Hagfish are pretty repellent by most standards: reputed to consume prey from the inside out, the animals look ghastly and, worst of all, they release gallons of disgusting slime in less than 0.1 s when under attack. ‘The slime lodges in and clogs the gills of any fish that tries to eat a hagfish’, says Doug Fudge from the University of Guelph, Canada. But that hasn't deterred Fudge from investing 16 years of his life in studying the revolting gunge. He explains that irritated Atlantic hagfish release minute volumes of concentrated mucous and microscopic skeins of protein filaments that rapidly unravel when mixed with seawater and the mucous to engulf their victims. ‘If either of these [the mucous or the mixing] is missing, skein unravelling is compromised,’ says Fudge. And he had no reason to think that the same would not be true for Pacific hagfish until his undergraduate student, Isdin Oke, decided to test their skein unravelling, only to discover that these protein filaments unravelled spontaneously in seawater. Fudge had another hagfish mystery on his hands (p. 1263).
Thinking about possible mechanisms that could rapidly liberate the tightly bound skeins, Fudge came up with two alternatives: that the threads swell in contact with water to burst the skeins open; or that the stiff protein fibres are coiled into skeins that are secured by an adhesive that dissolves in contact with water, releasing the skeins to spring open.
Curious to find out which theory held water, Oke and Mark Bernards gently anaesthetised Pacific hagfish to collect minute samples of the unexploded mucous and carefully extracted the coiled skeins of slime fibre. Then they tested different strengths of simulated seawater to find out which stabilised the skeins and which burst them open. Monitoring the proportion of ruptured bundles as they increased the temperature and salt concentration, the team was reassured to see that the skeins rapidly unravelled in conditions that simulated the animal's natural aquatic environment but were reluctant to unravel in dilute conditions and at high temperatures. And when the team took a close look at the skeins before and after unravelling with a scanning electron microscope, they could clearly see fluffy clumps coating the tightly coiled skeins that vanished after the skeins burst apart. Could they be some sort of protein adhesive?
Stabilising the skeins in dilute seawater, the team added a protein-digesting enzyme, trypsin, and waited to see whether the skeins sprung apart – which they did. And when the team scrutinised the surface of the trypsin-treated skeins, the enzyme had removed the clumpy coating that had restrained the coiled skeins.
So, the Pacific hagfish slime fibres are coiled into tight skeins that are restrained by a water-soluble protein adhesive that dissolves on contact with seawater to release the strain energy stored in the stiff fibre coils. Having confirmed that the Pacific hagfish deploy slime fibres in a completely different way from their Atlantic cousins, Fudge is keen to learn more about the adhesive that keeps the skeins intact. He also hopes that hagfish slime fibres will eventually adorn the catwalks of the fashion world. ‘We are currently working on a project whose aim is to produce protein fibres that are as strong and tough as hagfish slime threads in the hope that we could one day replace petroleum-based polymers like Nylon with more eco-friendly protein-based materials,’ he says.