Although it has the word "jelly" in its name, the blue button jelly (Porpita porpita) is not a jellyfish or sea jelly. It is a hydroid, which is an animal in the class Hydrozoa. They are known as colonial animals, and sometimes just referred to as "blue buttons." The blue button jelly is made up of individual zooids, each specialized for a different function such as eating, defense or reproduction.

The blue button jelly is related to jellyfish, though. It is in the Phylum Cnidaria, which is the group of animals that also includes corals, jellyfish (sea jellies), sea anemones, and sea pens.

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 that look like tentacles. The tentacles have stinging cells called nematocysts. So in that respect, they can be like jellyfish species that sting.

Blue Button Jelly Classification

Here's the scientific classification nomenclature for a blue button jelly:

Kingdom: Animalia

Animalia Phylum: Cnidaria

Cnidaria Class: Hydrozoa

Hydrozoa Order: Anthoathecata

Anthoathecata Family: Porpitidae

Porpitidae Genus: Porpita

Porpita species: porpita

Habitat and Distribution

Blue button jellies are found in warm waters off Europe, in the Gulf of Mexico, Mediterranean Sea, New Zealand, and southern U.S. These hydroids live on the ocean surface, are sometimes blown into shore, and sometimes seen by the thousands. Blue button jellies eat plankton and other small organisms; they are typically eaten by sea slugs and violet sea snails.

Reproduction

Blue buttons are hermaphrodites, which means that 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 develop 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.

Blue Button Jellies...Are They Hazardous to Humans?

It's best to avoid these beautiful organisms if you see them. Blue button jellies do not have a lethal sting, but they can cause skin irritation when touched.

Sources:

Climate Watch. Blue Button: Porpita porpita.

Larsen, K. and H. Perry. 2006. Sea Jellies of the Mississippi Sound. Gulf Coast Research Laboratory - University of Southern Mississippi.

Meinkoth, N.A. 1981. National Audubon Society Field Guide to North American Seashore Creatures. Alfred A. Knopf, New York.

SeaLifeBase. Porpita Porpita.