Imagine an organism that has four hearts, one nostril, no eyes, no stomach but with teeth on its tongue! This may seem like a creature from another world, but a fish with all these attributes exists. It is the hagfish, one of the most primitive fish on our planet.
Hagfish are scavengers that usually burrow their head into dead or injured sea creatures to eat their flesh and internal organs. Using the teeth on their tongue, they often even enter their victims and devour them from inside out.
The most surprising thing about hagfish is that they can loop themselves into a knot which they press against the body of their prey, as shown in the figure. Although most often they make the half hitch knot (halve knoop), they are even seen making the figure eight knot!
Living in cold deep waters to depths to over 1,300m, they have a slow metabolism and can survive months without feeding. In contrast, when feeding they can eat several times their own weight in flesh in just a few hours.
Interestingly, hagfish are actually not fish and not even vertebrates. The 50 or so living species are the only survivors of the first group of animals to have evolved a simple skull. During the past hundreds of millions of years they only evolved very little.
Next to their vigour feeding habits, Hagfish are the world champions of slime production, which they use to escape predators. These remarkable characteristics have led both the scientific and popular media to often nominate the hagfish as the most “disgusting” of all sea creatures.
I think hagfish are fascinating species and a good example of the huge diversity that exists among animal species and they way in which animal species evolve and specialize.