In the Web of the Velvet Worm

I used to think that velvet worms were kind of cute. Living members of a very old lineage of multi-legged invertebrates – Onychophora, which stretches back over 505 million years to the underwater explosion of animal life during the Cambrian – they seemed like charming little critters that hid in the undergrowth. Then I saw how they consume their prey.

Hollywood monster makers could learn a thing or two from velvet worms. Though many species eat small insects, at least one Peripatus velvet worm was recently observed after snagging a tarantula. They are frighteningly formidable predators.

Scientists have been trying to pick up a few lessons from velvet worms, too, specifically regarding how these soft-bodied predators ensnare their prey. For decades, naturalists have been puzzled by the goo velvet worms shoot from their modified legs. The slime spurts out as a fluid, but then solidifies into a web which is consumed along with the velvet worm’s hapless victim. (Waste not, want not.) How the hell does that work?

A few different slime recipes have been proposed. Perhaps the sticky stuff is a primitive version of silk, or maybe it’s a solution filled with collagen fibers. Then again, it could be some kind of silk-slime combination functionally similar to what some spiders use. No one knew for sure. Studies of the slime itself showed that it was about 90% water, and about 5% of the rest was a mix of proteins rich in the amino acids proline, glycine, and hydroxyproline. (I would direct you to Wikipedia for more about each, but the pages for these organic compounds are useless unless you already have a doctorate in biochemistry.) This composition seemed to indicate that the velvet worms created a silk or collagen facsimile, but a study published by Victoria Haritos and colleagues last year showed something different.

In order to investigate the mystery of the slime, Haritos’ group studied Euperipatoides kanangrensis velvet worms captured from Tallanganda State Forest in Australia. The scientists harvested slime directly from the animals and also collected the slime glands themselves from dissected specimens. Together, this allowed them to study the composition of the slime as well as the gene expression involved in slime production.

The slime itself behaved as it did in the wild. When squirted onto glass, the goo quickly set and adhered, but could also be drawn into sticky threads when wet. As for the organic compounds found within the slime glands, Haritos’ team found an array of proline-rich, carbohydrate-binding, and antimicrobial proteins. In functional terms, the velvet worm’s protein soup is highly disordered and can’t form the same kind of tiny, highly-ordered sheets seen in — among other things — spider silk. The slime is a chaotic mix in which these particular proteins are well-coated with water molecules, which is why the velvet worm’s goo is about 90% water. Inside the velvet worm’s body these proteins float around in the soup, but when jetted onto prey the water evaporates and the disordered proteins form a sticky gel.

The paper by Haritos’ team breaks it down like this. Once the velvet worm detects prey, it shoots the protein-laden goo onto its victim. This slime spreads quickly over the target – especially since the gak contains specific elements that allow it to stick to the waxy exoskeletons of insects – and as the prey struggles it draws slime threads around itself. Now exposed to the air, the goop loses water and the threads start to become a strong, solid net, by which time the velvet worm has already set to work with its pincer-like jaws. This all happens in moments – blink and you will miss it – and it makes me glad that there aren’t any giant, human-eating velvet worms around. (That we know of, anyway.)

[Ed Yong also blogged about this discovery at Not Exactly Rocket Science when it was first released last year.]

Top Image: The velvet worm Peripatus. (Ant Boy/Flickr)

Video Clip: From Life in the Undergrowth.

References:

Dias, S., & Lo-Man-Hung, N. (2009). First record of an onychophoran (Onychophora, Peripatidae) feeding on a theraphosid spider (Araneae, Theraphosidae) Journal of Arachnology, 37 (1), 116-117 DOI: 10.1636/ST08-20.1

Haritos, V., Niranjane, A., Weisman, S., Trueman, H., Sriskantha, A., & Sutherland, T. (2010). Harnessing disorder: onychophorans use highly unstructured proteins, not silks, for prey capture Proceedings of the Royal Society B: Biological Sciences, 277 (1698), 3255-3263 DOI: 10.1098/rspb.2010.0604