Jellyfish: Scyphozoa

Physical characteristics: The medusae of thimble jellies are only 1 inch (2.5 centimeters) high. These jellies are shaped like a thimble, which is a hard cap some people put on a fingertip while sewing. There is a shallow groove near the top of the bell. Thimble jellies have eight very short tentacles and eight sense structures alternating between sixteen folds at the bell margin. The outside of the bell is transparent and has numerous warts of stinging cells. The inner part of the bell is white with greenish brown spots. The polyps form colonies and are covered by a thin, hard sheath.

Geographic range: Thimble jellies live in oceans all over the world.

Habitat: The medusae of thimble jellies live near the surface in warm near-shore waters. The polyps live on coral rubble.

Diet: The medusae of thimble jellies catch a variety of animal plankton on their folds. The colored spots in the bell are filled with algae that transfer nutrients to the medusa. Algae (AL-jee) are plantlike growths that live in water and have no true roots, stems, or leaves.

Stings from the larvae or new medusae of thimble jellies cause a disorder called seabather's eruption. This disorder is characterized by a prickling sensation and red bumps that last for seven to twelve days. It is irritating but not dangerous. (© Doug Perrine/SeaPics.com)

Behavior and reproduction: Thimble jellies usually live in large groups just beneath the surface. They are very active swimmers, moving in circles. The fertilized eggs of thimble jellies form large larvae that live as plankton for three to four weeks. They settle and form an unbranched colony of polyps. Each polyp can produce as many as forty medusae.

Thimble jellies and people: Stings from the larvae or new medusae of thimble jellies cause a disorder called seabather's eruption. This disorder is characterized by a prickling sensation and red bumps that last for seven to twelve days. It is irritating but not dangerous and becomes worse when the jellies become trapped under a swimsuit.

Conservation status: Thimble jellies are not threatened or endangered. ∎