus) is an exception, having the widest geographic range of any reptile species, reaching the western coasts of North and South America.
These marine reptiles inhabit a variety of aquatic environments. Many species prefer shallow coastal waters, thriving in areas such as coral reefs, seagrass meadows, and muddy seafloors. Estuaries and even river mouths are also common habitats for some species, with reports of them swimming up to 160 km from the sea. While most true sea snakes are fully aquatic and give birth to live young at sea, marine kraits maintain a semi-aquatic lifestyle, regularly returning to land to lay their eggs in limestone caves or rock crevices.
Understanding Their Nature
Most sea snakes are highly venomous, possessing potent neurotoxins and myotoxins. Despite their powerful venom, sea snakes are generally docile and non-aggressive. Bites to humans are rare and typically occur when the snakes feel threatened, such as when accidentally caught in fishing nets or handled. Their fangs are relatively small, and venom is not always injected with every bite.
Sea snakes are carnivores, mainly eating fish and eels. Some species specialize in particular prey, such as burrowing eels or fish eggs. They play a role in marine ecosystems as predators, contributing to the balance of fish populations. While many species are common, several sea snake populations face threats, including habitat degradation, incidental capture in fishing nets, and the illegal pet trade. Conservation efforts are underway for species like the dusky sea snake and the leaf-scaled sea snake, which are listed as endangered or critically endangered due to these pressures.