Thousands of tourists holidaying in the Mediterranean have been stung by jellyfish after vast swarms of the creatures invaded the coasts during the hot summer.
According to a report from the BBC, some beaches in Spain have been closed because they are unsafe due to the numbers of jellyfish in the water. Tourist resorts in Sicily and North Africa have also seen large numbers of jellyfish. Experts believe that up to 30,000 people may have been stung this summer.
Although the stings can be painful, the Mediterrannean jellyfish are not thought to be particularly dangerous and the stings are not fatal, however, allergic reactions are possible.
Hot, dry weather is believed to be responsible for the population explosion of the creatures which have reached levels of 10 per square metre in some parts of the Spanish coast, says the report. A jellyfish swarms is technically known as a "smack" or "bloom".
Francesc Peters of the Institute of Marine Science told BBC Europe Today that the hot summer and lack of rain had made the sea warmer and saltier than usual. The low river flows mean that jellyfish that are normally washed offshore are drifting in much closer to the coast. Peters believes that global warming and overfishing could mean that jellyfish swarms like these become much more common:
"Probably because of overfishing, populations of jellyfish offshore will increase and then these special environmental conditions... higher temperatures and higher salinity near the coast, may bring these swarms of jellyfish close to the beach."
Are they fish?No. They're invertebrates and they're in the same Phylum as corals and anemones. Some people call them sea jellies instead. Technically, they're Scyphozoan invertebrates from the phylum Cnidaria.
How do they sting?Jellyfish are opportunist feeders and primarily eat passing fish. The "bell" of the jellyfish - that's the big, blobby bit at the top - contains the stomach and its surface doesn't sting. The tentacles and the oral arms, which flap gently beneath the bell are covered in thousands of microscopic stinging cells called nematocysts.
These contain a little trigger mechanism called the cnidocil which is attached to a coiled sting inside the nematocyst. When the tentacles or oral arms are touched, the cnidocils trigger and fire a tiny harpoon-like sting into the victim. This often has a barb on the end and injects toxins into the prey. The arms are then used to pull the fish into the mouth of the jellyfish so it can be eaten.
How dangerous are they?Most aren't actually that dangerous, although the stings can be painful. Some of them, such as Cyanea capillata and Carukia barnesi, can be deadly and swimmers have been killed after being touched by their tentacles.
Perhaps the best-known dangerous "jellyfish" is the Portuguese Man o' War, Physalia physalis, a species with very long tentacles that pack a powerful sting. It has a characteristic floating air-bladder to help it stay near the surface.
However, contrary to popular belief, it is not actually a true jellyfish at all. It's a siphonophore, a member of the Cnidarian Class Hydrozoa. It gets its name because it apparently resembles a Portuguese naval ship with its sail up.
Can they be kept in an aquarium?Yes, quite a few of the marine public aquaria in the UK have jellyfish exhibits and they frequently breed when conditions are right. A common species called Cassiopeia xamachana, the Upside-down jellyfish, is sometimes kept in reef aquaria. It is non-pelagic, so it spends its time sitting on the bottom, rather than bobbing around in the middle of the water column.
How should you treat a jellyfish sting?Serious stings may need hospital treatment, as some can be lethal and anaphylactic shock is a possibility. Stings should always be washed in salt water, not freshwater. Vinegar is also recommended, and baking powder is said to be useful in neutralising nematocysts in the victim that haven't discharged venom (however, this is controversial as it can make the wound even more painful). Rubbing alcohol or urine onto the wound can also encourage the release of venom, which means the pain doesn't last as long.
Most people think that the Portuguese Man o' War is a jellyfish and try to treat its stings in the same manner, which can lead to problems.
Can you get freshwater jellyfish?Yes. Practical Fishkeeping contributor, Pete Liptrot, recently found some baby jellyfish in a freshwater display tank in the Bolton Museum Aquarium which he runs.