Hagfishes are marine, bottom-dwelling fishes that release large amounts of slime into the surrounding water when disturbed. The mass of slime consists of mucins (a type of protein with high water-holding capacity), long protein threads, and the seawater that these two interacting components temporarily trap amongst them.
The formation of this slime starts in glands lining the hagfish body, along which slime gland openings are visible as pores. Protein threads are initially tightly coiled into gland thread cells, resembling skeins of yarn, while mucins are held within vesicles (membrane-bound sacs) in gland mucous cells. Both types of cells are packed into each slime gland, which is surrounded by muscles. When the hagfish contracts these gland muscles, the thread cells and mucous cells are pushed out as a concentrated mixture through the gland duct. Travelling through the duct and actively mixing with seawater outside the gland opening cause the cells to burst and release their packaged contents into the water. The bundled protein threads unravel and the mucins from ruptured vesicles attach to them, and this network rapidly expands into a large, dilute mass of slime that temporarily holds water like a fine sieve.
Watch this video to see hagfish thread bundles spontaneously unravel in seawater.
Researchers believe that the slime functions as a defense against predators with gills, which can become clogged with the fibrous slime. Check out this underwater video footage demonstrating that hagfish can slime their way out of a predatory encounter.
Video from Bernards et al. 2014. J. Exp. Biol. 217:1263-1268
Video from Zintzen et al. 2011. Sci. Rep. 1:131
“Hagfishes are known for producing large volumes of slime when stressed. Their slime is believed to act as a defence mechanism against gill-breathing predators, as it has been shown to reduce water flow over the gills of fish. Hagfish slime is composed of two interacting components, slime thread skeins and mucin vesicles, which are both released from glands along the ventrolateral length of the animal. Each slime gland is surrounded by striated muscle and a connective tissue capsule, and contains large numbers of gland thread cells and gland mucous cells. Gland thread cells contain skeins of tightly coiled polymers rich in intermediate filaments, while gland mucous cells produce vesicles containing mucins, a class of glycoproteins. Both cell types rupture partially as they pass through the slime gland duct, causing each to lose its plasma membrane, and releasing both thread skeins and mucin vesicles into the external environment. The mucin vesicles are released by holocrine secretion rather than the more typical mechanism of mucus secretion through fusion of vesicles with the membrane of the mucous cell and release of mucin granules by exocytosis. In this way, the mucin vesicles remain intact until they come into contact with seawater in the external environment.
“The mature slime is formed when exudate released from the hagfish contacts convectively mixing seawater. Agitation during mixing causes the thread skeins to uncoil to lengths of 10–17 cm, providing a large surface area to which the mucins released from the ruptured vesicles can attach. The fully formed slime is a complex network capable of confining seawater to channels between the slime threads and ruptured mucins like a fine sieve. The interaction between the thread skeins and ruptured mucins is critical for the production of the mature slime.” (Herr et al. 2010:1092; in-line citations removed from quote)
“When agitated, Atlantic hagfish (Myxine glutinosa) produce large quantities of slime that consists of hydrated bundles of protein filaments and membrane-bound mucin vesicles from numerous slime glands. When the slime exudate contacts seawater, the thread bundles unravel and the mucin vesicles swell and rupture. Little is known about the mechanisms of vesicle rupture in seawater and stabilization within the gland, although it is believed that the vesicle membrane is permeable to most ions except polyvalent anions. We hypothesized that the most abundant compounds within the slime gland exudate have a stabilizing effect on the mucin vesicles. To test this hypothesis, we measured the chemical composition of the fluid component of hagfish slime exudate and conducted functional assays with these solutes to test their ability to keep the vesicles in a condensed state. We found K+ concentrations that were elevated relative to plasma, and Na+, Cl– and Ca2+ concentrations that were considerably lower. Our analysis also revealed high levels of methylamines such as trimethylamine oxide (TMAO), betaine and dimethylglycine, which had a combined concentration of 388 mmol l–1 in the glandular fluid. In vitro rupture assays demonstrated that both TMAO and betaine had a significant effect on rupture, but neither was capable of completely abolishing mucin swelling and rupture, even at high concentrations. This suggests that some other mechanism such as the chemical microenvironment within gland mucous cells, or hydrostatic pressure is responsible for stabilization of the vesicles within the gland.” (Herr et al. 2010:1092)
“…[hagfish] slime originates as a two-component glandular exudate comprised of coiled bundles of cytoskeletal intermediate filaments (thread skeins) and mucin vesicles…Deployment of the thread skeins involves their unraveling in a fraction of a second from a 150 µm-long ellipsoid bundle to a thread that is 100x longer. We hypothesized that thread skein deployment requires both vigorous hydrodynamic mixing and the presence of mucin vesicles, both of which are required for whole slime deployment. Here we provide evidence that mixing and mucin vesicles are indeed crucial for skein unraveling.” (Winegard & Fudge 2010:1235)
“Our video deployments confirmed the hypothesis that hagfish use slime secretion as an effective defence mechanism against predation. The footage we obtained showed that slime can affect gill-breathing predators by clogging gills, likely by increasing the resistance to water flow.” (Zintzen et al. 2011:2)
“What triggers the slime secretion is likely to be direct skin stimulation by a predator. Active slime secretion was not observed when the potential predator approached the hagfish, but only began when the predator either tried to bite or engulf the hagfish. The mechanism of localised control and coordination of slime glands, as observed in our video footage, has been documented in laboratory studies and was highly effective to allow a full escape. It was also fast enough to prevent any injury to the hagfishes.” (Zintzen et al. 2011:3)