Blue Button Jelly (Porpita porpita)
Although it has ‘jelly’ in its name, the blue button jelly (Porpita porpita) is not a jellyfish, or sea jelly. It is a hydroid - an animal in the class Hydrozoa, which are colonial animals. The blue button jelly is made up of individual zooids, each specialized for a different function, such as eating, defense or reproduction.
Blue button jellies are relatively small, and measure about 1 inch in diameter. They consist of a hard, golden brown, gas-filled float in the center, surrounded by blue, purple or yellow hydroids, which look like tentacles. The tentacles have stinging cells called nematocysts.
Blue buttons are hermaphrodites - each blue button jelly has both male and female sex organs. They have reproductive polyps that release eggs and sperm into the water. The eggs are fertilized and turn into larvae, which then turn into individual polyps. Blue button jellies are actually colonies of different types of polyps - these colonies form when a polyp divides to form new types of polyps. The polyps are specialized for different functions, such as reproduction, feeding and defense.