Obelia Geniculata

Obelia geniculata, commonly called the knotted thread hydroid, is a species of cnidarian in the family Campanulariidae. It is distinguished from other Obelia species by a notably thicker protective outer layer called the perisarc, and it alternates between two life forms, sessile feeding polyps called hydranths and free-living medusae produced from reproductive structures called gonangia; it can also produce a blue bioluminescent glow through a photoprotein called obelin. It is epiphytic, growing encrusted on aquatic plants rather than settling on bare surfaces, and its colonies feed by using tentacles armed with stinging cells to capture bacteria, zooplankton, and fish larvae. It has a cosmopolitan distribution, and population genetics studies have identified three distinct geographic groups, in the North Atlantic, the Pacific near Japan, and New Zealand, with the North Atlantic populations having become established more recently than the Pacific ones. It reproduces both asexually, by budding medusae from its blastostyles, and sexually, through broadcast spawning that produces free-swimming larvae which eventually settle and develop into new polyps.