A mushroom cap jelly on Sanibel.

A mushroom cap jelly on Sanibel.

by Johnny Rader

In recent weeks, some large jellies have washed up on our beaches. They are called mushroom cap jellies, and they are completely harmless. This week, let’s take a closer look at the more common jellies found in our area, learn about their basic biology, and find out which ones to avoid contact with.

An upside-down mushroom cap jelly.

An upside-down mushroom cap jelly.

Jellies – the term “jellyfish” is incorrect because they are not fish – belong to a group of invertebrates called Cnidarians, and are closely related to corals, anemones and hydroids. Many of these animals have nematocysts, stinging cells that help them capture prey and provide defense against predators. Jellies are active swimmers and hunters that feed on fish, plankton, and even other jellies. Unless you are sure a species is harmless, you should avoid coming into contact with jellies whenever possible.

True jellies are Scyphozoans, or free swimming Cnidarians. Many are bell or mushroom shaped with tentacles around the bell. The mushroom cap jelly has no stinging tentacles, so it is safe to touch. However, we always suggest looking instead of touching, as they are similar in appearance to more dangerous species, like the moon jelly.

Two Portuguese man o’ wars on a beach.

Two Portuguese man o’ wars on a beach.

When we hear the term jellies, many of us think of the deadly Portuguese man o’ war, but it is not a true jelly at all, rather a type of Hydrozoan. These animals are Cnidarians with some of the most potent nematocysts on the planet. They are a colony of many single-celled organisms, and can cause severe symptoms if they sting you. They are most easily recognized by the clear, bluish bubble that keeps the colony floating at the surface of the water. Use extreme caution if you come close to a Portuguese man o’ war.

A comb jelly.

A comb jelly.

Comb jellies, or Ctenophores, are one more group that resembles jellies, but are classified separately. These animals lack tentacles completely and propel themselves through the water using ciliated hairs along their sides. Comb jellies tend to be quite small and use sticky cells (kind of like slime) to capture the prey, which is mostly plankton.  

More Facts About Jellies:

  • They are made up of nearly 95% water.

  • Sea turtles sometimes confuse jellies with plastic bags, which can cause illness or death if ingested. Don’t forget to bring your reusable bags to the grocery store.

  • Many Jellies will relocate tentacles around their bell after losing others. This helps the animals rebalance themselves to swim more efficiently.

  • Fossil Records indicate jellies have been on our planet for about 500 million years.

  • The box jelly’s nematocysts are potent enough to kill an adult human.

  • Jelly stings are best neutralized with white vinegar.

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