The life cycle of the scyphozoans jellyfish are cataloged as simple or complex. The complex life cycles are represented by a metagenetic model where individuals alternate different live forms and reproductive models due to the highly seasonal environment. We can start describing the life cycle when males and females of adult jellyfish release sperm and ovules into the ocean. The fertilized eggs undergo a metamorphosis into planulae, which sits on the benthic substrate and gives rise to a polyp in a slow process.

The morphology of the polyps varies according to the species, but they are generally concave upwards with a lot of tentacles that contain stinging cells called cnidocytes, used to catch plankton and feed. The reproduction of the polyp is asexual. During this stage, parental polyps generate new polyps through buds and stolons. When the resources are limited, the polyps are able to adapt to the environment for long periods of time in resting stage. However, when the environmental conditions are optimal for their development, the polyps through an asexual reproduction process known as strobilation, fissions perpendicularly from the oral-aboral axis giving rise to multiple ephyrae that develop in a pelagic environment corresponding to the medusa phase. The amount of newly released ephyrae from a polyp varies according to the species. In the case of the Aurelia genus, a polyp can release between six and 21 ephyrae into the ocean in each strobilation. This reproduction strategy implies that the survival of the species is ensured, because only a low percentage of the ephyrae will grow to become adult jellyfish capable of reproducing sexually and starting the biological cycle again.

Although there is no clear evidence, some studies suggest that the increase of artificial structures mainly in coastal areas, such as dykes or docks, provide more space for the larvae to settle, giving rise to new polyp populations of for example the most common jellyfish species in the world, Aurelia aurita. Therefore, coastal habitat modifications as well as other human activities like eutrophication (when a body of water becomes overly enriched with minerals and nutrients), global warming, translocation and overfishing have been considered significant reasons of jellyfish outbreaks seen most frequently in recent years. What a paradox, right? We as humans and the consequences of our activities are responsible for jellyfish blooms.

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Birth of a jellyfish: Why blooms are on the increase in the Mediterranean - Oceanographic - Oceanographic Magazine