How hagfishes defend themselves

This video says about itself:

Attacking Shark Gagged by Slime | National Geographic

January 12, 2012 — For the first time, scientists have recorded the defense abilities of the hagfish, which secretes slime from hundreds of pores across its body when attacked. In this video you can see sharks and other predators gagged by the slime as they try to eat the hagfish.

From the University of Wisconsin-Madison in the USA:

Unraveling threads of bizarre hagfish’s explosive slime

January 15, 2019

Summary: Biologists have modeled the hagfish’s gag-inducing defense mechanism mathematically.

Hundreds of meters deep in the dark of the ocean, a shark glides toward what seems like a meal. It’s kind of ugly, eel-like and not particularly meaty, but still probably food. So the shark strikes.

This is where the interaction of biology and physics gets mysterious — just as the shark finds its dinner interrupted by a cloud of protective slime that appeared out of nowhere around an otherwise placid hagfish.

Jean-Luc Thiffeault, a University of Wisconsin-Madison math professor, and collaborators Randy Ewoldt and Gaurav Chaudhary of the University of Illinois have modeled the hagfish‘s gag-inducing defense mechanism mathematically, publishing their work today in the Journal of the Royal Society Interface.

The ocean-dwelling hagfish is unique for all the strangest reasons. It has a skull, but no spine or jaw. Its skin hangs loose on its body, attached only along the back. Its teeth and fins are primitive, underdeveloped structures best described with qualifiers — “tooth-like” and “fin-like.”

But it has an amazing trick up that creepy, loose sleeve of skin: In the blink of an eye (or flash of attacking tail and teeth) the hagfish can produce many times its own body’s volume in slime. The goop is so thick and fibrous, predators have little choice but to spit out the hagfish and try to clear their mouths. “The mouth of the shark is immediately chock full of this gel,” Thiffeault says. “In fact, it often kills them, because it clogs their gills.”

The gel is a tangled network of microscopic, seawater-trapping threads unspooled from balls of the stuff ejected from glands along the hagfish’s skin. These “skeins” are just 100 millionths of a meter in diameter (twice the width of a human hair), but so densely coiled that they can contain as much as 15 centimeters of thread. Curious scientists have looked at the unraveling before, putting the skeins in salt water to see how long it took them to come apart.

“The hagfish does it in less than half a second, but it took hours of soaking for the threads to loosen up in experiments,” says Thiffeault, whose research is focused on fluid dynamics and mixing. “Until they stirred the water, and it happened faster. The stirring was the thing.”

The slime modelers set out to see if math could tell them whether the forces of the turbulent water of a bite-and-shake attack were enough to unspool the skeins and make the slime, or if another mechanism — like a chemical reaction providing some pop to the skein — was required.

Ewoldt, a mechanical engineering professor, and his graduate student Chaudhary began unraveling skeins under microscopes, watching the process as loose ends of thread stuck to the tip of a moving syringe and trailing lengths spun out from the ball.

“Our model hinges on an idea of a small piece that’s initially dangling out, and then a piece that’s being pulled away,” says Thiffeault. “Think of it as a roll of tape. To start pulling tape from a new roll, you may have to hunt for the end and pick it loose with your fingernail. But if there’s already a free end, it’s easy to catch it with something and get going.”

Unrolling requires a big enough difference between the drag on the free end and an opposing push on the skein — a ratio larger than a tipping point the researchers refer to informally as the “peeling number” — to free more thread.

“That’s unlikely to happen if the whole thing is moving freely in water,” says Thiffeault. “The main conclusion of our model is we think the mechanism relies on the threads getting caught on something else — other threads, all the surfaces on the inside of a predator’s mouth, pretty much anything — and it’s from there it can really be explosive.”

It doesn’t even have to be a single snag.

“Biology being the way it is, it doesn’t have to be exact. Things get to be messy,” says Thiffeault. “That leading bit of thread can get caught a little bit, then slip, then get caught again. As long as it’s happening to enough skeins, it’s pretty fast that you’re in the slime.”

The skeins may get a boost from mucins, proteins found in mucus that could speed the breakup of packed thread, “but those kinds of things would just help the hydrodynamics,” says Thiffeault, who once calculated the extent to which swimming marine life mix entire oceans with their fins and flippers.

“It’s just hard to imagine there’s another process other than hydrodynamic flow that can lead to these timescales, that burst of slime,” he says. “When the shark bites down, that does create turbulence. That creates faster flows, the sorts of things that provide the seed for these things to happen. Nothing is going to happen as nicely as in our model — which is more of a good start for anyone who wants to take more measurements — but our model shows the physical forces play the biggest role.”

The hydrodynamics of hagfish slime is not just a curiosity. Understanding the formation and behavior of gels is a standing issue in many biological processes and similar industrial and medical applications.”

One of the things we’d love to work on in the future is the network of threads. I love thinking about modeling materials as big random collections of threads,” Thiffeault says. “A simple model of entangled threads may help us see how that network determines the macroscopic properties of a lot of different, interesting materials.”

A grant from the National Science Foundation (CBET-1342408) helped support this research.

This 15 January 2019 video says about itself:

Unraveling hagfish slime

Researchers unravel a tiny ball (called a “skein”) of microscopic thread produced by glands along the side of the eel-like hagfish. In less than half a second, tens of thousands of the threads can tangle, trapping seawater in a slimy gel that chokes attacking predators.

