The appearance of the mammoth creatures in Sepetiba Bay, Rio de Janeiro, has alarmed local authorities who have issued an alert about the dangers of the marine animals in the coastal area.
The sightings are believed to be the first registered cases of the venomous sea monsters emerging from the surf onto beaches in the region.
In a statement, the government said: "According to their morphology these jellyfish have a high concentration of toxins and can cause serious injury or even allergic reactions when in contact with the skin.
“As a precaution, it is important to keep away from the species and not swim when they are in the area.”
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“It is important to keep away from the species and not swim when they are in the area”Local government
A video has emerged of one of the huge jellyfish emitting poisons as it floats near the shore.
The footage was captured last Thursday (December 28) by swimmers who can be heard reacting with fear at the size of the soft-bodied beast.
One onlooker can be heard saying: “She’s leaving a trail of venom behind her so don’t go near her.”
The massive gelatinous umbrella-shaped creature was snapped gliding next to a fisherman’s boat and appears to be at least a third of the size of the vessel.
Concerned tourists initially believed the jellyfish were the Lion’s Mane, the largest and most poisonous species normally found in the Arctic and the North Sea around the UK and Norway.
But marine biologists from the universities of Sao Paulo and Rio de Janeiro claim the specimens are Lychnorhiza Lucerna.
This particular variety is also large and frequently found in tropical waters.
Biologist Sergio Stampar said “We will be analysing the DNA to identify the species. From what we have seen we believe it is Lychnorhiza Lucerna because of the different staining patterns.”
Marcelo Szpilman, president of Aqua-Rio, the Marine Aquarium in the city warned: “The species has an active poison that can cause problems for those who are hit.”
The wild animal releases a toxin that can be very painful, severely irritating the skin leaving sore bruised marks.
Victims who have been stung are advised to wash the area with seawater or vinegar but not to rub the affected region.
Mr Szpliman said: “A sufferer shouldn’t use fresh water or other liquids such as alcohol or olive oil, for example, as this will further irritate the skin and not ease the pain.
“Anyone stung should also be kept under observation for other symptoms such as dizziness, vomiting, fainting, difficulty breathing or closing of the throat. If this happens, it may be sign of an allergic reaction and the person should be seen by a doctor immediately,” he added.
Lifeguards said there have been no reports of injuries so far.