Jellyfish Swarms

Sep 1, 2011
John Blatchford

Australian Spotted Jellyfish - Papa Lima Whiskey - Wikimedia Commons

Stinging jellyfish are becoming more of a nuisance around the world, sometimes even killing bathers.

Swarms of jellyfish are becoming common, and many species are extending their range. Swimming among jellyfish swarms can result in stings which range in severity from unpleasant to excruciating – causing death in extreme cases.

Swarms are becoming more common because:

overfishing continues to remove fish from the sea, so jellyfish numbers can increase

climate change lowers the amount of freshwater coming down rivers, so jellyfish can move in closer to the shore (as the water there becomes salty enough for them)

Many jellyfish species are also increasing their range. Sometimes climate change is responsible (warmer water suits many species, while others exploit the changes in ocean surface currents), and in other cases the sessile young (polyps) travel to new areas attached to ships.

Australian Spotted Jellyfish in Spain

In July 2011 six beaches in Spain were closed to swimmers after more than 100 bathers needed treatment for stings. The jellyfish to blame was the Australian spotted jellyfish (Phyllorhiza punctata).

This is a big jellyfish (45-50 cm in bell diameter) originally only found in the southwestern Pacific around Australia. Recently there have been swarms in the Gulf of Mexico and they have been recorded in southern California, around the Hawaiian islands, and in the Caribbean.

Spotted jellyfish damage fisheries by eating young fish and their planktonic food, and by clogging fishing nets. They also sting swimmers, but fortunately their venom is mild (not considered a threat to humans life) – but still painful!

Some other jellyfish have much more dangerous stings.

Three Dangerous Australian Jellyfish

Some Australian 'Box Jellies' are notorious; the sea wasp (Chironex fleckeri) is considered the most lethal jellyfish in the world, and the small 'Irukandji Jellyfish' (Carukia barnesi and Malo kingi) can also give fatal stings.

The sea wasp has stinging tentacles eight foot long, with a body (bell) the size of a football. It feeds on shrimps and fish, hunting by day and resting on the sea bed at night. Some Australian beaches are protected from sea wasps by nets, but these are of no use against the Irukandji.

Irukandji are very small (less than a cubic centimetre in diameter), and impossible to spot when swimming. They can easily pass through nets protecting beaches.

Stings in Spain and France

In most recent years there have been summer swarms of jellyfish off the Spanish and French coasts (in 2006 the Spanish Red Cross treated 19,000 swimmers along the Costa Brava), and the Mediterranean Sea is becoming home to more and more jellyfish.

While not strictly a jellyfish (a colonial hydroid) the Portuguese Man of War (Physalis physalis) is regularly blown towards atlantic shores, and has recently entered the Mediterraneab to join the jellyfish. It is easily spotted since it waves a large bluish float above the water (its 'sail'), but it has very long 'tentacles' and gives a very painful sting if you swim too close.

It is important to realise that jellyfish can sting long after they are dead, so touching stranded jellyfish (or pulling tentacles off bathers who have been attacked) can be very painful.

As the sea warms up holidaymakers go in for a swim – then along come the jellyfish!

Sources: