The humble hagfish is an ugly, gray eel-like creature best known for its ability to release a cloud of sticky slime at unsuspecting predators, plugging its gills and suffocating said predators. This is why it is affectionately known as the “snake snake”. Hagfish also like to burrow into deep-sea sediment, but scientists haven’t been able to observe exactly how they do this because the murky sediment obscures the view. Chapman University researchers built a special tank of transparent gelatin to overcome this challenge and get a complete picture of burrowing behavior, according to a new paper published in the Journal of Experimental Biology.
“We’ve known for a long time that hagfish can burrow into soft sediments, but we had no idea how they did it,” said co-author Douglas Fudge, a marine biologist who leads the lab at Chapman that studies hagfish. “When we figured out how to get the hagfish to voluntarily burrow into the transparent gelatin, we were able to get the first ever insight into this process.”
As previously reported, scientists have been studying hagfish slime for years because it’s such an unusual material. It’s not like mucus that dries up and hardens over time. Hagfish slime remains slimy, giving it the consistency of semi-solidified gelatin. This is due to the long fiber-like fibers in the mucus, in addition to the proteins and sugars that make up mucin, the other main component. These fibers are coiled into “skeins” that resemble balls of yarn. When the hagfish is released with a shot of slime, the skeins unfurl and combine with the salt water, inflating to more than 10,000 times their original size.
From a materials perspective, hagfish slime is a fascinating thing that could one day prove useful for biomedical devices, or for weaving light but strong fabrics for natural Lycra or bulletproof vests, or for lubricating industrial drills that tend to clog in deep soil and sediments. In 2016, a group of Swiss scientists studied the unusual liquid properties of hagfish slime, specifically looking at how these properties provided two distinct advantages: helping the animal defend itself from predators and tying itself into knots to escape from its own slime.
Hagfish slime is a non-Newtonian fluid and is unusual in that it is also shear-thickening in nature. Most hagfish predators use suction feeding, which creates a unidirectional stream of shear, the better to clog the gills and suffocate said predators. However, if the hagfish needs to get out of its own slime, its body movements create a shear-weakening current, collapsing the slimy web of cells that make up the slime.
Fudge has been studying the hagfish and the properties of its slime for years. For example, in 2012, when he was at the University of Guelph, Popletal’s lab successfully harvested slime from hagfish slime, dissolved it in a liquid, and then “spun” it into a strong but flexible thread, similar to spinning silk. It is possible that such fibers could replace petroleum-based fibers currently used in hard hats or Kevlar vests, among other potential applications. And in 2021, his team found that slime produced by larger hagfish contained much larger cells than slime produced by smaller hagfish—an unusual example of cell size scaling with body size in nature.
Sedimentary solution
This time, Fudge’s team turned their attention to burrowing hagfish. In addition to elucidating hagfish reproductive behavior, the research could also have broader ecological implications. According to the authors, the burrow is an important factor in sediment turnover, while venting the burrow changes the chemistry of the sediment so that it can contain more oxygen. This in turn would change which organisms are likely to thrive in that sediment. Understanding burrowing mechanisms could also help in the design of soft robots.
DS Fudge et al., 2024
But first Popletal’s team had to figure out how to see through the sediment to observe burrowing behavior. Other scientists studying various animals have relied on transparent substrates such as the mineral cryolite or hydrogels made of gelatin, the latter of which has been successfully used to observe the behavior of polychaete worms. Fudge et al. chose gelatin as a sediment substitute placed in three custom transparent acrylic chambers. They then filmed the behavior of 25 randomly selected hagfish while burying the gelatin.
This allowed Fudge et al. identify the two distinct phases of movement that hagfish use to create their U-shaped burrows. First, there is the “thrash” phase, in which the hagfish swims vigorously and moves its head from side to side. This not only serves to propel the hagfish forward, but also helps chop the gelatin into pieces. This could be how hagfish overcome the problem of creating a hole in the sediment (or gelatinous substrate) through which to move.
Next comes the “wriggling” phase, which seems to be driven by the “internal harmonic” common to snakes. It involves shortening and strongly stretching the body, as well as exerting lateral forces on the walls to strengthen and widen the burrows. “A snake using accordion movements will move smoothly through a narrow channel or burrow in alternating waves of lengthening and shortening,” the authors wrote, and the hagfish’s loose skin is well suited for such a strategy. The wriggling phase lasts until the rake sticks its head out of the substrate. Hagfish took about seven minutes or more on average to complete their burrows.
Naturally, there are a few caveats. The walls of the acrylic containers may have affected burrow behavior in the laboratory or the final shape of the burrows. The authors recommend repeating the experiments using sediments from the natural environment, introducing X-ray videography of hagfish implanted with radiomarkers to capture movement. Body size and substrate type can also influence burrowing behavior. But overall, they believe their observations “are an accurate representation of how hagfish burrow and move in the wild.”
DOI: Journal of Experimental Biology, 2024. 10.1242/jeb.247544 (About DOI).