Image credit: the_tahoe_guy/Flickr
They're commonly known as "jellyfish" but the name is a bit of a misnomer: In fact "jellies," or "gelatinous zooplankton," are not fish at all.
What they are, is amazing. Without bones, brains, or blood, they manage to thrive in all of the worlds oceans, in an astonishing range of depths. This versatility, however, is becoming a problem as other species are strained by steadily collapsing ecosystems.