The jellyfish Obelia belongs to the family Campanulariidae (Hydrozoa, Leptomedusae), an important and widely distributed family of hydrozoans. Obelia is a very popular and widespread medusa, however first report on its complete life cycle was only published in the late nineties in Northern Japan. This is probably due to its size, as the medusa umbrella diameter is lower than 1mm. This Hydrozoa presents a bentho-pelagic life cycle with both polyp and medusa stages. In Thau lagoon, Northwestern Mediterranean Sea, populations of Obelia occur each year. Some polyps colonies were identified and sampled in the lagoon on Zostera noltii leaves, indicating that the populations realise their full development cycle within the lagoon. Morphological and genetical (COI) approaches, on both polyps and medusae, have revealed that main populations are composed by O. dichotoma individuals (side branches typically irregular in length; hydroteca bell-shaped, usually not very deep, thin walled, often thrown into fine longitudinal folds; hydrotecal rim with smooth or with shallow cusps \u2013crenate, slightly flared; diaphragm transverse to oblique) but that O. bidentata (lateral branches roughly in right angles pairs are given on both sides; slightly oblique diaphragm; hydrotecal rim with bimucronate cusps) is also present in lower abundances. An in situ pelagic monitoring every two weeks since 2008 have allowed understanding the populations dynamics of the genus and the environmental factors that seemed to be particularly appropriated for promoting optimum growth conditions.
Baseline information regarding seasonal cycles and historical abundances were provided from a 8 years monitoring, which has allowed detecting blooms. Indeed, the medusae were found at very low densities during the study period with main abundances under 5 ind.m-3, in accordance to previous reports for Obelia spp. medusae around the world. Nevertheless, 2 exceptional events took place in June 2008 and May 2013 were blooms reached 1232 and 660 ind.m-3 respectively. Such data raise questions regarding the potential importance of this small and understudied hydrozoa in the pelagic community of Thau lagoon. Consequently, it is necessary to improve the available databases documenting medusae blooms, as this will provide baseline information about these little-studied events
