Upside Down Jellyfish

Image: Lyndi&Jason via Flickr

OK, Cnidarians. So you got your polyps which are like the Sea Anemones. You turn it upside down and you got a medusa, which is like a jellyfish. But what happens when a medusa goes upside down again and tries out the polyp life without actually being a polyp? You just got yourself an Upside Down Jellyfish!

These rebels are found in shallow, sunlit waters in tropical regions. There are 5 or so species, the most well known of which is from the Caribbean and reaches around 30 cm (1 ft) in diameter.

When they want to, Upside Down Jellyfish can go right side up and swim about in the oddly mesmerising way common amongst jellyfish. The thing is, they prefer not to do that. They spend most of their time with their flattened bell resting on the floor and tentacles waving around above them. Why is this?

The answer lies within the extraordinary oral arms that branch out and look like some kind of cauliflower or broccoli or something. Within all that fluffiness is algae that lives inside the Upside Down Jellyfish in a symbiotic relationship.

The algae gets food for itself via photosynthesis and gives the excess to its landlord. In exchange, the jellyfish provides the algae with safety. So those oral arms are even more vegetable than they first seemed!

BocasResearchStation

But the Upside Down Jellyfish still enjoys a more varied diet than what the algae can provide. Nutrients from their vegetable friends is cool enough, but this jellyfish wants meat in the form of tiny planktonic animals.

Image: Wikipedia

To capture them, the Upside Down Jellyfish releases mucus into the water. But, like the worst cold in the world ever, this mucus contains stinging cells. Once the tiny prey is caught they're moved on to the secondary mouths.

That's right, whereas most jellyfish have one mouth in the middle, the Upside Down one has dozens of tiny mini-mouths on its oral arms. That's the kind of thing that happens when a jellyfish is even more weird than other jellyfish.

It sounds like they barely have to do anything at all! In actual fact they still pulse their bell constantly like other jellyfish do, it's just that they mainly do it to circulate water for themselves and their algal guests. Just because you're stationary doesn't mean you have to be stagnant!