A school of baby jellyfish have arrived at Reef HQ Aquarium, but mystery surrounds their arrival.

The upside-down jellyfish polyps were found in a tank that previously held an injured jellyfish that had died four months earlier.

The jury is out on how they came to be - the polyps have grown into healthy baby jellyfish and are thought to be clones of their parent.

However, having started as polyps, it is also possible they are the result of sexual reproduction despite their parent being isolated in a tank.

"Jellyfish clone very easily – when some jellyfish are cut in half, you get two jellyfish," Reef HQ aquarist Krystal Huff said.

"Since the parent jellyfish was injured, it had damaged tissue cells which could have easily grown into other jellyfish.

"But, they have grown from polyps, so we don't know if the parent reproduced with another jellyfish without us noticing."

Upside-down jellyfish will grow to be about the size of a dinner plate.

They get their name from their habit of sitting on the seabed floor with their tentacles pointing up to get sunlight and food.

They are able to swim around, but choose to stay put.

The jellyfish join baby seahorses and some leopard shark eggs, turning Reef HQ Aquarium into a marine nursery.

The jellyfish, seahorses and leopard shark eggs will soon be on display for visitors to watch them grow into adults.

Reef HQ Aquarium is the Australian Government's national education centre for the Great Barrier Reef.