Cotylorhiza tuberculata

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Two examples of the species are shown here: one from the Ionian Sea in Italy (top) and one from Sant’Antioco, Sardinia (bottom).

Cotylorhiza tuberculata is a jellyfish in the phylum Cnidaria. It’s also called the Mediterranean jellyfish or fried egg jellyfish. It lives mainly in the Mediterranean, but also in the Aegean and Adriatic seas. It can reach up to about 40 cm across, but most are smaller than 17 cm.

Its sting is usually mild or not noticeable to people, though it can cause itching or allergies in sensitive individuals. The jellyfish has a smooth, raised central dome surrounded by a ring, with long, elongated marginal lobes. Its mouth arms split near the base and branch several times, and there are many short, club-shaped pieces with disk-like ends.

Cotylorhiza tuberculata is one of the most common jellyfish in the Mediterranean. It has an annual life cycle with summer population blooms. Its growth goes through four main stages: planula larvae, scyphistoma polyps, ephyrae, and adult medusae. Planulae are present from August to November; polyps stay year-round; ephyrae appear from May to August; and medusae are visible from July to November.

Planulae swim with tiny hair-like cilia and settle on hard sediment to become polyps. During early development, polyps take in symbiotic algae called zooxanthellae, which live in their body and help provide energy. The polyps reproduce asexually to make more polyps, and parts of each polyp can become ephyrae (1.7–4.2 mm in size). Young medusae reach about 3 cm in 8–10 weeks and then grow about 3–4 cm each week, eventually reaching around 35 cm in diameter.

Sexual reproduction happens in the late summer and autumn; females are fertilized by sperm from males, and planulae are released into the water. As they age, medusae can be damaged by waves and human activity, especially boats and fishing nets. Older individuals show more wear and may regrow damaged parts imperfectly.

A key part of their biology is the relationship with zooxanthellae. These algae provide energy through photosynthesis, helping the jellyfish grow and survive. They mainly feed on tiny organisms, including plankton, and their diet is largely made up of a few small types of microplankton. Much of their food comes from plankton carried in by the surrounding waters.

Blooms of C. tuberculata can interfere with tourism and fishing, leading authorities to remove large numbers of jellyfish in summer. On the flip side, scientists are interested in their potential. Some studies suggest these jellyfish have substances that can affect certain cancer cells, so researchers are exploring their possible use in cancer research. More work is needed to understand this potential and its safety.