The humble hagfish produces a sticky slime to defend itself from predators, as well as to hunt for its own food. Now a team of Swiss scientists has figured out the physics behind how the hagfish can use the same slimy substance for both purposes, according to a new paper in Scientific Reports. The seal shark Dalatias licha (a — c) and the wreckfish Polyprion americanus (d — f) fare poorly when they attack the hagfish. (Image: V. Zintzen et al/Scientific Reports 2011)
The hagfish is an ugly, grey, eel-like creature — clammy, stinky and eyeless — whose most interesting feature is the sticky slime it produces from pores all over its body when attacked. (Its Latin name, Myxine glutinosa, even derives from the Greek words for "mucus" and "glue".) That mucus is made of protein-and-sugar molecules known as mucin. Just 90mg of the milky concentrated "pre-slime" can produce more than a litre of sticky goo when it combines with the salty seawater — sticky enough to trap and suffocate aspiring predators.
It's different from typical mucus, because it doesn't dry out and harden over time. It stays slimy, in part because the mucin found in the hagfish also has long threadlike fibres, finer than spider's silk and just as strong. That's what gives the resulting mix the consistency of half-solidified jelly. Check it out:
Image: The Fudge Lab/University of Guelph
The stuff bonds to the gills of a predatory fish, and the would-be predator basically suffocates on snot. The more it struggles to escape, the faster the goo expands. Sometimes even the hagfish gets caught in its own slime, at which point it literally ties itself in a knot and pushes the knot down the length of its body to scrape the stuff off.
There have been prior studies on the unusual fluid properties of hagfish slime. But lead author Lukas Boni of ETH Zurich in Switzerland and his colleagues wanted to focus specifically on how those properties played a role in the animal's defence system — as well as its trick of tying itself in knots to escape from its own slime.
To retrieve the slime, they placed the hagfish in a bucket of cold seawater and used a mix of clove bud oil and ethanol to anaesthetise them. Then they put the hagfish on a dissection tray, blotted them dry, and zapped them with electricity to make the muscles contract and expel the milky pre-slime from the pores. Finally, the fish were transferred to nice, comfy recovery bath.
The Swiss scientists found that different types of fluid flow affect the overall viscosity of the slime — a property loosely defined as how much friction/resistance there is to flow in a given substance. A flowing liquid is essentially a series of layers sliding past one another. The faster one layer slides over another, the more resistance there is; and the slower the sliding, the less resistance there is.
Hagfish slime is an example of a non-Newtonian fluid, in which the viscosity changes in response to an applied strain or shearing force. Stir a cup of water, and the water shears to move out of the way, so the viscosity is unchanged. The opposite is true for non-Newtonian fluids. Applying a strain or shearing force will increase viscosity — in the case of tomato sauce, pudding, gravy or that classic mix of water and corn starch called "oobleck" — or decrease it, like non-drip paint that brushes on easily, but becomes more viscous once it's on the wall.
Hagfish slime can be both. It turns out that the suction feeding employed by many of the hagfish's predators creates a unidirectional flow. The elongated stress of that sucking flow increases the goo's viscosity, the better to suffocate said predators by clogging of the gills. But when the hagfish is trying to escape from its own slime, its motion creates a shear-thinning flow that actually reduces the viscosity of the slime, making it easier to escape. In fact, the slimy network quickly collapses in the face of a shear-thinning flow.
All these interesting properties make hagfish slime dead useful for lots of potential applications. There's even an online recipe for scones made with hagfish slime — courtesy of the Museum of Awful Food — for the culinarily adventurous. The slime is a handy substitute for egg whites. (By the way, that trendy "eel skin" wallet or purse in your collection might actually be hagfish leather.)
But the more promising applications are far less frivolous. It could be used to staunch bleeding in accident victims during surgery, for instance, expanding upon contact with the salty blood. The stretchiness — akin to the plastic rings that hold together six packs of beer — would make hagfish slime useful in biomedical devices, tissue engineering, or biosensors.
Back in 2012, Douglas Fudge's lab at the University of Guelph (he just moved to Chapman University this month) successfully harvested the sticky stuff from the fish, dissolved it in liquid and then "spun" it into a strong-yet-stretchy thread, much like spinning silk. Those threads might one day replace the petroleum-based fibres currently used in things like safety helmets or Kevlar vests. A first attempt at making artificial hagfish slime fibres was "quite terrible", by Fudge's own admission, but it still showed potential.
And perhaps hagfish slime might prove useful to to filmmakers in search of the perfect substance to represent spectral slime — the bane of any Ghostbuster: