No summer is complete without reports of terrifying marine creatures threatening holidaymakers – this year it’s the turn of the relatively harmless barrel jellyfish

A bloom of barrel jellyfish has been spotted on England’s south coast by snorkellers, divers and beach-goers. Cue reports of “plagues” of tens of thousands of giant “monster” jellyfish “invading” our beaches.

Not quite. The latest seaside scare is extraordinary and the animals can grow up to 5ft long, but there’s no danger. “Barrel jellyfish are harmless to us in spite of their size,” says Cathy Lucas, professor of marine biology at the National Oceanography Centre, Southampton. “All jellyfish sting, but the barrel sting is weak.”

Rhizostoma pulmo, AKA the barrel jellyfish, the dustbin-lid jellyfish or the frilly-mouthed jellyfish, reappeared in UK waters in 2014, with the highest recorded number since 2002 – but this year’s bloom looks even bigger. Marine photographer Steve Trewhella has spent 35 years diving off the British coast. “We saw at least 100, compared with one or two last year,” he says. “If you can see 100 in half a mile, it’s safe to say there are very high numbers of them.”

Facebook Twitter Pinterest Warning: great white sharks can seriously ruin your holiday. Photograph: Brandon Cole/Getty Images

The barrel bloom is just the latest in a rich history of British seaside scares, from the traditional – sewage spills, big waves – to the colourful, and the fanciful to the credible. Let’s start at the top: great white sharks have been spotted and recorded since the 1960s. Shark fever really took off following a few key sightings in Cornwall in 1999, but only seven out of 80 sightings over the last 15 years were found to be credible. Further causes of consternation include “mystery snakes” (Porthilly, 2014, just a grass snake), hand grenades thrown by a dog walker for her pet to fetch (Essex, 2014) and “killer” seals, after one was filmed gnawing on a porpoise (Pembrokeshire, 2015). A “sea monster” found in Weston-Super-Mare was actually a common ray, according to experts. Perhaps the most absurd was the “ghost ship crewed by cannibal rats heading straight for Britain”, which was found to be unlikely.

Is there a sense of panic on the beaches of Dorset? “I haven’t heard people leaping out of the water screaming,” says Trewhella. Barrel jellyfish could, in fact, be good for ecosystems: they are a food source for both endangered leatherback turtles and sunfish. Oh, we do love to be beside the seaside – but we love a good seaside scare, too.