Don't panic: no one's been stung!
But when we went to the beach the other day, the place was swarming with these clear, racquetball-sized balls of goo, which we could only assume were little jellyfish. Seeing as the lifeguard was totally complacent and there was a large crowd bathing and suspiciously little screaming going on, we deduced that the jelly fish were harmless. Of course, that still didn't make it any less freaky to brush against them in the water or to grab them by accident.
Later, a co-worker explained the jellyfish situation. It turns out, these little guys are juvenile jellyfish, and every year they start popping up around the middle of August. By the end of August, they've matured--and begun to sting. This is apparently the origin of the old wives' tale that anyone who goes swimming after the Obon holiday will be dragged under by a sea nymph.
Luckily, the mature jellyfish are a lot easier to spot: unlike the juveniles, which are almost completely clear, the adults are pink- or blue-tinged, with a cloverleaf pattern in the middle, and about the size of a small dinner plate. But the best plan, I think, is to follow the locals, and only swim where there are other people swimming (and not writhing in pain).
Bonus: Every autumn, the waters off Japan are invaded by the Nomura's jellyfish, one of the largest jellyfish in the world. Larger and heavier than a full-grown person, schools of Nomura's jellyfish often gets tangled in fishing nets, sometimes causing smaller boats to capsize.
Apparently, in the past, the Japanese have responded to these invaders by eating them. Nana and I have had Chinese-style jellyfish, and we can vouch for their tastiness if prepared the right way--that is, dried, salted, cut into tiny little strips, and marinated to heck.