Rare black jellyfish have now been spotted swimming off of the coast of California. The rare jellyfish stung swimmers over the 4^th of July weekend.

The black jellyfish, some as big as a hula hoop, have have a dark burgundy belly and black tentacles that can grow up to 30 feet long. They are also known as Chrysaora achlyos,

The black jellyfish can reach more than 3 feet in diameter and 30 feet in length...with painful 25 foot-tentacles.

The increase of the rare jellyfish may be due to warm ocean water. The rare black jellyfish species was found in 1998 for the first time. There was an influx of the jellyfish in 1989 near San Diego; they resurfaced again 10 years later. They were last seen in California back in 2010.

Swimmers came out of the ocean with dark jellyfish stingers stuck to their skin, the AP said. Numerous people were stung.

"And all of a sudden, I see this big jelly fish swimming right by me," Joe Richman, a kayaker who saw the rare jellyfish in La Jolla, Calif., said to press

 "They are not likely to kill you. They are not a deadly jellyfish, but they can give you a painful sting," Richman said.

The black jellyfish kill their prey by injecting toxins into their skin. In humans, it isn't enough to be lethal to people who aren't allergic to jellyfish-but it may be extremely painful for almost an hour.

The jellyfish usually swim in deep waters, but now, apparently, they are making their way closer to shore again.

The 25-foot-long tentacles pose a particular risk.

Professor Greg Rouse, from the Scripps Institution of Oceanography, said "If I saw one, I'd be wanting to snorkel all around it. But those tentacles are so long you have to really watch out for them."

Watch a video of the black jellyfish below: