Overfishing, rising temperatures and ocean acidification create ideal environments for jellyfish to thrive and multiply, and have led to jellyfish "blooms" in many waters.
Although jellyfish are low on the evolutionary chain -- entirely lacking brains -- the "enchanting and lovely" invertebrates can take a damaged ecosystem and make it worse.
"They need something a little bit funky to set them into a cascade of events where they end up in control," said Gershwin.
In her new book, "Stung! On Jellyfish Blooms and the Future of the Ocean," Gershwin explains that jellyfish often eat smarter, faster organisms higher up on the food chain. They also compete with whales and larger animals by eating small fish and plankton.
“So jellyfish can wipe out a whole food chain by eating down at the bottom,” Gershwin says. “And they’re doing this.”
She explains that jellyfish species Mnemiopsis leidyi was accidentally introduced into the Black Sea in the early 1980s, and proceeded to eat and compete with everything there, eventually becoming 95 percent of the biomass.
“Ninety-five per cent of every living thing was this one species of jellyfish," Gershwin said.
"I think it’s a very scary thing that we could be heading back to a situation where jellyfish are dominating the oceans," Gershwin said. She fears the biodiversity of the world's oceans is in danger of returning to a Precambrian era state, where jellyfish ruled the waters before reptiles and mammals existed.