Eels!

Sigmund Freud, before he became the well know psychologist he is today, studied eel sexuality. Freud tried to find eels testicles while researching them, but couldn't. More recently, strong concerns have arisen to the population of European eels. Their population is being destroyed by a foreign parasitic nematode. This parasite from East Asia arrived in Europe in the late 1980s. The parasite has even spread to America in Southern Texas and California. Other threats to eels are man-made structures and dams. Because eels spawn in salt water but live in freshwater, so they often have to swim to other freshwater locations. Since the 1970s, however, there has been an increasing number of eels ladders being constructed across Europe and North America to help the eels bypass the dams and other obstructions. Also, here is a video of two eels mating!