Sea jellies 101: What is a sea jelly?

Most people think of jellies as clear, squishy blobs of goo that sting and ruin your day at the beach. You can see them in several places throughout the Aquarium. The common image in most people's minds looks somewhat like an umbrella with long strings dangling. While this is what many jellies look like, to a biologist the term "jelly" can refer to a huge range of animals that have existed on the planet for around 650 million years! Let's explore...

Jelly Classification
When classifying jellies the best place to start is the phylum Cnidaria, so named because of the venomous cells called cnidocytes that are unique to animals in this phylum. Cnidaria is most commonly divided into four classes and perhaps the most well known class is Scyphozoa – sometimes referred to as "true jellies," or those umbrella-shaped jellies mentioned earlier.

Some examples at the Aquarium would be moon jellies and our pacific sea nettles. Another well represented class at the Aquarium that you may not have even realized is a cnidarian is the class Anthozoa – the corals and sea anemones. The last two classes are Hydrozoa (hydroids, Portuguese man-o-war) and Cubozoa (box jellies, sea wasps).

Moon jelly © Hans Hillewaert / CC-BY-SA-3.0 , via Wikimedia Commons

The second most prominent phylum referred to by the name "jelly" is Ctenophora, the "comb jellies." The most prominent feature of these animals is their comb-like cilia that they use to swim. Ctenophores differ from cnidarians in a few ways, perhaps the most important being that ctenophores do not have cnidocytes, and thus they cannot sting.

Comb jellies, credit: Überraschungsbilder, via Wikimedia Commons

Common Characteristics
Jellies are invertebrates made of about 95% water, so as you can imagine they are pretty fragile. Jellies consist of two or three layers of cells formed around a central opening – this organization of similar parts around an axis is known as radial symmetry. Despite the fact that they do not have a brain, heart, real digestive system, complex eyes OR a respiratory system they are still effective predators of mainly zooplankton. Jellies that move do so by pulsations of their bell, but many species are sessile and don’t move at all. Or, some species are sessile at one stage of their life and then begin to free swim.

Which leads us to sea jelly reproduction! As the weather warms up, you'll likely see loads of jellies in Boston Harbor (see this blog post for evidence). Keep checking the Exhibits Blog to learn how sea jellies reproduce and to find out why there are just so many jellies in oceans today!

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