The humble hagfish is an ugly, gray eel-like creature best known for its ability to release a cloud of sticky slime onto unsuspecting predators, clogging the gills and choking said predators. This is why it is affectionately called “snot snake”. Hagfish also like to burrow into deep-sea sediments, but scientists have not been able to observe precisely how they do this because the murky sediment obscures the view. Researchers at Chapman University built a special tank with transparent gelatin to overcome this challenge and obtain 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 didn’t know how they do it,” said co-author Douglas Fudge, a marine biologist who runs a lab at Chapman devoted to the study. hagfish. “By figuring out how to get the hagfish to voluntarily burrow into the clear gelatin, we were able to get a first glimpse of this process.”
As previously noted, scientists have been studying hagfish slime for years because it is a very unusual material. It’s not like mucus, which dries and hardens over time. Hagfish slime remains viscous, giving it the consistency of half-solidified gelatin. This is due to the long, threadlike fibers in slime, in addition to the proteins and sugars that make up mucin, the other major component. These fibers roll up into “skeins” that resemble balls of wool. When the hagfish unleashes a stream of slime, the skeins unfurl and combine with the salt water, swelling more than 10,000 times their original size.
From a materials perspective, hagfish slime is a fascinating substance that could one day prove useful for biomedical devices, or for weaving lightweight but strong fabrics for natural Lycra or body armor, or for lubricating industrial drills that tend to get clogged in deep soil and sediments. In 2016, a group of Swiss researchers studied the unusual fluid properties of hagfish slime, focusing specifically on how these properties provided two distinct benefits: helping the animal defend itself against predators and knotting itself to escape to his own drool.
Hagfish slime is a non-Newtonian fluid and is unusual in that it is both thickening and shear thinning in nature. Most hagfish predators use suction feeding, which creates a unidirectional shear-thickening flow, to better obstruct the gills and suffocate said predators. But if the hagfish needs to get out of its own slime, its body movements create a thinning flow, collapsing the slimy network of cells that makes up the slime.
Fudge 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 successfully harvested hagfish slime, dissolved it in a liquid, and then “spinned” it into thread. strong but stretchy, much like spun silk. It is possible that these threads could replace petroleum-based fibers currently used in safety helmets or Kevlar vests, among other potential applications. And in 2021, his team discovered that the slime produced by larger hagfishes contains much larger cells than the slime produced by smaller hagfishes – an unusual example of cell size changing with body size in nature.
A sedimentary solution
This time around, Fudge’s team turned their attention to the hagfish digs. As well as shedding light on hagfish reproductive behavior, the research could also have wider ecological implications. According to the authors, burrowing is an important factor in sediment turnover, while ventilation of the burrow changes the chemistry of the sediment such that it could contain more oxygen. This in turn would change the organisms that could thrive in these sediments. Understanding burrowing mechanisms could also help in the design of soft burrowing robots.
DS Fudge et al., 2024
But Fudge’s team first had to figure out how to see through the sediment to observe burrowing behavior. Other scientists studying different animals have relied on transparent substrates like the mineral cryolite or gelatin-based hydrogels, the latter of which have been used successfully to observe the burrowing behavior of polychaete worms. Fudge et al. opted for gelatin as a sediment replacement housed in three custom transparent acrylic chambers. Next, they filmed the gelatin-burrowing behavior of 25 randomly selected hagfish.
This allowed Fudge et al. to identify two distinct phases of movement that the hagfish used to create its U-shaped burrows. First there is the “thrash” stage, during which the hagfish swims vigorously while moving its head from side to side. the other. This not only serves to propel the hagfish forward, but also helps chop the gelatin into pieces. This may be how the hagfish overcomes the challenge of creating an opening in the sediment (or gelatin substrate) through which to move.
Next comes the “squirming” phase, which appears to be powered by an “internal accordion” common to snakes. This involves the forced shortening and elongation of the body, as well as the exertion of lateral forces on the walls to strengthen and widen the burrow. “A snake using accordion movements will progress steadily 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 to such a strategy. The writhing phase lasts until the burrowing hagfish raises its head from the substrate. The hagfish took on average about seven minutes or more to complete its burrow.
There are, of course, some caveats. The walls of the acrylic containers may have affected laboratory burrowing behavior or the final shape of the burrows. The authors recommend repeating the experiments using sediments from the natural habitat, implementing X-ray videography of hagfish implanted with radiotags to capture movements. Body size and substrate type can also influence burrowing behavior. But overall, they think their observations “are an accurate representation of how hagfish create and move through their burrows in the wild.”
DOI: Journal of Experimental Biology, 2024. 10.1242/jeb.247544 (About DOIs).