curling (Cotylorhiza tuberculata)
The species, commonly called fried egg jellyfish for its distinctive look, appears as a flattened umbrella with tawny, greenish hues and an orange central stripe. Its diameter ranges from 20 to 35 centimeters.
It inhabits both open waters and coastal areas, with a stronger presence on the shoreline during the summer and autumn in the Mediterranean. The Mar Menor region reports unusually high numbers.
Its danger is low. Short tentacles and fewer stinging cells limit its ability to provoke severe reactions, typically causing only mild skin irritation and itching.
glowing jellyfish (Pelagia noctiluca)
One of the most commonly observed species along the coast. It can reach diameters over 20 centimeters and has 16 long tentacles that can exceed 20 meters when expanded. The umbrella surface bears wart-like structures and a reddish pink coloration.
Its presence is common in open waters and it drifts toward the shore carried by sea breezes, especially in summer. It is frequently seen in both the Atlantic and the Mediterranean.
The sting can be high risk, causing skin irritation and itching and sometimes a wound that may become infected. With its abundance and long tentacles, large areas of skin can be affected, potentially leading to respiratory, cardiovascular, and dermatological issues that last for weeks or months.
bad water jellyfish (Rhyzostoma pulmo)
Known locally as aguamala or aguaviva, this large jellyfish can reach up to one meter in diameter. It has a bell-shaped form and pronounced purple markings, and it is common along Mediterranean and Atlantic coasts, especially in summer.
It stings, but serious injuries are uncommon. Caution is advised because tentacle fragments can remain dangerous even when the animal is in shallow water.
aurelia aurita
This jellyfish appears plate-shaped with scalloped arms and four purple horseshoe shaped centers. It is transparent with blue-white speckles. It can provoke irritation but its sting is generally not dangerous.
Coastal regions and lagoons such as Mar Menor, as well as fjords and enclosed bays with variable salinity, often host this species. It may irritate the skin but rarely causes serious harm.
compass jellyfish (Chrysaora hysoscella)
This species, known as the compass jellyfish or radiating akalephos in Spain, displays yellow and orange tones with a striped umbrella pattern. Long tentacles can reach up to five meters, and the umbrella often shows a radial chart reminiscent of a compass.
It is relatively common in the Mediterranean and Atlantic and may form groups offshore. In summer it can drift closer to shore as currents bring it in.
Among the higher danger level entries, bites typically cause itching and burning, followed by red lesions and swelling that may take time to resolve. This assessment follows guidance from the Ministry of Ecological Transition.
Portuguese man o war (Physalia physalis)
Technically a siphonophore rather than a true jellyfish, Physalia physalis features a blue to pink violet float and a set of venomous tentacles that can extend over 20 meters. The floating part is about 30 by 10 centimeters in the largest specimens.
It inhabits temperate Atlantic waters and is occasionally seen in the Mediterranean. It is driven by surface currents and winds.
The danger is very high according to the Ministry. Tentacle contact can cause serious outcomes for humans, including severe pain, skin burns, and possible respiratory or cardiovascular issues due to potent venom. Close contact can lead to burning sensations and skin injuries.
sailboat jellyfish (Velella velella)
This organism is not a true jellyfish but a floating hydrozoan. Its disk measures about 1 to 8 centimeters in diameter. It often forms large groups during winter and spring and poses no danger to people.
Its presence is a regular feature of coastal seas, and it is commonly noted as a harmless visitor in many regions.
Aequorea forskalea
This transparent jellyfish appears as a plate roughly 30 centimeters across with distinctive dark patterns. It is frequent in the Atlantic and can occasionally be seen in the Mediterranean. It is increasingly observed along Catalonia and the Balearic Islands. It does not sting, so it is not dangerous.
box jellyfish (Carybdea marsupialis)
Also called the sea hornet in the Mediterranean, it presents a cube-like shape with long, bright tentacles and a bioluminescent glow in blue or white. Its eye-like structures allow it to react to movement and light changes.
Its bite is serious and can pose a health risk. However, it tends to inhabit deeper waters, making encounters with beachgoers relatively rare.
American ctenophore (Mnemiopsis leidyi)
This invasive species originates from the Atlantic coast of the Americas. It entered the Black Sea and later spread to the Mediterranean. Spain first reported it in the summer of 2009.
It is a small organism with iridescent color and a bulb-shaped body about 10 centimeters long. It lacks tentacles and often gathers in dense concentrations, sometimes exceeding thousands of individuals per square meter. It does not cause pain and is not considered dangerous.
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Notes: Public health and environmental agencies monitor these jellyfish and related species to assess risks and guide beach safety practices. Researchers emphasize cautious behavior around jellyfish, avoidance of direct contact, and awareness of local advisories. The information above is provided with attribution to official environmental authorities and regional marine research bodies.