"This AM thousands of small jellyfish washed in to our beach covering the shoreline and prompting continuous questions from beach patrons," they wrote. "This happens about every three years."
Despite their appearance and similar characteristics, Velella are siphonophores and not true jellyfish. Where jellies are a single distinct organism, siphonophores are a colony of zooids attached together to act as a single organism.
Velella are not known to produce a painful sting, though some who touch them can feel the stingers on more sensitive skin, such as around the eyes. But lifeguards warned that some dangerous man-o-war (another siphonophore) were among the group.
Both creatures cannot move themselves – they have small "sails" protruding above water and are blown around the ocean by wind.
Thursday's mass stranding was the result of southeast winds blowing them toward beaches on Florida's southeast coast. Lifeguards reported another mass stranding to the north on beaches in Boca Raton, Delray Beach and Boynton Beach.
Velella and man-o-war look similar, but man-o-war are larger and have an oblong inflated "sail." Velella have a more triangular sail that is flat and not inflated.
Only one sting was reported in the first three hours on Hallandale Beach as crews began cleaning up the scene throughout the day.