Mucin Strands Enable Rapid Slime Formation

close up underwater photograph of hagfish

Biological Strategy

Atlantic hagfish

Jeanette Lim

Image: NOAA Okeanos Explorer Program / NOAA / CC BY - Creative Commons Attribution alone

Distribute Solids

Solids include resources such as food, seeds, spores, and leaves, which living systems must sometimes move to accomplish various activities such as pollinating, feeding their young, or building shelters. To effectively distribute solids, living systems must use strategies appropriate to the sizes and types of solids and the media in which they exist (air, water, or another medium). This may entail strategies beyond simply holding and carrying solids. For example, some seeds are carried by the wind to new locations for germination. Others–like the burdock plant’s hooked seeds–cling to animal fur to be transported.

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Fish

Class Agnatha (“without jaws”), Class Chondrichthyes (“cartilage fish”), Superclass Osteichthyes (“bone fish”): Sharks, eels, snapper, hagfish

The fish are a diverse group, comprising multiple classes within Phylum Animalia. The most well-known classes are Chondrichthyes, which has sharks and rays, and superclass Osteichthyes, which has all bony fish like cod and tuna. Unlike other vertebrates, fish only live in water. They use special adaptations like fins, gills, and swim bladders to survive. Most are ectothermic, meaning their body temperature depends on the water temperature around them. Over half of all vertebrates are fish. They’re found from the bottom of the sea to high mountain lakes.

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Mucin strands in hagfish slime enable rapid slime formation by transmitting forces from flowing water.

Hagfishes are marine fishes that are infamous for their slime. When threatened by a biting predator, a hagfish responds by releasing small amounts of thick slime concentrate out of s lining its body (see the related strategy here). This slime concentrate then explodes into a ~1L volume of dilute slime in a fraction of a second. What enables this rapid slime formation?

Mucins are and carbohydrate molecules that partly make up mucus. Hagfish slime contains thin protein threads interspersed in a dilute mucus. In the slime concentrate, those protein threads are initially wound up into 150 μm-long bundles (skeins). Part of the rapid slime expansion in seawater involves those skeins unraveling into threads 100 times longer. For the Atlantic hagfish, Myxine glutinosa, these threads won’t deploy on their own: active mixing and mucins are needed to enable thread unraveling. Hagfish slime mucins are contained within membrane-bound vesicles that are ~7 μm long, and these vesicles are released along with the thread skeins in the slime concentrate.

Image: Jeanette Lim / CC BY SA - Creative Commons Attribution + ShareAlike

As hagfish slime is stretched, the unraveled threads become visible.

One leading model for how whole slime forms in the Atlantic hagfish proposes that mucins swell and elongate into sticky strands when they come into contact with seawater. Ions and water rushing into the mucin vesicles cause them to swell, while fluid flow forces cause them to stretch. The mucin strands then attach to the thread skeins at multiple points, creating interconnected networks. The mucins are malleable enough to stretch and strong enough to withstand and transfer pulling forces between the flowing water and thread skeins. The threads, which are still wound up when mucin attaches, are unravelled by disruptive forces transmitted from the actively mixing water by the mucin strands.

Biologists are still uncovering how slime forms in other species of hagfish. In the Atlantic hagfish’s Pacific relative, Eptatretus stoutii, thread skeins can spontaneously unravel on their own. As vigorous thrashing often accompanies sliming during a predator attack, however, most hagfish naturally provide the mixing needed to form a large mass of slime.

Last Updated April 20, 2018

References

“…we propose a new mechanism of hagfish slime deployment in which elongated mucin strands transmit mixing forces to thread skeins to initiate their unraveling. According to this ‘mucin transmission hypothesis,’ hagfish slime deployment occurs via the following sequence of events: (1) expulsion of the slime exudate into convectively mixing seawater results in the swelling and subsequent elongation of mucin vesicles to form mucin strands, (2) these elongated mucin strands attach to the thread skeins, (3) the mucin strands transmit the hydrodynamic forces of mixing to the thread skeins, thereby initiating unraveling, and (4) entanglement of the threads and mucin strands results in the complete unraveling of thread skeins. The whole slime is therefore a highly complex network of mucin strands, slime threads and seawater.” (Winegard and Fudge 2010:1240)

Journal article