Lion’s Mane Jellyfish

I’ve had lots of life-first encounters here in the Pacific Northwest (PNW), one of these is Lion’s Mane Jellyfish. These jellies like colder water and are rarely seen below 42 degrees latitude on the US West Coast. The northern border of California is at 42 degrees, so that is probably why I’ve not seen this type of jellyfish before.  

Size. Lion’s mane is the largest jellyfish species in the world. The colder the water, the larger the lion’s mane. In frigid Arctic and Antarctic waters, they can grow a bell diameter of more than 8 feet and tentacles that reach 120 feet long! They are longer than blue whales, so are the longest known animal in the world. In the warmer waters of the PNW, lion’s mane jellyfish are much smaller, with bells up to 20 inches in diameter and tentacles up to 30 feet long.

Lifecycle. They breed in March through early May and the larvae develop into polyps that eventually grow into jellyfish within 30–40 days. Their life expectancy is only one year. Juveniles are lighter orange or tan and become red and darken with age. They are most often spotted during the late summer and autumn, when they have grown to a larger size and the currents bring them closer to shore.

Food sources. They are carnivorous, using their long tentacles to capture and inject venom into small fish and crustaceans, zooplankton, and even other jellyfish. Tentacles raise the stunned food sources to their mouth at the center of their dome.

Stings. The best first aid treatment for a lion’s mane sting is to stop the firing mechanism in the stingers by rinsing the affected area with vinegar and then immersing it in 113°F hot water (or use a heat pack) for 40 minutes. Although generally not fatal, some of those stung require medical attention. Stings from large, mature jellyfish and ones from the shorter tentacles nearest the dome are more potent. Initially, a sting may result in itching and pain that progresses and radiates to other areas of the body. Stings can result in an Irukandji-like syndrome (named after a type of box jellyfish) that causes back pain, nausea, abdominal cramps, sweating, and hypertension.

Our experience. The most dense population of lion’s mane that we’ve seen so far was in Princess Louisa Provincial Marine Park. Even with a warning sign about them at the dock, many people were still swimming. After a visual scan of the area, Sean even went into the water to check the anodes under our boat. After a few minutes, he shot out of the water because a lion’s mane was in the depths below him!