Rhizostoma pulmo is a species of jellyfish belonging to the class Scyphozoa, order Rhizostomeae and family Rhizostomatidae. Within this family, we find 4 genera, being Rhizostoma the one formed by Rhizostoma pulmo
and 2 other species (R. luteum and R. octopus). It is the most abundant scyphomedusa in the Mediterranean Sea, and is also present in the eastern Atlantic Ocean. It lives in open waters at various depths, and reaches coastal waters swept by currents.
The body of Rhizostoma pulmo is usually about 60 cm in diameter, although specimens up to 1 meter in diameter have been reported. The main part of the body, called the umbrella, is whitish with yellowish/pinkish tones and has a gelatinous appearance with a smooth surface. The rim of the umbrella is formed by 96 lobes fused together, which have an intense blue or lilac color. We will not find, as in other species, stinging tentacles on the edge of the umbrella. From the central part of the umbrella, and from the lower part of it, there are 8 robust arms that in its middle part take the shape of a cauliflower, and in its most apical part are completely smooth and unbranched.
It is common for Rhizostoma pulmo specimens to form large swarms of individuals. It is also common to observe juvenile specimens of different fish species establishing a close relationship. Fish of the genera Boops, Trachurus and Seriola obtain protection from the jellyfish against possible predators, since the tentacles of this species are loaded with stinging cells. It is not a very dangerous species since its sting, although irritating, has no consequences beyond skin irritation.
Rhizostoma pulmo feeds mainly on the plankton it is able to capture with its tentacles. The specimens, despite having some ability to direct their movement through coordinated and repetitive movements of the edge of the umbrella, are usually dragged by the currents, in whose journey they capture their prey.
There are individuals with separate sexes, which release their gametes into the water during the reproductive season. Therefore, the fertilization of this species is external, leading to the formation of a planktonic larva, which will later settle to the bottom giving rise to the polyp. This polyp, after developing, will give rise to juvenile jellyfish through a process of asexual excision.
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