cannonball jeliies

Hundreds of cannonball jellies have been washing up on Charleston area beaches. Anna Etheridge/Provided

Hundreds of dead jellyfish, some shrimp and crabs washed up Monday across the Sullivan's Island beach.

What's that all about? Summer breezes.

Cannonball jellies tend to turn up in the nearshore waters during April, pushed in from the Gulf Stream with washes of warmer water. They’re a sign that the surf is approaching that magic room-temperature zone a lot of people find plungeable on a hot day. 

When the wind blows as hard as it has, the jellies end up on the beach.

"Nothing unusual at all given the time of year and wind conditions since Sunday," said Mel Bell, S.C. Department of Natural Resources fisheries management director. As for the other critters, "when the winds are blowing as hard as they have been, anything on the bottom gets bounced around a good bit and pushed ashore with the tide." 

Jellies have been turning up on the sand at the Folly Beach pier for a couple of weeks, said Charleston County Park and Recreation aide Claire Seabrook. But staff at Isle of Palms County Park hasn't noticed any unusual numbers.

The water temperature at the Folly Beach Pier was 68 degrees on Tuesday.

Temperature changes, winds and high tides tend to lead to the beaching of large numbers of a variety of different finfish, shellfish or other sea life from time to time. It can be startling even to people who frequent the dunes.

"We were completely flabbergasted by the amount of dead sea creatures that were washed up," Anna Etheridge of Johns Island said about the Sullivan's Island beach. "We walked down about a mile and saw hundreds of dead jellyfish, crabs, shrimp and even a dead horseshoe crab."

The cannonballs are abundant and relatively innocuous among jellies. They can sting, despite the “harmless” rep they have cultivated over the years, but the sting is too mild for people to notice.

In fact, spring might be the safest time of the year to swim as far as jellyfish stings. Lion’s manes, which resemble cannonballs but have a vicious sting, tend to be cold-water critters that move on out as waters warm.

Sea nettles, viciously stinging jellies that somewhat resemble cannonballs but have trailing tentacles, don’t move in until the waters really warm. They’re heaviest in mid-summer. So are sea wasps, or box jellies, another notorious stinger with long, trailing tentacles.

But be careful of the Portuguese man o’wars, the alluring blue jellies with “sails” and screamingly painful stings. They tend to wash in with strong winds out of the east. A lot of sargassum, or seaweed, washing up is a sign there might be man o’wars around.

Sign up for our daily newsletter

Get the best of The Post and Courier, handpicked and delivered to your inbox every morning.

Reach Bo Petersen at @bopete on Twitter or 843-937-5744.

Science and environment reporter. Author of Washing Our Hands in the Clouds.