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Part of South Florida’s sandy coastline was covered with thousands of unwelcome beachgoers this week.

Lifeguards on Hallandale Beach, about 18 miles north of Miami, discovered the shoreline littered with jellyfish, according to the city of Hallandale Beach’s Facebook page.

Strong winds and ocean currents can bring these creatures onto shore. Unfortunately, there is no way to forecast their arrival, CNN meteorologist Allison Chinchar said.

These bluish, purplish jellyfish, called Velella velella, have bodies designed to allow them to sail away from shore. However, strong winds can catch the fins of the glassy-looking sea critters and carry them onto beaches.

“This happens about every three years. We are flying our Purple flag for dangerous marine life,” city officials said on social media. Similar incidents occurred on Pacific Northwest beaches in 2015.

Also known as “purple sailors” or “by-the-wind sailors,” these jellyfish have small tentacles for catching prey and are relatively benign. However, they often travel with Portuguese man-of-war jellyfish, which carry toxins that can leave a painful sting.

Despite being harmless to humans, it is advised that people avoid rubbing their eyes or putting fingers in their mouths after handling Velella jellyfish, according to reports.

“Public works beach tractor did rake up numerous, but they continue to wash up and cleanup will be a gradual process,” officials said.