Knotted Thread Hydroid

Obelia geniculata

Obelia geniculata is a colonial cnidarian characterized by its thick perisarc and distinctive branching pattern. The colonies consist of hydranths and gonangia, with the former being the feeding polyps and the latter responsible for reproduction. The hydranths have a hollow manubrium surrounded by approximately 24 tentacles, which are equipped with nematocysts for capturing prey. This species is typically found on green and brown algae, often encrusting on plants in shallow waters, and has a cosmopolitan distribution across polar, subtropical, temperate, and boreal climates. It inhabits depths ranging from 0 to 100 meters, although most occurrences are shallower than 40 meters. As an epiphytic organism, O. geniculata thrives on substrates like leaves of aquatic plants, making it an important species in its ecosystem. It feeds through suspension feeding, capturing a variety of bacteria and zooplankton with its tentacles. Reproduction occurs both asexually, through budding of blastostyles, and sexually, with gonangia releasing sperm and eggs into the water for fertilization. The resulting planulae eventually settle on a substrate to develop into new hydranths. Divers may encounter O. geniculata in areas of upwelling, where food availability is high, leading to increased abundance. The colonies can be seen encrusting on kelp and other plants, sometimes impacting the quality of these resources for human consumption. The conservation status of O. geniculata has not been evaluated by the IUCN.