Cannonball jellyfish invade Amelia Island

While pirates launched a cannonball on Amelia Island last weekend to celebrate the Shrimp Festival, thousands of cannonballs were washing up on the island’s beaches — cannonball jellyfish.

The gelatinous creatures were seen all along the island’s shores. Although commonly referred to as cannonball or cabbage head jellyfish, their scientific name is Stomolophus meleagris. The creatures are roughly five inches wide by seven inches long, weigh around 23 ounces and have a three- to six-month lifespan, according to the Georgia Department of Natural Resource’s Coastal Division. Cannonballs prefer water from 68 degrees to 76 degrees Fahrenheit.

Cannonballs are “by far” the most common species of jellyfish on the southeastern coast. During summer and fall, they can make up more than 16% of the biomass, GDNR said, and they primarily eat zooplankton and red drum larvae.
“When disturbed or threatened, the cannonball jellyfish will secrete a toxic mucus that will harm small fish, and drive away most predators,” GDNR said. “Some crabs, however, are not driven away by this mucus and will continue to attack/eat the cannonball.”

Mark Martindale, Ph.D., is the director of the Whitney Lab for Marine Bioscience at the University of Florida. He told the News-Leader that, while the numbers of cannonball jellyfish can vary from year to year, their arrival at this time is “pretty consistent.”

“The swimming jellyfish stage in these jelly fishes is derived from a benthic ‘polyp’ stage that lives in the bottom in shallow waters so the prevalence of the adult animals does depend on temperature and predation,” Dr. Martindale said. “This makes it difficult to predict when we are going to have gang-buster years.”

Even if the globe-like creatures gross you out, they cannot hurt you.

“The adult jellies are completely harmless,” Martindale said. “Their stinging cells will not penetrate human skin. The biggest problem is that they get caught in fishermen’s nets and, if there are a lot of them, their sheer weight can rip and tear the nets.”

University of Georgia Marine Extension said cannonball jellyfish play an important role in the marine food chain, serving as a food source for sea turtles, especially the leatherback sea turtle as well as fish such as spadefish, harvestfish and butterfish.

In his 1983 thesis, The ecology of Stomolophus meleagris, R.A. Roundtree shared some theories about the life cycle of cannonballs.

“Small populations of large adults appear offshore in the spring and move inshore by early summer. The origin of these populations of adults has not been determined, but it seems likely that they are survivors from the previous season,” Roundtree said. “It is also possible that they are early spawned medusae from more southern waters, which are carried north by prevailing spring ocean currents.”

Roundtree went on to explain two more theories regarding the jellyfish: First-generation populations move from offshore in the spring to inshore waters in the summer and finally into brackish waters in the fall, and second-generation populations move out from estuarine waters beginning in the mid-summer and continuing through the fall.
While the numbers of cannonballs on Amelia Island may have kept some people off the beach, Martindale said some research shows their numbers are not increasing.

“There was a big literature search a few years ago by a guy named Steve Haddock out in Monterey Bay, Calif., and he concluded that jellyfish blooms are not any more prevalent now than they were in the last century,” the biologist said. “They are just sporadic and people freak out when they see big years.”

jroberts@fbnewsleader.com