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The Box Jellyfish (Tripedalia cystophora) and its Rhopalia

Kenia Damian 
Lake Forest College
Lake Forest, Illinois 60045

Can you imagine a species that has existed since the dinosaur era and still persists in the 21st century forming part of our ecosystem? A species that consists mostly of water and has twenty-four eyes? That astonishing species is the box jellyfish (Tripedalia cystophora). These box-like cnidarians are composed of four different types of eyes with upper and lower lenses that reflect images. Regardless of the direction the jellyfish swim in, they have a full view of the environment; their upper lenses are always looking up, while their lower lenses are always looking down. Their twenty-four eyes are located in sets called rhopalia. Little research has been conducted regarding their visual system, but so far it has been demonstrated that their visual system is very elaborate. More biological studies on the eyes of the box jellyfish could be the gateway to incredible scientific discoveries about visual systems.


In 2011, box jellyfish,ere studied by Anna Lisa Stöckl, Ronald Petie, and Dan-Eric Nilsson in Sweden, and were found to have an advantage in having twenty-four eyes. Their research focused on how the box jellyfish can guide its behavior based on its eyes. They used two different types of box jellyfish: jellyfish with four rhopalia and jellyfish with one rhopalium. The researchers then exposed the jellyfish to four different light conditions: light off (panels turned on or off), light on (panels turned on or off), constant light (panels turned on) and constant darkness (panels turned off). With different light exposure conditions, they recorded the creatures’ pulses in seawater for 30 seconds. They recorded the swimming pulse of the jellyfish by placing them in an experimental tank (Figure 1). The tank contained LED panels attached on the outside to supply the light and a camera facing up to record the pulse of the rhopalia. The first recordings were done with four rhopalia and the second were done with one rhopalium.


Damian Fig 1

Figure 1. The experimental tank.
(A) The box jellyfish (Tripedalia cystophora) with the bell (B), manubrium (Ma), rhopalia (Rh), and tentacles (T). (B) The experimental tank (Et) with suction at the top (Te) and light panels (Lp).  (C) shows one rhopalium’s pulse in the light condition. 


The results of this experiment brought up many questions and showed that there is much more to find out about the box jellyfish’s elaborate visual system. For the jellyfish with four rhopalium, during the constant light and dark condition, there was no statistical significance between the two conditions. The four rhopalia’s pulse increased slightly for light off, and it decreased slightly for light on. On the other hand, the jellyfish with one rhopalium’s pulse frequency was not statistically significant; however, under the light off condition, the pulse significantly increased, then it decreased during the light on condition. The mean pulse frequency results are shown in Figure 2.

Damian Fig 2

Figure 2. (A) Pulse during constant light. (B) Pulse during constant darkness. (C) Pulse during light off (panels turned on or off). (D) Mean pulse frequency resulted equal for both rhopalia conditions, but the light off condition increased significantly for the four rhopalia vs. one rhopalium.


In conclusion, the box jellyfish is a unique species that awaits further research. The researchers in this study were surprised at some of the findings in this experiment. Their data shows the reliance the jellyfish has on its eyes to navigate and change behavior. The four rhopalia showed a higher pulse rate in all of the conditions, which indicates how much the jellyfish relies on their visual cues. The researchers also strongly suggest that being able to see their environment controls their swimming speed, since their pulse increases with more rhopalium. The box jellyfish (Tripedalia cystophora) is one of the oldest aquatic animals that is still around long enough to do more research. Since some of the twenty-four eyes that are in the jellyfish’s bell have a similar structure to human eyes, we could examine how their central nervous system works with their eyes to interpret all of the different visual cues at the same time. Future studies on the box jellyfish could reveal how their eyes and nervous system correlate to allow it to thrive in its environment. Perhaps discovering those connections can show us how to help patients that are brain dead or develop new technology to help them recover. Furthermore, discoveries in the least expected animals- like the box jellyfish- can help unravel the science of tomorrow.



Stöckl AL, Petie R, Nilsson D-E (2011) Setting the Pace: New Insights into Central Pattern          Generator Interactions in Box Jellyfish Swimming. PLoS ONE 6(11): e27201.            doi:10.1371/journal.pone.0027201


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