Hagfish, also known as slime eels, are eel-shaped, jawless fish belonging to the family Myxinidae of the class Myxini. They are part of the superclass Cyclostomi, which also includes lampreys (another group of jawless fish), and thus are collectively classified as cyclostomes.
Despite their resemblance to eels, hagfish are not true fish. They are among the most primitive vertebrates, possessing a flexible notochord instead of a true vertebral column for structural support. Their bodies are embedded with numerous slime glands, which secrete a viscous, elastic mucus when threatened. This mucus expands rapidly in water, forming a thick, suffocating barrier that can choke predators.
Approximately 70 species of hagfish exist worldwide, with the highest concentrations found in the North Pacific, Atlantic, and Southern Oceans. They typically burrow into muddy seafloor sediments, emerging to scavenge on polychaete worms, cephalopods, brittle stars, marine birds, and even the decaying flesh of sharks and whales.
Fossil evidence suggests that hagfish have remained relatively unchanged for millions of years, making them an important group in studying vertebrate evolution.
They are sexually dimorphic, with the females being considerably larger than the males. On average, hagfish grow to about 19.7 in (50 cm) in length. The largest species, the goliath hagfish (Eptatretus goliath), can reach up to 4 ft 2 in (127 cm), while smaller species, such as Myxine kuoi and Myxine pequenoi, measure around 7 in (18 cm). Some individuals have been recorded as small as 1.6 in (4 cm).
Hagfish have elongated, finless bodies that range in color from pink to blue-grey. Their distinctive features include six to eight whisker-like barbels surrounding the mouth, a single nostril, and a paddle-like tail. Their mouths are equipped with two pairs of horny, comb-shaped teeth attached to a cartilaginous plate, which helps them grasp food and direct it toward the pharynx.
Their soft, scaleless skin is loosely attached to the body, particularly along the center ridge of the back. Embedded within the skin are approximately 100 slime-producing glands, which secrete a milky, fibrous mucus through 90 to 200 pores along their sides. This mucus rapidly expands in water, forming a protective slime barrier against predators.
Hagfish have a true innervated heart (like jawed vertebrates), along with multiple accessory pumps or auxiliary hearts scattered throughout their bodies.
These fish draw water into their pharynx and channel it to their internal gill pouches, which number between 5 and 16, where gaseous exchange occurs. Though each of these pouches has a separate opening, in members of the genus Myxine, these openings merge into a single branchial opening on the underside. Additionally, the left branchial opening connects to the esophagus via a pharyngocutaneous duct, which helps remove large particles from the pharynx but does not directly aid in respiration.
Scientists have also proposed that these fish may be capable of subcutaneous respiration.
Their digestive system is unique since, unlike many vertebrates, where food moves freely through the gut, hagfish ensheath their food particles in a permeable membrane (similar to the peritrophic matrix found in insects). This membrane protects the digestive tract from potentially harmful or abrasive particles that might be present in the ingested food.
They have a mesonephric kidney, which drains through a mesonephric duct. Unlike in many vertebrates, this duct remains separate from the reproductive tract.
The proximal convoluted tubule of the nephron is connected with the coelom and is constantly washed by the celomic fluid.
Interestingly, these fish are the only known vertebrates that maintain the same salt concentration as their surrounding water (isosmotic).
Hagfish have brains similar to lampreys, and they lack myelin in their neurons. Their spinal nerves connect to the dorsal and ventral body muscles, while those that innervate the pharynx grow separately to reach the organ.
They are functionally blind. Unlike lampreys, which have well-developed eyes, these fish possess rudimentary eyes that lack a lens, extraocular muscles, and key motor cranial nerves (III, IV, and VI) found in most vertebrates. Additionally, they do not have a parietal eye, which is present in many other vertebrates. If eyespots are present, they have poor light detection ability and cannot form clear images.
In members of the genera Myxine and Neomyxine, the eyes are partially covered by trunk muscles, further limiting their function.
Both hagfish and lamprey constitute the superclass Cyclostomi, distinguished by keratinous tooth plates and movement of postotic myomeres to the orbitals. They are placed under the infraphylum Agnatha owing to the lack of jaws.
These two groups are estimated to have diverged during the Late Ordovician Period (~450 million years ago).
Currently, around 70 species of hagfish are divided into 3 subfamilies and 7 genera.
The only known fossil of these fish is Myxinikela siroka, dating back to the Pennsylvanian Period (around 323 to 298 million years ago).
Hagfish are found in most oceans of the world but are most abundant in the North Pacific, Atlantic, and Southern Oceans. They inhabit temperate and tropical waters at depths ranging from 53 to 3,788 ft (16 to 1,155 m). Mostly, these fish remain hidden in the crevices of the muddy seafloor.
These animals feed on a diverse range of food items, including polychaete worms (primary food), hermit crabs, shrimp, brittle stars, and cephalopods. They also feed on sea birds, bony fishes, sharks, and the flesh of whales.
As scavengers, they also feed on dead and decaying fish (a trait only rarely found in lampreys).
When perturbed, these fish release copious amounts of slime from the mucous glands along their body. Once released into the water, this slime expands to 10,000 times its original size in as little as 0.4 seconds. It is elastic and comprises numerous proteinaceous fibers, which aid in ensnaring potential predators. When a predatory fish attacks them, the released slime clogs the gills of the attacker, thereby asphyxiating it. Hence, the enemy is forced to set the hagfish free.
Even if a hagfish gets captured, it ties its body into an overhand knot and then slides the knot from its head to its tail to escape the grip of the predator.
Hagfishes usually tend to devour their prey from within. While scavenging, they often enter prey larger than themselves and eviscerate the inner fluids and tissues from their bodies. They then absorb the nutrients through their skin and gills.
In captivity, they pull out chunks of flesh from their prey using the overhand knot mechanism.
Although the exact lifespan of these animals is not well documented, estimates suggest that they live up to 40 years or more in the wild.
While some species, like the Atlantic hagfish, may reproduce seasonally, others, like Eptatretus burgeri, reproduce annually. The females typically lay between a single and 30 yolky eggs, which tend to aggregate owing to their tuft-like ends. Unlike lampreys, these eggs do not undergo a larva stage and instead directly transform into adults.
Hagfish embryos undergo a long development period, taking up to 11 months to hatch. However, this is relatively shorter compared to other jawless vertebrates.
Since these fish release mucus, only a few animals pester them. They are typically preyed upon by octopuses, sea birds, and marine mammals like sea lions, dolphins, and porpoises. Additionally, larger fish, like kitefin sharks and spiny dogfish, also consume hagfishes.
Humans feed on hagfish, particularly in countries like North and South Korea.
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