The homely hagfish might look like just your average bottom feeder, but it has a secret weapon: it can unleash a full liter of sticky slime in less than one second. That slime can clog the gills of a predatory shark, for instance, suffocating it. Scientists are unsure just how the hagfish (affectionately known as a "snot snake") accomplishes this feat, but a new paper in the Journal of the Royal Society Interface suggests that turbulent water flow (specifically, the drag such turbulence produces) is an essential factor.

Scientists have been studying hagfish slime for years because it's such an unusual material. It's not like mucus, which dries out and hardens over time; hagfish slime stays slimy, giving it the consistency of half-solidified gelatin. That's due to long, thread-like fibers in the slime, in addition to the proteins and sugars that make up mucin, the other major component. Those fibers coil up into "skeins" that resemble balls of yarn. When the hagfish lets loose with a shot of slime, the skeins uncoil and combine with the salt water, blowing up more than 10,000 times its original size.

Yet the precise mechanism for slime deployment is still poorly understood, according to co-author Gaurav Chaudhary of the University of Illinois, Urbana-Champaign. Recent research showed that sea water is essential to the formation of the slime and that hagfish skeins can unravel spontaneously if ions in the sea water mix the adhesives that hold the fibrous threads together in skeins. Chaudhary says that what's missing in this earlier work is taking the fast time scales into account. A 2014 study, for instance, showed that any spontaneous unraveling of the skeins would take several minutes—yet the hagfish deploys its slime in about 0.4 seconds.

G. Chaudhary et al.

G. Chaudhary

G. Chaudhary

For the current study, Chaudhary et al. opted to focus on how the hagfish managed to achieve that rapid unraveling of the slime skeins. The critical factor is the mixing. If you're going to get those short time scales for deployment, "The hydrodynamic stress on the thread and the skein are enough to peel the thread from the skein within a second," said Chaudhary.

Specifically, the movement of the surrounding water as a predator attacks helps trigger the uncoiling. Skeins have a loose end; tugging on that triggers the unraveling. But drag from flowing water as a predator thrashes about makes this process happen even faster. Working with colleagues at UIUC and the University of Wisconsin, Madison, Chaudhary first tested the hypothesis by immersing skeins in salt water. He found that they fully unraveled only when he applied a drag-like force on the loose ends of the skeins.

The team relied on mathematical models to explore the various hydrodynamic forces at work. They concluded that the fastest rate of unraveling occurs when the skein is stuck to a surface, like a shark's mouth (or, even better, its gills), because it creates the most drag. Chaudhary et al. also made several testable predictions, simple enough for lab testing, although Chaudhary acknowledges that setting up well-designed experiments will be challenging. Developing a synthetic version of the slime would also be a boon to future experiments.

"This paper takes a detailed theoretical look at how hydrodynamic forces and collisions with the gills of an attacking predator might influence the unraveling process," said Douglas Fudge, a marine biologist at Chapman University and a leading expert on hagfish, who has collaborated with Chaudhary in the past (although he was not involved with the current study). "Their analyses suggest that unraveling, and therefore slime deployment overall, should be fastest when either the skein or the partially unraveled thread is pinned to a solid object—a prediction we are eager to test in the lab."

Hagfish slime might one day prove useful for biomedical devices, or weaving light-but-strong fabrics for natural Lycra or bulletproof vests, or lubricating industrial drills that tend to clog in deep soil and sediment. "There is considerable interest in replicating the material properties of hagfish slime for diverse practical applications, and the insights provided in this paper get us closer to making that goal a reality," said Fudge.

DOI: J. R. Soc. Interface, 2019. 10.1098/rsif.2018.0710 (About DOIs).

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