Even though hagfish are titled as 'fish', they do not fulfill several conventional characteristics that are commonly associated with fish species. They are also known by many names like 'jaw-less fish' or 'slime eels', despite the fact that they are not eels. In fact, their classification as fish is also debatable.
Average hagfish have a length of about half a meter, which is equivalent to 18 inches. The longest ever recorded hagfish was an Eptatretus goliath species, which measured about 127 cm. Other species that grow up to a rather significant length include the Myxine kuoi and Myxine pequenoi. For an animal that is classified as a fish, the hagfish does not look like a fish at all.
It is their physical appearance that has earned them the nickname, 'slime eel'. Their body is large and elongated just like that of an eel. These fish have a group of 4 hearts and are powered by 2 brains. Their skeleton has a wired construction. These fish are the only animals which have a skull, but do not have a vertebra (spine). Their skulls are cartilaginous in nature, where Keratin based tooth-like structures are attached for consumption of food.
The skin color differs from species to species. It often ranges from pink to blue-gray. The eyes are not very well-developed and thus, they come across as very simple eye spots. The underdeveloped eyes or rather, eye spots cannot revolve or resolve complex images, and that leaves the species as good as blind.
The sense of smell, on the other hand, is very well-developed. In the absence of fins, locomotion is facilitated by six to eight barbels that surround a single nostril. Their locomotion is facilitated by their paddle-shaped tails. Hagfish do not have jaws like other species of fish. Instead, these fish have two structures that move horizontally. Their tentacles have a tooth-like structure attached to the end that helps them pull pieces of food. Interestingly, they can survive for months without food.
The circulatory system of hagfish is not well-developed either, and is similar to the respiration of worms. It is made up of open and closed blood vessels. The 4 hearts are brachial in nature and act as pumps. The first three hearts fulfill the function of the circulation of blood. The fourth heart, on the other hand, is used to circulate blood between the intestines and livers.
While studying hagfish anatomy, it is important to study their defense mechanism. As compared to other marine animals, their defense mechanism is very different. If a hagfish is in danger or is caught by a predator, it secretes a huge amount of mucus. This mucus reacts with water and forms a micro-fibrous slime, which in turn makes its body very slippery. It confuses the predator, thus giving the fish ample time to escape. Hagfish get rid of the slime on their body by tying themselves into a knot and again slowly coming back to their initial position. An average adult hagfish is capable of secreting enough mucus to produce 20 liters of water slime.
Though the hagfish is often considered gorse and disgusting creature, its study has helped marine scientists to know more about ancient and prehistoric marine life. That can be attributed to the fact that the anatomy of hagfish is very similar to that of prehistoric species. In fact, many of their organs―their eyes for instance―have remained primitive and underdeveloped even today.