When the waters off of San Diego rise in temperature during the summer months, so does the number of jellyfish and jellyfish encounters. While not every species of jellyfish are harmful to humans, there are a few species that can cause painful stings if touched, even when dead or on the shore. This year, a rare black jellyfish species, also called a black sea nettle (Chrysaora achlyos), has made their appearance in numbers off the southern California coast.
Jellyfish live in every ocean in the world, though they are more frequently encountered in some areas more than others. They are a simple form of life with a simple nervous system and venomous tentacles for catching and stinging prey. They have no eyes and can only sense light and dark and chemicals in their environment. They hunt mostly by stinging prey they encounter while floating or swimming by propelling themselves by contracting and expanding their smooth bells. They are likely to sting, out of instinct, most animals they encounter whether they are prey or not. Most of the stinging is done by their tentacles, but many species also can sting from their bells. Jellyfish can still deliver a painful sting even when dead or in pieces on the shore. Most jellyfish stings in the San Diego area are not fatal unless one is allergic to the venom, but can still be quite painful. Be sure to visit a lifeguard or first aid station to be properly treated if you get stung.
Most jellyfish die off as the water cools down after summer after they breed and lay their eggs in the fall. Their larval young float to the bottom where they attach to rocks and other hard objects. In the spring, they become new jellyfish to be carried with the tides or their own propulsion through the ocean and, perhaps, near a San Diego beach.