These skeins are just 100 millionths of a meter in diameter (twice the width of a human hair), but so densely coiled that they can contain as much as 15 centimeters of thread.

UW-Madison mathematician Jean-Luc Thiffeault and University of Illinois materials scientists Gaurav Chaudhary and Randy Ewoldt modeled the unraveling of skeins to show that sticking threads pulled by moving water and thrashing fish could indeed produce clouds of slime in less than a second.


Elephantnose fish, electrical fish

This 14 January 2019 video from the Natural History Museum in London, England says about itself:

Elephantnose fish are usually found in murky waters. Although it would be difficult for us to find our way around in their preferred habitat, these fish have developed a special sense that helps them see.

Most dangerous marine animals, video

This 12 January 2019 video says about itself:

In this Blue World Academy, Jonathan talks about the most dangerous animals in the sea–and sharks are not even on the list! Animals like the flower urchin, blue-ringed octopus, cone shell, box jellyfish, stonefish and sea snake are extremely dangerous sea creatures!

Young sharks, old sharks, who’s toughest?

This 2016 video says about itself:

A Port Jackson Shark egg washed up on Semaphore beach, South Australia with baby shark inside. Baby released back into the water and swims away.

From Florida Atlantic University in the USA:

Who’s tougher? Baby sharks or daddy sharks?

Study examines mechanical behavior of sharks‘ vertebrae under biological conditions

January 3, 2019

Summary: One would assume that since humans and many animals tend to get stiffer and perhaps tougher as they reach adulthood, the same would be true for sharks. A new study finds the opposite in these swift-swimming marine predators. The youngest sharks were stiffer and tougher than older sharks. Another key finding is that while scientists have historically looked at alternating patterns of mineralization on sharks‘ vertebrae to determine their age, these patterns are not related to time.

It’s not just their teeth and jaws that people find intriguing. It’s also their funky shapes and unique skeletal makeup that capture attention. Unlike humans and most land animals, sharks have mineralized cartilage skeletons instead of bones. This allows them to move at unbelievable speeds through the water. Since cartilage weighs less than bone and is less dense, sharks can bend, swim, and maneuver in the ocean much differently than their bony fish counterparts.

Because sharks vary in size and shape, there is great diversity in their morphology, physiology and how they swim. For example, the common thresher shark relies on its tail to stun prey when feeding, and the size of its vertebrae and their mechanics may explain why it depends on a strong and long tail that operates like a whip. To move this way requires low stiffness and toughness, or a lower resistance to deformation and ability to absorb energy, respectively.

A shark’s vertebral column is governed by dynamic and complex interactions among tissue composition and morphology, and there are many differences in growth, mineralization and mechanical properties.

Scientists from Florida Atlantic University’s Charles E. Schmidt College of Science and the National Marine Fisheries Service, National Oceanic and Atmospheric Administration (NOAA), predicted that the solid central part of the vertebrae in mature, older sharks would be stiffer and tougher. So they decided to put their theory to a test.

They examined cartilage mechanics from six species of sharks under biologically relevant conditions along the length of their bodies and over a range of ages. They looked at small, infant (young of year) sharks, immature sharks and mature sharks from each of six different species: the dusky shark (Carcharhinus obscurusp); the porbeagle (Lamna nasus); and charismatic sharks like the great white shark (Carcharodon carcharias); the shortfin mako (Isurus oxyrinchus) and the common thresher shark (Alopias vulpinus).

For the study, the researchers divided the sharks’ bodies into two regions: anterior (pectoral fin insertion and first dorsal fin origin) and posterior (second dorsal fin origin and pre-caudal pit). They conducted mechanical testing, used an imaging technique called X-radiography, and evaluated the relationship between stiffness and toughness using a simple linear regression.

One would assume that since humans and many animals tend to get stiffer and perhaps tougher as they reach adulthood, that the same would be true for sharks. Much to their surprise and contrary to their hypothesis, the researchers discovered the opposite in these swift-swimming marine predators.

Results of the study, published in the Journal of Experimental Biology, show that the youngest sharks were stiffer (able to resist compression) and tougher (able to absorb more energy) than older sharks. The researchers speculate that cartilage from younger sharks has fewer “interruptions” in the mineral matrix within the cartilage. They also discovered that the cartilage was stiffer and tougher in the posteriorly-located vertebrae (toward the back of the body), suggesting that this body region is better equipped to handle the mechanical loading that occurs during swimming.

In addition, although scientists have historically looked at alternating patterns of mineralization on sharks’ vertebrae to determine their age, a key finding from this study reveals that these patterns are not related to time.

“Our results suggest that toughness and stiffness, which are positively correlated, may be operating in concert to support lateral body undulations, which is how a shark moves its body and tail from side-to-side to propel itself forward, while providing efficient energy transmission and return in these swift-swimming apex predators,” said Marianne E. Porter, Ph.D., co-author of the study, an assistant professor in FAU’s Department of Biological Sciences, and director of the Biomechanics Laboratory in the Charles E. Schmidt College of Science.

Porter worked with graduate student and first author of the study Danielle I. Ingle; and Lisa J. Natanson, Ph.D., co-author, Apex Predators Program, National Marine Fisheries Service, NOAA.

“These comparative data from our study really highlight the importance of better understanding cartilaginous skeleton mechanics under a wide variety of loading conditions that are representative of the swimming behaviors that we see in the wild,” said Ingle.