The hydrozoan Clytia brevithecata (Thornley 1900) was described from Papua New Guinea, and a very similar form was described from Martinique as C. hummelincki (Lekoup 1935). Genetic evidence indicates that these two species are the same, with wide distributions in in the tropical to warm-temperate Atlantic and Pacific. Clytia spp., like many hydrozoans, has a sessile hydroid form and a planktonic medusa. The hydroids grow from a stolon spreading over the substrate with unbranched stalks topped by cup-shaped hydrothecae, enfolding a mouth with ~20 threadlike tentacles. Reproductive polyps (gonothecae) also grow from the stolons, and produce medusa buds. The medusae are sexual and release eggs and sperm, which fuse to form planula larvae and settle to form new hydroid colonies. Clytia brevithecata grows in spreading colonies on stones, shells, vegetation, and man-made structures. Clytia brevithecata is now considered to have a wide native and cryptogenic range in the tropical to warm-temperate Atlantic and Pacific. In the Atlantic, it ranges from Bermuda to Brazil, and the Mediterranean to South Africa, and in the Pacific from the Hawaiian Islands to Papua New Guinea. It is considered to be a possible introduction in the Western Mediterranean and the Tropical Pacific, where it has been recently found on the Galapagos Islands and on Cocos Island, Costa Rica. This hydrozoan has the potential to be introduced in ballast water or fouling.