Swimming by a black sea nettle may be a bit of a shock. These large jellyfish can grow to be over three feet in diameter, and their tentacles can be over 25 feet long. Despite their large size, much about them is still unknown, and they are relatively new to science: black sea nettles weren't an official species until 1997! This is because they are rarely observed in the wild, preferring calm deep water, and are difficult to raise in containment.