The New Zealand Longfin Eel is a freshwater fish species endemic to New Zealand.
These slippery fellas are named after the long dorsal fin that stretches all the way down to the tip of their tails, something that the mob editor unfortunately doesn't allow me to do.

Longfin Eels breed only once, at the end of their life. When they are ready to breed they leave New Zealand and swim all the way to the sub-tropical Pacific Ocean to spawn, probably in very deep ocean trenches. When they reach their destination, the females lay millions of eggs that are fertilised by the male. The Eels use up every last bit of energy to swim to their mysterious spawning location and by the time the reach it they're essentially just skin and bones.
After hatching, leaf-shaped larvae are taken by the ocean currents all the way back to New Zealand were the transparent, glass-like eels arrive by the thousands in the spring. The eels soon turn a grey-brown and migrate upstream, climbing waterfalls, dragging themselves across land, facing any obstacle that gets in their way.

The NZ Longfin eels themselves are not a rare species, however big, healthy, fertile adult females are becoming increasingly scarce. Commercial fishing since the 1960's has had a significant impact on the eel numbers and average size. Habitat loss and pollution has also been a contributing factor to their decline. These odd fish are a threatened species are really should not be fished.