A rare stinging jellyfish capable of forming toxic chains of up to 30 metres long has been discovered off the coast of Cornwall for the first time.

The pearl chain or string jellyfish is normally found only in deep water but is believed to have been blown inshore. Shoals of the jellyfish, which can deliver a nasty sting, were seen between Land’s End and the Isles of Scilly by a diver taking pictures of basking sharks.

Neil Hope, 44, from Torpoint, near Plymouth, said: “I was just waiting for the basking sharks to come back when I saw this thing. What attracted me was there was a small fish floating alongside it.”

The orange jellyfish had formed chains between six and ten inches long. Mr Hope said: “I just wish I could have seen a 30-metre-long one.” The string jellyfish is a close relative of the Portuguese Man o’War. Its sting contains a powerful toxin that can kill large fish. Divers have compared the sensation to that of a wasp sting, but it is possible that the toxin could inflict serious injury. In 1997 a colony caused the deaths of a large number of farmed salmon off the coast of Norway.

The rare jellyfish was also recorded in Plymouth Sound by Keith Hiscock, of the Marine Biological Association. Dr Hiscock said: “They’ve been off Plymouth for about three or four weeks. It’s an unusual event — there’s no record of anything which would match that species.”

Dr Hiscock warned people not to handle the jellyfish, but added: “It’s always useful for divers and people in boats to report their observations and send in photos. We welcome those sorts of records.”

Rory Goodall, from Elemental Tours and Atlantic Adventures, which operates wildlife-spotting trips, said that he had noticed the creatures in lengths ranging from a few centimetres to three or four metres.

Mr Goodall took a specimen ashore and, with help from experts Stella Turk, Raymond Dennis and Joana Doyle, of Cornwall Wildlife Trust, the species was identified as Apolemia uvaria.

The jellyfish work as a highly organised machine, with a specific job allocated to each colony member. A colony can start from just one individual through asexual reproduction.

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