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Question of the Week: Why do box jellyfish have eyes?

By Kyle Lee-Crossett, on 10 May 2018

If you meet me during one of my shifts as a PhD Student Engager in the Grant Museum, you’ll find me next to the Micrarium, facing a case packed full of jellyfish and their ghostly relatives. I’ve never had an interest in jellyfish before, but hours and hours of staring at them over the time I’ve been an Engager has inspired my admiration (as well as a previous blog post).

Box jellyfish specimen at the Grant Museum, photo by author.

In recent weeks, I’ve had a number of visitors ask me about box jellyfish eyes, because it’s surprising to find out that something which often looks and moves like a floating plastic bag has eyes. And not just one or two eyes, but 24 in total. Their eyes are bundled into four structures called rhopalia, which sit around the bottom of its bell. Two of the eye types have the capability to form images, while the other two types help with swimming navigation, avoiding obstacles, and responding to light. Fun fact: Box jellyfish can regenerate their eye bundles (rhopalia) in as quickly as two weeks’ time.

On the specimen in the Grant Museum, you can only see two of it’s eyes because it’s been carefully bisected to reveal its internal anatomy.

Grant Museum specimen with eyes highlighted by author.

Like other jellies, box jellyfish have no brain, perceiving the world only through their nervous systems. Most jellyfish catch their prey without having either brains or eyes, just by floating transparently through the sea until prey run into their tentacles. So, our question should actually be: why do box jellyfish even need eyes?

There are at two main possibilities:

1)  Habitat: Unlike most jellies, which live on the open sea, box jellyfish tend to live in shallow water, which has many obstacles. Scientists have shown that box jellies near Puerto Rico can navigate around the dense mangrove swamps where they live, and also make sure that they don’t drift away to where there is less prey. Their upper lens eye can actually peer through the water’s surface to navigate from landmarks above the water, and perhaps celestial ones as well! Some scientists think these kinds of jellies actively hunt rather than passively encounter prey.

2) Reproduction: Among jellyfish, box jellies also have unusual mating practices, involving the precise transfer of sperm, which might involve the use of their complex eyes to identify mates.

Many things about jellyfish biology and behaviour are still a mystery to scientists, so keep a lookout for ongoing discoveries.

 

Bonus fact: box jellyfish also need to rest their eyes

Scientists have only recently discovered that jellyfish appear to sleep at night—an activity usually only associated with vertebrates. Some reasons why they might do this include is because of their reliance on vision for hunting (they don’t see well enough to hunt in the dark) or because they jellies simply need to take a break from the neural processing their eyes require.