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They’re the source of minor panic when swimming in the Swan River. They have no legs. They have no teeth. Or bones. And you’ve probably thrown one at a friend.
What could it be?
A jellyfish, of course.
WHAT IS A JELLYFISH?
Jellyfish are not fish at all, and scientists prefer using the term ‘sea jelly’.
Sea jellies are part of the phylum Cnidaria, a group of roughly 9000 animal species that include corals and anemones.
Sea jellies get their name from their jiggly body, a substance called the mesoglea, that makes up their ‘skeleton’. It’s not a skeleton in the human sense, but it gives the animal structure and protects its organs.
YES, ORGANS
It may not seem like it because they’re 95% water (that’s more than a watermelon), but sea jellies do have organs.
These are their nervous system, digestive system, eyes, gonads (sex organs) and tentacles.
The sea jelly’s nervous system is relatively simple but is responsible for setting the rhythm that they swim at, their vision and knowing where they are in the water using gravity sensors.
Their digestive system is simple too. They’re not hunting down large antelopes or setting up large banquets. Instead, sea jellies are passive feeders, meaning that they just float or swim through the water and eat whatever floats past them.
This food gets munched on and processed in a single cavity, called the coelenteron, which can extract nutrients and sort out the waste.
SWAN RIVER JELLIES
If you see a jelly in the Swan River, there’s two different species it could be: the Australian spotted jellyfish or white-spotted jellyfish (Phyllorhiza punctata) or the moon jelly (Aurelia aurita).
Even though these jellies don’t live in the ocean, the Swan River is a saltwater estuary, so there’s no need to change their name to river jelly.
The Australian spotted jellyfish can be varied in colour, ranging from orange-brown to purple.
In comparison, the moon jelly is clear and looks the same all over the world.
The four semi-circles seen through the moon jelly are their gonads, in charge of making lots of little sea jelly babies.
While these jellies have a lot in common, there’s one crucial difference to keep in mind.
The Australian spotted jellyfish can’t sting humans, but the moon jelly can give you a mild zap. Still, you can save the panic for when you’re snorkelling in Ningaloo Reef, home to deadly Irukandji jellies.
