A snake at sea isn't automatically a sea snake. Lots of unrelated species frolic in our oceans from time to time. Reticulated pythons, for example, will swim between islands along the coasts of southeastern Asia, crossing distances that could wear out an Olympic gold medallist. That doesn't, however, make them sea snakes. When naturalists talk about sea snakes, they're usually referring to two very specific groups of reptiles that are part of the cobra family: true sea snakes and sea kraits. We've already met one of the former species. Yellow-bellied sea snakes, those oddball knot-tiers, are classic hydrofoils. True sea snakes like these have sworn off dry land altogether. Fully marine, they give birth to live young at sea. Scientists recently learnt that Australia's Cleveland Bay is a biological nursery where pregnant spine-bellied sea snakes come to deliver their broods. Terrestrial births just aren't an option. Hydrofoils never exit the water voluntarily because they lack the wide belly scales other snakes use to crawl over solid ground. Drop one on a beach and the poor creature will struggle to move under its own power. Sea kraits are a bit less streamlined, but they're more competent on land. They mate, shed and digest some of their meals outside the water. Equipped with the requisite belly scales, the animals are free to hit the turf, and like sea turtles, they lay eggs in beachside nests.