A bloom of Velella velella jellyfish took over the sea at Dwejra yesterday. Photo: Sean Azzopardi

A harmless type of jellyfish that usually reaches the islands at this time of year was spotted in “extraordinary” blooms in Gozo yesterday, particularly at Dwejra.

The Velella velella species is normally found on the north and northwest coast. Sightings have also been recorded in Għar Lapsi, Xlendi, Ġnejna and Xatt l-Aħmar over the past couple of days.

Velella velella normally appear in spring, mostly between April and June, but this is the biggest bloom ever witnessed.

Marine biologist Alan Deidun, who heads the Spot the Jellyfish team, said it is “impressive”, running into the tens of thousands.

Its appearance could be due to recent northwesterly winds, as this is a wind-propelled, surface species.

He said that when on shore, the jellies lose their blue colour and turn into what looks like plastic and are likely to cover parts of the country’s north and west coast over the coming days.

The species is a floating colony propelled by a six centimetre-wide triangular flap that acts as a sail, beneath which are thousands of three-millimetre individuals surrounding a large central mouth.

Sightings can be reported to Spot the Jellyfish at www.ioikids.net/jellyfish or via SMS on 7922 2278.

The data is fed into the initiative’s new app that offers information about weather conditions and facilities on different sandy beaches, as well as details of different species and how to treat stings. In the coming weeks, the app will also give users a forecast of possible jellyfish encounters on different beaches.