On a calm summer morning in southern Australia, the water can look deceptively clear, until you see thousands of gelatinous shapes washing ashore.

In January, thousands of pink lion’s mane jellyfish washed into Port Phillip Bay, prompting beach warnings and startling swimmers more accustomed to cold water than the shock of stinging tentacles.

The same month, unusually high numbers of moon jellyfish were reported across southern Tasmanian coastal waters.

If you’re swimming in southern seas and have an encounter with a jellyfish swarm, you may well wonder what led to it. Could a cold spike, a marine heatwave or other changes in the ocean have triggered the bloom?

Are these blooms normal?

Many Australians associate jellyfish with the dangerous stings of northern species such as box jellyfish and Irukandji. But jellyfish aren’t just confined to tropical waters. In southern Australia, species such as lions mane and moon jellyfish are more common, particularly this summer.

In fact, hundreds of species of jellyfish are found in Australian waters, ranging in size from a mere speck to nearly two metres wide. And it’s natural for jellyfish numbers to boom and bust. When conditions are favourable, they can bloom into superabundance. Their millions of mouths can strip every particle of food out of the water, from........

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