Outside tropical waters, one of the biggest offenders is the mauve stinger. It is particularly prevalent in the summer months around the Mediterranean, when the jellyfish gather in huge swarms that sometimes drift close to popular beaches. Their victims can experience pain for up to two weeks, so the species has become known as a holiday wrecker. And in November 2007 a swarm of billions of mauve stingers wiped out an entire fish farm of 100,000 salmon in Northern Ireland.

It’s worth noting though that where they end up is governed by the currents. On a more positive note, these jellyfish do offer a spectacular light show at night. Owing to bioluminescence, they emit an ethereal glow, particularly when disturbed in the water by a moving boat. Different types of nematocyst are found on all parts of the mauve stinger – not just the tentacles – so give it a wide berth!

DANGER FACTOR – A sting will cause blistering and scabs, and can lead to nausea, vomiting, muscle cramps and even breathing disorders.

Bell diameter: 10cm (4in)

Tentacle length: < 10m (33ft)