Box Jellyfish: Anatomy, Movement & Adaptations

Instructor: Lauren Posey

Lauren has taught intermediate reading in an English Language Institute, and she has her Master's degree in Linguistics.

In this lesson, you will learn about the anatomy of box jellyfish, including how they move and some interesting adaptations that make them different from other jellyfish species.

Box Jellies

What comes to mind when you think of a jellyfish? Maybe you picture a round top with tentacles hanging down. What about a cube?

There is a whole class of jellyfish, the Cubozoa group, that have cube-shaped bells (the top part of a jellyfish). The common name for these is box jellyfish. They are also known as sea wasps or marine stingers. Within this group there are a number of different species. However, they share many features that make them distinct from other types of jellyfish.

Basic Anatomy

All jellyfish, box jellies included, are invertebrates, meaning they have no spine. In fact, they are entirely soft, and have no bones of any kind. The name box jelly comes from their square bells, which differentiates them from round-belled species.

Box jelly tentacles hang down from the four corners of the bell. Some species have multiple tentacles at each corner, while others just have one. All of them have a long tube, much like an elephant's trunk, that hangs down from the center of the bell. This is called the manubrium, and the mouth of the jellyfish is at the tip.

Inside the bell is the gastrointestinal cavity, which holds the stomach and guts of the jellyfish. The guts are separated into the stomach, and four distinct sections, known as gastric pockets.

The gonads, or reproductive organs, of the jellyfish are also inside the bell. There are eight of them, with four on either half of the divided gastrointestinal cavity. These gonads are what release the sperm or eggs when the jellyfish is reproducing.

The basic anatomy of a jellyfish
Basic jellyfish anatomy

The Nervous System

Box jellies also have a number of anatomical adaptations that set them apart from other jellyfish. One is the development of their nervous system, which is far more advanced than other types of jellyfish.

Box jellies have a ring of nerves around the base of their bell. This helps them contract and expand the bell, moving them purposefully rather than drifting with the current.

In addition, box jellies have eyes, which other jellyfish do not have. These are set in groups all the way up the outside of the bell. They may not be as advanced as our eyes, but they can see light and dark, as well as some images. This helps them find prey and escape predators.

The presence of eyes in a box jelly is particularly interesting because they have no brains! There seems to be nothing there to process the images. However, by observing how they react to prey and predators, we can see that their eyes are clearly functional.

Adapted for Movement

Another adaptation that sets the box jelly apart from other types of jellyfish is its bell. The underside of the bell is shaped like a shelf. This helps the jellyfish catch and expel water when it pulses the bell, and (along with its developed nervous system) allows it to move more quickly. Box jellies can move up to six meters a minute. That may not sound like a lot, but most jellyfish can only drift with the current. In the jellyfish world, box jellies are like race cars!

The shape of the box jellyfish bell allows it to move more quickly.
Box jellyfish

Other Adaptations

Another adaptation of the box jellyfish is its toxin. Most jellyfish can sting, but the toxin of a box jelly is extremely strong, and can quickly kill an adult human. This may have developed as a response to very large predators that the box jelly once had to deal with.

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