How hagfish choke their enemies with slime

The eel-like hagfish has survived on Earth for at least 300 million years, thanks to its ability to fend off predators by producing and choking them with slime. Now, researchers from the University of Illinois at Urbana-Champaign are uncovering the mystery of just how this defence mechanism works.

“Hagfish are both amazing and disgusting at the same time,” said Associate Professor Randy Ewoldt, who has studied the hagfish for about a decade. “The reason that they are disgusting explains how they have survived all this time.”

The fact that hagfish are slimy has been known for a number of years, but it wasn’t until this decade that there was finally evidence of the hagfish using its slime to clog up the gills of some suction-feeding predators, confirming suspicions as to why the hagfish produces the slime.

“When the hagfish create this slime, they do so in an incredibly efficient way in that the amount of material they put into the water grows in volume by a factor of up to 10,000 to make the final slimy gel,” Assoc Prof Ewoldt explained. “The volumetric increase is astounding, and as far as I know untouched by anything else in nature or anything we’ve done as engineers.”

What the team discovered was that the hagfish threads, 100 times thinner than a human hair and initially wound up like a skein of yarn, can unravel in the blink of an eye due to fluid flow. In doing so, it can go from a size of about 0.1 m to 10 cm in length, or a water incorporation in a magnitude of 10,000:1. Their work has been published in the Journal of the Royal Society Interface.

“The hagfish slime doesn’t maintain the same topology while expanding in volume; it really fundamentally changes as threads unravel and its volume increases,” Assoc Prof Ewoldt said. “The focus of this study was on this unique mechanism.”

The colleagues hypothesised that the fluid flow was actually making this happen rather than it being like a chemical explosion. Using material generated by an actual hagfish, they created a model using both mathematics and physics to test the hypothesis and discovered that the time and volume of the slime’s unravelling was correlated to the viscosity and flow of the surrounding liquid. In other words, if the slime were put in still water, it wouldn’t be able to unravel, but in the midst of some attack, there would be enough fluid flow for the slime to unravel.

Assoc Prof Ewoldt noted that because attacks on the hagfish happen under different conditions, his team had to simplify to come up with a model scenario. Another driving factor likely affecting this process has yet to be measured — that is the stickiness inside the thread, which could provide some resistance.

“Because that physical resistance in the hagfish case has never been measured, we had to really generalise our mathematical model and use some reasonable range of values to do this,” he said. “One really powerful way to do this is to write down governing equations and use dimensional analysis to take the complicated high dimensional equations and find a way to simplify them. The robustness of the conclusion is that it wasn’t just a single value of a property, instead a range of things that we could cover by framing the mathematics of this in a powerful way.”

In the end, under various conditions, the team concluded that this unravelling from about 0.1 mm to 10 cm can happen in less than a few hundred milliseconds — and that while the resulting slime gel is a solid, it is soft enough to take the form of its container, making it almost undetectable by the eye in a bucket of water. It is also strong enough and non-permeable enough to virtually stop the flow of water.

In the future, Assoc Prof Ewoldt and his team want to further study the slime’s hydraulic permeability and ultimately play a part in developing materials with the same property, which could be of use to a number of real-world applications. The US Navy is interested in using the material in defence, for instance.

“I would love to think more about the material design aspect,” Assoc Prof Ewoldt said. “We still haven’t figured out how to make a material like this, which swells up in water and clogs things up in a marine environment.”

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