Scientists are still learning interesting new things about gooey hagfish slime

Meet the humble hagfish, an ugly, gray, eel-like creature affectionately known as the “snot snake”, due to its unique defense mechanism. Hagfish can release a full liter of sticky slime from pores all over its body in less than a second. This is enough to, say, clog the gills of a predatory shark, suffocating the potential predator. A new article published in the journal Current Biology reports that the slime produced by large hagfish contains much larger cells than those produced by small hagfish – an unusual example of cell size scaling with the size of the hagfish. body in nature.

As we reported earlier, scientists have been studying hagfish slime for years because it is such an unusual material. It’s not like mucus, which dries up and hardens over time. Hagfish slime remains viscous, giving it the consistency of half-solidified gelatin. This is due to the long, threadlike fibers present in the mud, in addition to the proteins and sugars that make up mucin, the other major component. These fibers wind up in “skeins” that look like balls of wool. When the hagfish slack off with a slime, the tangles unroll and combine with the salt water, detonating more than 10,000 times its original size.

From a material standpoint, hagfish slime is fascinating. In 2016, a group of Swiss researchers studied the unusual fluid properties of hagfish mud, focusing specifically on how these properties provided two distinct benefits: helping the animal defend itself against predators and attaching itself in tying knots to escape his own mud. They found that different types of fluid flow affect the overall viscosity of the slurry. A flowing liquid is essentially a series of layers sliding over each other. The faster one layer slides over another, the more resistance there is, and the slower the slide, the less resistance there is. As I wrote for Gizmodo at the time:

Hagfish slurry is an example of a non-Newtonian fluid, in which the viscosity changes in response to applied stress or shear force. … Applying a force or shear force will increase the viscosity – in the case of ketchup, pudding, gravy or that classic mixture of water and cornstarch called “oobleck” “- or decrease it, like an anti-drip paint that applies easily, but becomes more viscous once on the wall.

Hagfish slime can be both. It turns out that the suction feeding employed by many hagfish predators creates a one-way flow. The prolonged stress of this suction flow increases the viscosity of the glue to better suffocate said predators by obstructing the gills. But when the hagfish tries to escape from its own slime, its movement creates a shear flow that actually reduces the viscosity of the slime, making it easier to escape. Indeed, the viscous network quickly collapses in the face of a lightening flow.

Scientists are still learning the precise mechanism by which hagfish produces the slimy substance. Previous work has shown that seawater is essential for silt formation, and skeins of hagfish can spontaneously fall apart if ions in seawater mix the adhesives that hold the fibrous threads together in the tangles. . But deadlines also matter. A 2014 study, for example, showed that any spontaneous untangling of the tangles would take several minutes, but hagfish deploys its slime in about 0.4 seconds.

A 2019 article in the Journal of the Royal Society Interface suggested that the turbulent flow of water (in particular, the drag that this turbulence produces) is a critical factor. The movement of the surrounding water during a predator’s attacks helps to trigger the unfolding. The skeins have a free end; pulling on it triggers fraying. But the trail of flowing water as a predator struggles makes this process even faster.

This new article summarizes the results of the latest research from Douglas Fudge, a marine biologist at Chapman University who has been studying hagfish and the properties of its slime for years. For example, in 2012, while at the University of Guelph, Fudge’s lab managed to harvest hagfish slime, dissolved it in a liquid, and then “spun” it into a thread. strong but stretchy, much like silk spinning. It is possible that such yarns could replace the petroleum-based fibers currently used in safety helmets or Kevlar vests, among other potential applications.

For this last article, Fudge et al. took samples from 19 different species of hagfish, large and small, took microscopic images, and carefully measured the size and shape of the wire cells in those images. The resulting database incorporated measurements from over 11,700 cells harvested from 87 hagfish (the latter measuring between 10 and 80 cm in length).

They found that these thread-like cells were extremely large, compared to similar cells in vertebrates – larger than abdominal fat cells in elephants, in fact. Even more intriguing, the size of these cells turns out to be highly dependent on the body size of the hagfish. There are other examples in nature of this type of scaling.

For example, geckos and other creatures that use sticky pads to climb exhibit a scale exponent of about 0.35 when it comes to the size of their pads in relation to body mass. And some species of spiders produce dragline silk that varies in diameter with body mass with an exponent between 0.37 and 0.39. But the exponent of scaling Fudge et al. found in their hagfish wire cells was 0.55, significantly greater than any other known scaling exponent in vertebrates.

“Our work showed the greatest exponent of scaling known in animal cells,” said co-author Yu Zeng. This means that, on the hagfish evolution tree, the large species all make large, thread-like cells, despite being far apart.

The authors hypothesize that the unusual characteristic could be the result of an evolutionary selection linked to the mechanical properties of the wire cells. “Very little is known about the behavioral ecology of hagfish, particularly how it changes with body size,” Yu said. “Our study suggests that body size-dependent interactions with predators have resulted in profound changes in the defensive slime of the hagfish and these changes can be seen at the cellular and subcellular level. “

The team’s models showed that the threads get thicker and longer in the larger cells of large hagfish, which can produce threads about 4 micrometers thick and 20 centimeters long. It is the largest intracellular fiber known in animals, comparable in size to keratin fibers and spider bristles. And like these examples, the threads in hagfish slime rely on the coordination between many cells. At some point in their growth cycle, the intracellular protein fibers of hagfish mucus “undergo a phase transition,” according to the authors, “where individuals [fibers] condense with their neighbors into a much larger intracellular fiber superstructure. “

So what could this scaling feature (and the resulting larger threads) provide a scalable advantage? “There are several ways that the larger threads can be useful for larger hagfish,” Fudge said. “Thicker threads can withstand more force before breaking and make the slime stronger and better able to stay on the gills of a large and powerful fish predator. Longer threads have a similar advantage in this sense. that they can extend for greater distances between the gill arches of large predators. Longer wires are also more likely to produce larger volumes of mud, which improves its use as a deterrent against large predators. .

Future studies will focus on studying how each wire wraps such a complex structure in a tiny cell, according to Yu.

DOI: Current Biology, 2021. 10.1016 / j.cub.2021.08.066 (About DOIs).