How hagfish burrow in deep-sea sediments

Zoom in / Hexagonal hagfish (Eptatritus hexatrema) in False Bay, South Africa.

The humble hagfish is an ugly, grey, eel-like creature known for its ability to unleash a cloud of sticky slime on unsuspecting predators, clogging its gills and suffocating said predators. That’s why it’s affectionately called “Snake snot“Hagfish also like to burrow in deep-sea sediments, but scientists haven’t been able to observe precisely how they do it because the murky sediments obscure the view.” Researchers at Chapman University built a special tank of transparent gelatin to overcome this challenge and get a full picture of their 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 in soft sediment, but we had no idea how they did it.” said co-author Douglas Fudgea marine biologist who He heads the laboratory At Chapman he devoted himself to the study of hagfish. “By learning how to make hagfish voluntarily burrow into clear gelatin, we were able to get our first look ever at this process.”

As mentioned previously, scientists were Study of hagfish slime For years because it is an unusual material. It’s not like mucus, which dries up and hardens over time. The hagfish’s mucus remains sticky, giving it the consistency of semi-hardened gelatin. This is due to the presence of long, thread-like fibers in the slime, as well as the proteins and sugars that make up mucin, the other main component. These fibers are twisted into “tufts” that resemble balls of yarn. When a hagfish releases a dose of sticky substance, the threads unwind and combine with the salt water, exploding more than 10,000 times their original volume.

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From a materials standpoint, hagfish mucus is a fascinating material that may one day prove useful for biomedical devices, weaving light but strong fabrics for natural Lycra or bulletproof jackets, or lubricating industrial drills that tend to get clogged in deep soil and sediment. In 2016, a group of Swiss researchers He studied the properties of unusual liquids From the slime of hagfish, with particular emphasis on how those properties provide two distinct advantages: helping the animal defend itself from predators and tying itself into knots to escape its own slime.

Hagfish mucus is a non-Newtonian fluid and is unusual in that it is shear-thick and shear-thin in nature. Most hagfish predators use suction feeding, creating a thick, unidirectional shear flow that is better for blocking the gills and suffocating said predators. But if the hagfish needs to get out of its own gunk, its body movements create a thin flow, collapsing the network of sticky cells that make up the gunk.

It was bullshit Study of hagfish And the properties of its slime for years. For example, in 2012, when he was at the University of Guelph, Fudge’s lab Successfully harvested Hagfish mucus, dissolved in a liquid, then “spinned” into a strong but stretchy thread, much like spinning silk. These filaments could potentially replace petroleum-based fibers currently used in safety helmets or Kevlar vests, among other potential applications. And in 2021, his team Found it The slime produced by the larger hagfish contains much larger cells than the slime produced by the smaller hagfish – an unusual example of cell size increasing with body size in nature.

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Sedimentary solution

This time, Fudge’s team turned their attention to digging hagfish. In addition to shedding light on the reproductive behavior of hagfish, the research could also have broader ecological implications. According to the authors, burrowing is an important factor in sediment circulation, while burrow aeration changes the chemistry of the sediment so that it can contain more oxygen. This, in turn, would change the organisms that are likely to thrive in those sediments. Understanding burrowing mechanisms can also help in the design of soft burrowing robots.

Drilling sequence of a hagfish digging through clear gelatin.
Zoom in / Drilling sequence of a hagfish digging through clear gelatin.

D. S. Fudge et al., 2024

But first, Fudge’s team had to figure out how to see through the sediment to observe the burrowing behavior. Other scientists studying different animals have relied on transparent substrates such as mineral cryolite or gelatin hydrogels, the latter of which has been used successfully to observe the behavior of polychaete worms. Nonsense et al. She chose gelatin as a replacement for the sediment housed in three custom clear acrylic rooms. They then filmed the gelatin-burrowing behavior of 25 randomly selected hagfish.

This enabled Fudge et al. To identify two distinct phases of movement that hagfishes use to create their U-shaped burrows. First, there is the “pecking” phase, in which the hagfish swims vigorously while moving its head from side to side. This not only propels the hagfish forward, but also helps break the gelatin into pieces. This may be how the hagfish overcomes the challenge of creating an opening in the sediment (or gelatinous substrate) through which it can move.

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Next comes the “wriggle” phase, which appears to be supported by an “internal concertina” common in snakes. It involves forcefully shortening and elongating the body, in addition to exerting lateral forces on the walls to support and expand the burrow. “A snake using barbed movements will make steady progress through a narrow channel or burrow through alternating waves of elongation and shortening,” the authors wrote, and the loose skin of the hagfish is well suited for such a strategy. The evasion phase continues until the burrowing hagfish pokes its head out of the substrate. The hagfish took about seven minutes or more on average to complete its burrow.

Of course there are some caveats. The acrylic container walls may have influenced the burrowing behavior in the laboratory, or the final morphology of the burrows. The authors recommend replicating the experiments using sediment from natural habitats, and performing X-ray videography of hagfish implanted with radio tags to capture movements. Body size and substrate type may also affect burrowing behavior. But overall, they believe their observations “are an accurate representation of how hagfish grow and move within burrows in the wild.”

DOI: Journal of Experimental Biology, 2024. 10.1242/jeb.247544 (About digital IDs).

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