Hydrogen sulfide is not a gas that is usually described as life sustaining. Even at low concentrations it smells like rotten egg and exposure to high levels can cause all kinds of health problems, such as nausea, loss of smell and even death in extreme situations. However while its toxic fumes might knock most of us out, there is a creature that thrives on this toxic gas – the giant shipworm, a mysterious mud-dwelling creature that has eluded scientists till now.

Found in a lagoon laden with rotting wood in the southern Philippine island of Mindanao, the giant shipworm (Kuphus polythalamia) has been playing a game of catch-me-if-you-can with scientists since the 18th century. While its empty shells, which can measure up to five-foot long, are fairly common, the creature itself – and a live specimen at that – is not, says lead investigator Daniel Distel, a research professor and director of the Ocean Genome Legacy Center at Northeastern University.

As luck would have it, scientists were handed a clue when a collaborator alerted them to a Filipino documentary on the creature (known to locals as the giant tamilok). An expedition was duly organized and live specimens were eventually found and transported to the University of the Philippines for analysis.

"I was awestruck when I first saw the sheer immensity of this bizarre animal," says Marvin Altamia, a researcher at the marine sciences institute, University of the Philippines. Compared to its skinny, pale-hued wood-boring cousins, which are typically a foot long (30 cm), the ink-black giant shipworm looks like the dismembered tentacle of a larger and scarier creature, reaching lengths of 155 cm (5 ft) and making it the longest bivalve known to scientists. (Despite its name, it is actually a clam, not a worm.)

Bizarre appearance aside, what sets the giant shipworm apart from its cousins is the way it sustains itself. Unlike the latter, which feed on the wood they burrow into, its digestive system is disproportionately tiny compared to the rest of its body. As the researchers discovered, the giant shipworm does not have the large sac-like cecum, found in all other members of the same bivalve family, which stores digested wood particles. In addition, only trace quantities of fecal matter were found in its digestive system, which rules out the possibility that its much bigger size is due to ingestion of wood, even though it was found in a place with plenty of wood debris.

Furthermore, it lives enclosed in its shell, which has a cap that covers its mouth, thus ruling out the possibility that it excavates and feeds on the sediments in the mud, like earthworms.

So if it doesn't eat wood or mud, how did it become so much larger than other shipworms? The answer to this mystery lies in its outsized gill (see illustration below), within which live bacteria.

While this in itself is not unusual - normal shipworms also have bacteria living in their gill that help them digest the wood they eat - what the researchers found when studying the giant shipworm was that its bacterial genome contains features, such as sulfur globules, similar to those in other sulfur-oxidizing bacteria. This led them to surmise that that the creature gets its energy from the carbon the bacteria produce when they break down the hydrogen sulfide in its habitat. And since the giant shipworm doesn't have to use its digestive organs at all, this would also explain the size of the organs.

Transitioning from wood to sulfur

Back in 2000, Distel published a study proposing the theory that mussels found living in deep sea vents had evolved from specimens commonly found on sunken whale bones and rotting wood. This transition was made possible by the sulfur-oxidizing bacteria they harbored, which enabled them to survive on the gas produced by the vents when they sank to the ocean floor. Similarly, the researchers believe that the giant shipworm evolved from wood-eating ancestors that used wood as a "stepping stone" between habitats. Eventually, they evolved when they traded the wood-processing bacteria in their gills for the sulfur-ingesting variety, thus allowing them to thrive on the toxic gas, of which there is no shortage given the abundance of rotting wood and organic matter in these marine environments.

Apart from giving them a complete makeover in the size and anatomy departments, the researchers also believe that this evolutionary transition led to "a fundamental change" in the relationship between the shipworms and sulfur-oxidizing bacteria. In wood-eating shipworms, the bacteria acquire organic carbon from the host and in return provide digestive enzymes that help it to process the wood it eats, note the authors in their study. In the case of the giant shipworm, the bacteria require no organic carbon from the host, but instead provide it with the carbon they produce.

"We suspected the giant shipworm was radically different from other wood-eating shipworms," says senior author Margo Haygood, a research professor in medicinal chemistry at the University of Utah. "Finding the animal confirmed that."

The study was published in the Proceedings of the National Academy of Sciences.

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