A Blog by Ed Yong

A Thousand Worms Merge Into a Living Tower

What’s creepier than a worm rearing up on its tail to snag a passing insect? A thousand worms uniting into a single living, writhing, waving tower to snag a passing insect.

Pristionchus pacificus is a nematode or roundworm—one of 25,000 species that are among the most numerous animals on the planet. This particular nematode infests the bodies of scarab beetle larvae. It’s not a parasite as such, and its habits are positively tame compared to the creatures that often feature on this blog. It simply waits for its host to die of natural causes, and then eats the microbes that grow on its carcass.

But first, it has to get into a beetle. At some point during its early days, P.pacificus pauses its growth and becomes a dauer—an especially tough larva that’s adapted to survive through harsh conditions. The dauers stand on their tails and wave their body about in the hopes of latching onto passing beetles.

But Sider Penkov and Akira Ogawa from the Max Planck Institutes found that groups of P.pacificus can merge to form a single waving “dauer tower”, composed of up to a thousand individuals.

Each individual worm is just a quarter of a millimetre long but the towers can grow up to a centimetre. Some are so big that you can see them with the naked eye and photograph them with a macro lens, even though their members are all microscopic.

Such teamwork! Such togetherness! Such low odds of ever appearing on a motivational poster!

Other nematodes like C.elegans—that darling of biologists—also form dauer towers, but these constructions are small and fall apart easily. By contrast, P.pacificus’s towers are incredibly strong. Penkov and Ogawa tried prodding them with a metal wire, and they didn’t fall apart. They stuck the towers in water and the larvae started to swim, but they still kept together as a cohesive mass. The only thing that worked was a dash of detergent. When they added that to the water, and the towers disintegrate into a mass of individual larvae.

The team reasoned that the worms must be sticking together with a fatty or waxy chemical that repels water but can be dissolved by detergent. Indeed, they saw that the worms exuded small droplets over their skins, soon after they transformed into dauers. The droplets contained a huge wax molecule (C60H100O2N), one of the longest found in any animal or plant. The team called it nematoil.

Nematoil is the glue that gives the dauer tower its power. When the team synthesised the chemical and applied it to C.elegans, they found that even this nematode could unite into a sturdy spire.

Penkov and Ogawa suspect that the tower’s height gives its constituent larvae better odds of hitching onto a beetle. Once one of them snags some cuticle, the entire tower can get pulled along for the ride, held together by their nematoil secretions.

But their discovery raises many more questions. What brings the worms together in the first place? How do they coordinate their movements to produce a cohesive wave? And are there cheats?

P.pacificus reminds me of another microscopic creature called Dictyostelium discoideum, or Dicty for short. It’s not an animal but a slime mould. It mostly spends its time as single-celled amoebae, but these can merge into a many-celled slug. The slug slowly stretches skywards, forming a spore capsule atop a long stalk. Any amoebae that form the spores will survive, but those that create the stalk go nowhere and eventually die. This leads to conflict. Some amoebae are cheats, which make more than their fair share of spores and are rarely contribute to the stalk.

Does the same apply to P.pacificus? Do the nematodes at the base of the stalk perhaps get left behind? Do some of them secrete less nematoil (which, after all, takes a lot of energy to make), relying instead on their neighbours’ glue?

Reference: Penkov, Ogawa, Schmidt, Tate, Zagoriy, Boland, Gruner, Vorkel, Verbavatz, Sommer, Knolker, Kurzchalia. 2014.  A wax ester promotes collective host finding in the nematode Pristionchus pacificus. Nature Chemical Biology http://dx.doi.org/10.1038/nchembio.1460