Author: Lee Jean
Institution: Integrative Biology
Date: April 2002
Hagfish are among the many curious organisms living in the deep oceanic benthos. At first glance, the hagfish's dark, quiet environment appears extremely stable and unchanging - even quite dull. It's not always so still, though. Scavenging brittle stars may stampede over the soft silt mounds created by creatures that live within the sediments, destroying the mound and possibly trapping the inhabitant beneath the mix of silt and mud. "Benthic storms" of sudden water flow can occur from either falling objects or large marine fauna. Sinking human vessels or gigantic food falls such as whale carcasses can cause large disturbances.
Looking closer, one might discover an alarming sight: Those dead organisms resting on the deep sea floor are actually pulsating! What could cause such movements? Usually, it's a passel of scavenging hagfish feeding on the carcasses from the inside out.
The muddy bottom of the deep sea can seem quite creepy, and hagfish only enhance this impression. In the deep sea's high-salinity and high-pressure environment, there is little light, except that coming from eerie bioluminescent creatures who make their livings in complete darkness. Water flow over the soft silt is nearly nonexistent. The pointy tracks of polychaete worms remain for long periods of time, and even the indentations left by deep-sea submarines can be identified months later. Hagfish favor the soft muck of these highly saline waters, and tend to burrow in the mud or under rocks; they cannot withstand bright light or warm water. In fact, hagfish are mostly found near the mouths of rivers or at depths of 25 m or more, with the deepest-dwelling species (Myxine circifrons) found at more than 1000 m below the surface.
Primary productivity (plant growth) does not occur in the deep sea. Rather, organisms subsist upon particulate organic matter, the "marine snow" that slowly falls from the ocean's surface, or survive as lowly deposit feeders, swallowing 99% inedible debris for every 1% of edible organic substance. The natural foods of hagfish include worms and benthic infauna (creatures that live within seafloor sediments) or epifauna (creatures that live upon the sea floor), but hagfish are more infamous as voracious scavengers. Preferring the soft, inner flesh of dead and decaying marine life, hagfish can actually burrow into an organism's body and devour it from the inside out, resulting in the pulsating appearance of the dead body.
"Hagfish should be called fish' with the knowledge that they are only distantly related to the world's fishes," says Dr. Steven Webster, senior marine biologist at the Monterey Bay Aquarium in California and 24-year veteran of invertebrate zoology. According to Dr. Webster, hagfish are "an ancient group that branched off from the chordate line probably before the vertebrate column appeared in the tree of life." Webster appreciates hagfish for their evolutionary and taxonomic heritages. As members of the class Agnatha and order Cyclostomata, they are one of a handful of jawless fishes with odd sucker-shaped mouths. Agnathids such as lampreys and hagfish appear quite worm-like, but they are actually scaleless, chordate fish (embryologically related to vertebrates and others in the phylum Chordata). The hagfish at the Monterey Bay Aquarium reside in the "Mysteries of the Deep" exhibit, where they lie happily curled on the bottom next to a fake fish carcass.
Hagfish have yet another curious feature: They produce a viscous white slime. While lampreys do not have slime glands, hagfish have 75 to 100 glands from head to cloaca, with ducts that release thick globs of exudate when the hagfish is disturbed. Hagfish slime, being thick and sticky, is known to be extremely unfriendly to the touch and can appear shockingly suddenly. Moreover, seawater enhances the viscosity and thickness of the slime. It is said that when you have one bucket of angry hagfish, you will soon need another bucket for the overflowing slime.
Hagfish produce slime via two pathways: holocrine (slime glands) and merocrine (epidermal, through the skin). The scaleless hagfish epidermis consists of three cell types, all tightly knitted together with collagen fibers to form a flexible grayish "sheath": small mucous cells, epidermal thread cells, and large mucous cells. An extensive capillary network supports the dispersal of bio-products quickly and effectively, with capillary loops forming ball-like complexes around each slime gland.
Slime production occurs when the hagfish is alarmed or disturbed. Gland cells, which include gland mucous cells (GMCs) and gland thread cells (GTCs), rupture and release their contents to the outside seawater. GTCs release intermediate filament (IF) biopolymer, resulting in IF polypeptide sequences which are similar to skin keratins. GMCs expel mucins. The IF threads and mucins interact with each other, and have a high capacity for absorbing water. Following contact with seawater, the aggregate forms thick cables, not unlike the pre-ovulatory cervical mucous that protects the human uterus. Water is then expelled and the formation of the slime mass is complete.
Other than for individual defense and for frightening humans, the slime may be used to deter predators from hagfish nest eggs. Hagfish also defend themselves by tying their bodies into knots, but this may also help to wipe off any excess slime from their eyes and nostrils. Sneezing also aids this process. If a pulsating mass in the benthos sneezes, it is certain that hagfish are hiding inside!
Dr. Webster himself has experienced what he calls, "the gooiest, gloppiest, most amazing slime," during his study of ichthyology in the 1960s. Having completed research on these fascinating creatures long ago, Webster remarked that they are very long-lived animals, a fact discovered through research at the Monterey Bay Aquarium Research Institute (MBARI). The specimens currently residing at Monterey Bay Aquarium have not been replaced since they were first obtained years ago.
Hagfish have also been known to attack and suck the flesh of hooked or netted fish. This leaves a jagged hole in the hapless victim, a horrid sight that might suggest to some that hagfish are ugly, nasty animals. However, hagfish are also active predators in addition to scavengers. Although not beautiful creatures, it's somehow reassuring to know they are not only found within rotting carcasses.
Presently there is a significant interest in the use of hagfish in scientific study and in the economy. Hagfish have proven useful in the study of tumors. The cooperation of body parts in producing slime (essentially polypeptides) is also important in examining functionality, morphology, and metabolic actions.
A great commercial value is placed upon hagfish, especially in Korea, where hagfish meat is eaten. Skins are fashioned into "eel" leather items as well; however, due to their low fecundity and turnover rate, hagfish may be in danger of depletion from over fishing. Some possible future uses of hagfish slime include use as a replacement for egg whites in cooking, and oils in various ointments. Imagine frying up some bacon and slime for breakfast!
Hagfish do not have a cerebellum or eye muscles; however, to deem these animals helpless and dumb based on poor visual ability is wrong. As chemosensory and mechanosensory specialists, hagfish possess extraordinary sensitivity and compound sensory organs. An elaborate scheme of free nerve endings innervates their skin. The nervous system contains axons of different diameters for varying nervous signal speeds. Hagfish swim with ease and flexibility, using gliding, "figure eight," and vermiform (worklike) motion. They also possess a lateral line system and an advanced chemosensory system with Schreiner organs, which resemble taste buds.
General human disgust with these creatures is apparent in the generally low level of interest in hagfish. But perhaps one day, with more research, hagfish will enable us to cure cancers of the skin. Or, the slime may indeed end up as a favorite egg substitute. Then, fisheries management will realize the importance of maintaining their populations, and we will all have a bit more interest in the lowly hagfish.
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Pers Comm. Steven K. Webster, Ph.D. Senior Marine Biologist, Monterey Bay Aquarium 886 Cannery Row Monterey, CA 93940