Blue Zoo: Jellies


Featuring one amazing marine animal per week.

All true jellies are part of the phylum Cnidaria, which they share with other animals including soft and hard corals, hydroids, and anemones. Within this phylum, the true jellies belong to the order Scyphozoa.  Although many people call them jellyfish, scyphozoans aren’t really fish at all! 

Jellies may be most famous to beachgoers for their sting. They have specialized stinging cells called cnidocytes – a trait that they share with their other cnidarian cousins. The jelly's tentacles are lined with thousands of cnidocytes and inside each one is a nematocyst. When it bumps into something, pressure builds inside the nematocyst and this causes it to fire out a long thread with a barb on the end, injecting venom. 

Despite what you may have heard, the best way to treat a jelly sting is usually to pour vinegar on it.

A sea nettle. Photo by The Pug Father via Flickr, Creative Commons License. 

The most dangerous sting comes from the box jelly, also known as Irukandji, which is capable of killing humans. These guys are not, in fact, scyphozoans –  but they are close relatives known as cubozoans.

Other jellies, however, are perfectly harmless. The common moon jelly, for example, has a very weak sting that is really only useful for catching plankton – humans normally can't even feel it. Palau also has a famous species of “stingless” jelly; while they do still have nematocysts, they have become weaker due to the lack of  predators in the isolated sea lakes they call home.

This jelly found in Palau is harmless to humans. Photo by aSIMULAtor via Flickr, Creative Commons License.  

Amazingly, scyphozoan jellies are extremely diverse and hardy animals – even though they are made of 98% water and don't have brains! They can be found in almost every ocean environment around the world – from the Arctic to the deep sea; some are just millimeters long and others are a meter wide with tentacles hundreds of feet long. Some are clear, others are brightly colored; some look green due to a symbiosis with tiny algae that perform photosynthesis inside the jelly's tissues, while others produce bioluminescence so they glow in the dark!

Some scientists have predicted that changes in ocean conditions, such as climate change and ocean acidification, are leading to increasing jelly populations. This is because those changes have negative impacts on most animals, including jelly predators, but the jellies themselves are still able to thrive.

This unusual species is known as a cauliflower or crown jelly. Photo by Derek Keats via Flickr, Creative Commons License. 

Several other soft-bodied organisms (like salps and comb jellies) are easily confused with scyphozoan jellies even though they aren’t directly related and have different physiologies.

Learn more about jellies (and their relatives) in the video below!

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