Not all jellyfish inflict a painful sting. Coming in contact with certain types of jellies, like the moon jelly, is relatively painless, even though some harmless jellyfish are still venomous. By contrast, a sting from the box jellyfish Chironex fleckeri is severely painful and can be fatal.
But why? In a new study in PLOS ONE, researchers from Tokyo University of Marine Science and Technology say they have found that the longer the jellyfish’s stinging organ, the more painful its sting. They propose that the deeper in the body the jellyfish stinger penetrates, the more severe the pain it causes.
Jellyfish immobilize their prey and defend themselves against attackers using nematocysts, needle-like stingers usually located within the tentacles. When jellyfish make contact with another species (like a fish or a human swimmer), these nematocysts fire, injecting venom-carrying tubules into the unlucky recipient. (Watch the process close-up in this video.)
In the study, the researchers compared the length of discharged nematocyst tubules from different types of jellyfish found off the coast of Japan, both species whose stings cause severe pain and those that are relatively harmless. Dangerous species like Chironex yamaguchii (a box jellyfish that has caused several deaths) had significantly longer nematocyst tubules than harmless species like the moon jelly. Moderately painful stinging jellyfish like Chrysaora Pacifica had tubule lengths between 100 micrometers and 200 micrometers, compared to the deadly species, which had venom injectors more than 200 micrometers (one species, Carybdea brevipedalia, shot tubules longer than 600 micrometers).
X, Y, and Z correspond to nematocyst tubules from not-painful, moderately painful, and severely painful jellyfish stings, respectively. The two boxes (“a” and “b”) correspond to the distribution of nerves under the skin. Image Credit: Kitatani et al., PLOS ONE (2015)
The authors hypothesize that because these longer stingers penetrate deeper below the skin, where there are more nerve endings, people feel more pain from these stings. The more harmless species of jellyfish still inject venom, but since they don't go as deeply into the skin, it’s harder to feel. More painful jellyfish stings may come from the physical stimulation of the pain receptors below the skin. In addition, “the subsequent persistent pain in the region of the sting might be generated due to the destruction of tissue by the deeply injected venom,” they write.
This study only examined a handful of jellyfish species in Japan, so it’s possible that a larger sample of international species might confound these results. And this study doesn’t examine the differences in venom between the species, which also might create a more painful affect (some snake venom, for instance, contains a combination of toxins that attacks pain-sensing nerves). However, it might be a good idea to stay away from jellyfish with super-long venomous needles stored in their tentacles regardless.