In July of 2012, while doing a marine life survey of sea urchins off the coast of Pt. Loma, San Diego, to assist local marine biologists who track urchin populations in local kelp forests, we were astonished to find ourselves surrounded by a ‘swarm’ of giant black Sea Nettles, large, pelagic sea jellies otherwise known as Chrysaora achlyos—some of them over twice the length of an average diver.
Looking towards the surface, we could see dozens of them, ranging in size from a mere 3 feet (91 cm) long to much larger adults up to 15 feet (4.5 m) long, their tentacles gently drifting in the current high above us.