The Last Word On Nothing

There is an old story about a scorpion and a turtle. Variants abound, but the basic tale revolves around an unusually talkative scorpion that asks a turtle for a lift across a river. The turtle refuses at first, fearing the scorpion’s sudden but inevitable betrayal. The scorpion insists, the turtle relents, and the two get halfway across before the scorpion predictably stings the turtle. As they sink to their mutual deaths, the turtle asks, “Why did you do it?” The scorpion simply replies: “It’s my nature.”

This story is similar, except an octopus plays the role of the scorpion, and no one talks.

Moreton Bay, on the eastern coast of Australia, is home to around 20,000 green turtles. Kathy Townsend found one of them on October 11, 2008, washed up on a sand bank and dead. Townsend had been studying the links between human activity and sea turtle deaths, but it was clear that this turtle was not killed by people. On the surface, it had no signs of injuries, and it seemed perfectly healthy. “By all rights, it should have still been alive,” says Townsend.

She started cutting. The turtle had normal amounts of muscle and a thick typical layer of fat. Whatever killed it had acted quickly. The liver was normal. Heart: normal. Bladder and genitals: normal. Things only got interesting when Townsend cut into the animal’s digestive tract.

Sea turtles often eat jellyfish, and they hold down their slippery morsels with throats that look like nightmarish pits, lined with fiendish, backward-pointing spines. Among the spines, Townsend found a large ball of undigested seagrass. Nothing unusual there: green turtles find good eating among Moreton Bay’s lush seagrass beds. But within the grass ball, Townsend found the turtle’s killer – a tiny blue-lined octopus.

The blue-lined octopus (Hapalochlaena fasciata) is one of at least three closely related species that live in Australian waters. Its skin is usually brownish-yellow but when it’s threatened, it flashes with rings and lines of electric blue. The colours are a serious warning, for the blue-lined octopus is armed with tetrodotoxin, one of the most potent natural poisons in the world. Half a milligram of the stuff could kill a human, and a tiny 25 gram octopus has enough to kill ten. There is no known antidote.

A variety of animals wield tetrodotoxin including several newts, frogs, worms, crabs and snails, and all of them rely on bacteria to manufacture their poisons. The blue-lined octopus is no exception. Its salivary glands are loaded with bacterial pharmacists that dispense tetrodotoxin, which the octopus delivers with a sharp, parrot-like beak.

Tetrodotoxin kills by preventing nerve cells from firing. This paralyses the muscles, including those that expand and contract the chest. With these muscles weak and flaccid, victims are unable to draw breath and quickly suffocate. This is the fate that befell Townsend’s turtle. The animal’s tissues tested positive for tetrodotoxin; its throat was red and inflamed; and its lungs were filled with water and red, frothy blood. The octopus’s bite paralysed the turtle, preventing it from swimming and lifting its head out of the water. Unable to breathe, it drowned, taking the octopus with it.

Blue-lined octopuses have bitten humans before, but no one has ever recorded one killing a wild sea animal. Townsend found the first. Then she found the second.

Two years later, and five kilometres away, Townsend found a second green turtle with no obvious cause of death. She cut it open and to her utter disbelief, she found a second blue-lined octopus, entombed within a ball of seagrass. “Needless to say, I was beyond excited,” she says. “To observe it once was serendipity, but to observe it twice was quite unbelievable.”

The fact that two turtles died in this way, out of the hundred that Townsend eventually dissected, suggests that blue-lined octopuses may present a greater threat to grazing turtles than anyone had imagined. Still, Townsend thinks that the deaths were accidents. Blue-lined octopuses only use their venomous bite when absolutely necessary, relying instead on camouflage and concealment. The seagrass beds of Moreton Bay provide great hiding places – they’re veritable octopus’s gardens.

The two turtles probably snapped up the octopuses by accident, while feeding on the grass. Sloshing around the turtles’ throats, the octopuses used their weapon of last resort, and doomed both them and their inadvertent attackers in the process. It is, after all, their nature.

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Ed Yong is an award-winning British science writer, who writes the blog Not Exactly Rocket Science. He slept once, back in 2006, and vaguely recalls enjoying it.

Photo credits:  Kathy Townsend

Source: DOI: 10.1007/s00227-011-1846-9