Rebellious jellies and crafty crustaceans

Giovanna Fasanelli | VOLUME 28, ISSUE 4
Down is up (and vice versa) for the upside-down jellyfish, which occasionally hitches a ride on the carrier crab (above).
Just because everyone else swims upright doesn’t mean you have to, especially if you happen to be an upside-down jellyfish!

For members of the Cassiopea genus, life is not lived in the normal jelly way – that much is instantly obvious when one encounters these beasties whilst snorkelling. Instead of heading nonchalantly towards the surface, they make a determined beeline for the bottom!

These odd creatures earn a living primarily through their symbiotic algae tenants, otherwise known as zooxanthellae. These single-celled, photosynthetic power-houses are the same type of algae that reside within many hard corals, providing up to 98 per cent of their host’s nutritional requirements for life.

The upside-down jellies are also rather fond of this easy-going, if not lazy, lifestyle where all they have to do to keep their end of the bargain is swim down to the shallow, sandy bottom, lie on their ‘heads’ and bathe their algae-laden oral arms in the sunshine. Why waste all that extra energy swimming about hoping to bump into something to eat when you can enjoy the fare of millions of live-in chefs?

As they are primarily reliant on their zooxanthellae for food, a powerful sting is unnecessary, but snorkelling over meadows of these jellies may yield some itchy discomfort as the surrounding water will inevitably carry layers of mucus sloughed by these lazy layabouts. By pulsating the rim of their bells they are able to earn a more honest living by drawing in and capturing passing zooplankton, which helps make up any nutritional shortfall.

As a result of the algae living within their tissues, they are usually golden brown or green in colour but, upon closer inspection, their undersides branch off into beautiful lacy extensions that aid incamou flaging the jelly against the substrate, where they end up looking a lot like an anemone.

Upside-down jellies can be found in a variety of sandy-bottomed habitats all around the warm coastal waters of the world.

In a further bizarre twist to the story, the upside-down jellies occasionally fall victim to the crafty, kidnapping ways of a certain family of decapod crustaceans commonly known as the carrier crabs.

Let’s face it: crabs are pretty darn tasty. And we are not the only ones who think so. From octopus, triggerfish and wobbegongs to groupers, emperors and eels … a crab admittedly has a lot to be worried about. So when you know you are likely to be on the menu for just about everything that’s bigger than you, you have to get creative. What better way to protect yourself than to cover your body in a blanket of stinging jellyfish, or spiny sea urchin?

There’s rarely a more humorous sight to behold whilst snorkelling than watching one of these carrier crabs, its carapace measuring a mere few centimetres across, hauling around a huge, heavy upside-down jelly.

It walks using two front pairs of legs, whilst the back two cling onto its unwitting bodyguard, which keeps pulsating downwards, making movement even more laborious.

When not feeding, carrier crabs are usually in a hurry and are frequently bumping into unseen obstacles and trying to sneak in under rocks where their pet jellyfish can’t fit! It’s underwater comedy at its best.


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