Moon Jellyfish

A translucent saucer 25 to 40 cm (10 to 16 in) across, with four horseshoe-shaped gonads showing through the top of the bell like a clover, a fringe of short fine tentacles around the rim, and four frilled oral arms hanging beneath. The sting is too weak to matter to a diver; the animal is mostly a lens of water held in shape by protein.

The bell is not what most of the life cycle looks like. A fertilized egg becomes a larva that settles on hard bottom and turns into a polyp a couple of millimeters high, and that polyp can sit there for years, cloning itself into a colony. When conditions turn right - temperature, food, day length - each polyp constricts into a stack of discs like a pile of plates, and the discs peel off one at a time and swim away as tiny jellyfish. A bloom is not a population that bred; it is a hidden carpet of polyps releasing at once.

That is why blooms appear overnight and why they recur in the same bays: the polyp beds are still down there between events, attached to rock, pilings, hulls and pontoons. Harbor construction adds hard surface, which adds polyps.

It has no brain and no blood. A ring of nerves around the bell coordinates the swimming pulse, eight balance organs tell it which way is up, and feeding is done by mucus on the bell surface, which traps plankton and moves it to the rim. This is the 🌊 Red Sea species of a genus that used to be lumped worldwide under one name.