Obelia is used in many zoology textbooks to exemplify the Hydrozoa life cycle. The body plan of Obelia medusae, however, is unique among the Hydrozoa. Contrary to all other hydromedusae, Obelia medusae do not derive from a medusary nodule, have neither a velum nor a subumbrellar cavity, the umbrella is flat, and swimming is achieved by umbrellar flapping and does not involve jet propulsion. The medusae of most Obelia species do not grow well on a diet of Artemia salina nauplii, the usual food for laboratory-reared hydrozoans, and their rearing is often difficult. Previous observations reported that Obelia medusae might eat phytoplankton, driven to the mouth by movements of both the bell and the tentacles. To evaluate the efficiency of this feeding mechanism, we gave a suspension of genetically modified Escherichia coli expressing GFP to starved, newly released medusae of both O. dichotoma and Clytia hemisphaerica (the other campanulariid genus with medusae). Obelia medusae are able to concentrate bacteria in their gastric cavity, whereas Clytia medusae are not. Contrary to other Hydromedusae that are macrophagous, Obelia is a microphagous and filter-feeding medusa, at least at the onset of its medusan life. As Cnidaria evolved before their current metazoan prey, their ancestral diet was probably microphagous. Obelia’s microphagous feeding habits could thus represent an example of the ancestral feeding condition in Cnidaria, but the apomorphic characters of its medusa suggest that its body architecture is not ancestral, and that its microphagy evolved relatively recently.
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