A fisherman spotted the creatures in a pond off of the San Joaquin River.
"We were right over here, and my dad leaned over and saw these white things and said I bet those are jellyfish, just playing around, and he looked over and he said, oh those really are jellyfish," said Timmy LeBar, who was expecting to reel in a fish from the pond his family decided to fish in.
Thought jellyfish only live in the ocean?
The freshwater jellyfish in the San Joaquin River look like snowflakes on the surface of the water.
Timmy and his family have fished this pond for years, but have never seen this before.
At first they approached the creatures more with caution than curiosity.
Timmy says he was thinking, "Are they poisonous, I don't want to go swimming anymore."
"Never seen them before, why would I touch them. I watch Animal Planet enough to know the ones in the ocean aren't good to touch," said Matthew Kemp, who also spotted the creatures.
The jellyfish are only about the size of a penny, they do have stingers, but because they're so small, they're harmless to most people.
The last time they were seen in the river was in 2007.
"It's the only species that occurs in fresh water," said Pete Blumenshine, a Fresno State biology professor.
He says these jellyfish like calm waters.
Most of the time they're in a resting stage at the bottom of the lake, but after a spate of hot weather and when the food supply is right, they'll come to the surface.
"It's kind of an interesting phenomenon, because they have this bloom under the right conditions, and it's so unusual to think of jellyfish in fresh water," said Blumenshine.
Their presence in any lake, river, or pond is sporadic and unpredictable, so this unusual sighting won't last long.
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