Slimiest fish on the planet
This bottom-dwelling fish feeds on both small, live prey and on the bodies of dead or dying fish, burrowing inside and rasping at flesh. It can also claim the dubious honour of being the slimiest organism on earth. Pores along either side of its body generate copious amounts of incredibly strong, sticky mucous, which can suffocate predators and act as a lubricant for the hag to slip in and out of carrion.
Discharges slime by sneezing
As if it weren't peculiar enough, it is the only fish capable of sneezing (which it does to clear its own slime from its single nostril) and the only vertebrate animal that can tie itself in a knot (which it does to provide leverage when boring into flesh and to clear slime from its body).
Juniors choose sex as per requirement
Hagfish live in the North Atlantic and Mediterranean in very dense groups (up to 15,000 in an area). Because females tend to produce large eggs in small numbers, their population sizes suggest a low death rate. Newly hatched hagfish look just like the adults, but have both male and female sex organs. As they mature, they have the ability to select their sex to best fit into the population structure.