Moon Jellyfish (Auerlia auritas)


The moon jellyfish is one of the most famous jellyfish in the world, and certainly the most popular jellyfish in captivity. They are recognized around the world by their mesmerizing pulsing movements through the water. They are admired by many for their simplicity, and studied by scientists to learn more about their regenerative abilities. 

These simple-bodied creatures have since the Precambrian days, long before dinosaurs walked the earth. It’s likely they will long after us, gently pulsing through the water in oblivious tranquility.

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The moon jellyfish is characterized by a pale round bell with four distinctive horseshoe shapes on the tops. The horseshoe shapes are actually their stomachs, which can be seen colouring up as the moon jellyfish feeds. They have four oral arms located underneath their bells, with the bells surrounded by a fine fringe of tentacles. They are coated in a gooey mucus to capture prey the brushes alongside them or their tentacles.

Their tentacles are incredibly short in comparison to other species, only averaging about three inches (roughly seven and a half centimeters) long at most, and that would only be in the largest specimens. Instead of growing long tentacles, the moon jellyfish tend to have their bells grow in diameter! These jellyfish are often between two and fifteen inches, or five to thirty-eight centimeters in diameter in the wild. They often only reach their largest size in the wild, as moon jellyfish grow and shrink based on how much they are fed.

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To feed, moon jellyfish stun small prey with their tentacles, and bring the food to their oral arms. These four oral arms then bring the food up to their mouths, located in the center of their bells. Like a sea cucumber, their mouths serve two purposes, taking in and food and pushing out excrement.

The most incredible thing about these jellyfish has to be their simplicity. These jellyfish, like most others, have no brains, eyes or lungs. Instead, they have stomachs, gonads, oral arms, a mouth and tentacles. In fact, only up to five percent of their total bodies are actually composed of anything solid, the rest being made up of the saltwater they live in!


The habitat of the moon jellyfish is vast, expanding each and every ocean. That said, they are mostly found in the Pacific and the Atlantic. They are most often found around coastal areas, but they can be found anywhere near the surface of the ocean. They can be found both in warm and cool water. These jellyfish are by many considered the hardiest species of jellyfish. They adapt to their habitat, and seem to thrive wherever they go. They have been found in water temperatures as low as -6°C. However, on the contrary, they have been found in waters reaching temperatures as high as 31°C! Of course, neither temperature extreme is what the jellyfish really thrives in. In fact, the moon jellyfish seems to do best between 9°C and 20°C. However, as usual, what jellyfish like most is stability. They thrive in stable temperatures. 

Life Cycle

The moon jellyfish life cycle is fascinating. Unlike what many believe, moon jellyfish actually are divided into two sexes, male and female. In the wild in Fall, when temperatures become cooler, the males release sperm into the water that is picked up into the female’s gonads to fertilize her. The eggs then incubate in a pouch on the female, before they are released onto the seabed as planula larvae.

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When the planula larvae settles into the seabed, it begins to change. It forms into an odd little thing called a polyp. Attached to the seabed on one end and searching for food on the other, the polyp slowly begins to seem like it forms more of itself, in a stage known as the budding polyp. The top of the budding polyp seems to look like a stack of crepes held together. Eventually, the top of this ‘crepe stack’ will begging to pulsate, and slowly but surely, pull away as a small ephyra. The ephyra is rather different from the final adult phase, as not only is it absolutely tiny, the small ephyra have been described as looking almost like ‘snowflakes’. As the ephyra feed and grow off of the zooplankton around them, they begin to take shape, and take their final phase as an adult medusae. 

When fall comes, the cycle begins again.

In Captivity

In captivity, I find moon jellyfish behave almost identically to how they do in the wild. They pulsate mindlessly through the currents, collecting food provided to them and growing and shrinking depending on how much they eat.

However, in our home aquariums, there are some ways to make the lives of these mysterious and strange creatures a little bit better. The first way to encourage your moon jellyfish to live a long and healthy life is to provide them with stability. They require a stable temperature all year long, and prefer to be kept at stable salinity levels. I find that the best choice is to keep them between 9°C and 20°C, and at a salinity level of 30ppt, or 1.0226. But really, chasing numbers does more damage to your jellyfish than good. I realize that many of us are desperate to keep specific numbers at all times, but far too often I’ve seen this be the downfall of even the best aquarist. I’ve seen people be more likely to make mistakes in dosing the water, or in the more common case, change the water and damage the livestock in the aquarium. In some cases, some people have even gone so far as to interrupt the nitrogen cycle in the aquarium, and the tank ultimately crashes. Keeping a stable temperature all year, and ensuring that your jellyfish are kept in clean, stable salt water is really what’s best for them. And if you ask me, worrying about numbers takes away from enjoying watching your relaxing jellyfish.

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However, it’s a good idea to remember that in the oceans, jellyfish are provided with fresh, clean water at all times. In our home aquariums, nitrates can easily build up quickly, so it’s a wise move to measure your water levels on a regular basis, and change your aquarium water frequently. Having worked with these magnificent creatures, I like to advise at least 25% once a week. I wouldn’t go too far, and do a full water change, not unless you notice a reading of nitrites or ammonia. In the case that there is a spike of ammonia or nitrite, a 50-75% change of water in the aquarium is needed.

Feeding these beasts is also very simple. I find that I can feed them either once a day, or once every other day. The best diet to feed them on is freshly hatched brine shrimp, which can easily be raised at home. The other best option is to feed them on our diet of specially formulated jellychow, which is essentially freeze-dried zooplankton. A small spoonful once a day is really all that’s needed. However, as our jellychow is not live, it does not stay in the water column as long as freshly hatched brine shrimp, and therefore is more likely to dirty your aquarium. At Jellyfish Aquarium, we liked to feed our jellyfish with freshly hatched brine shrimp once a day every day, except on Saturdays, the day before we do a water change on the tanks. On Saturday, we feed them our jellychow, and the next day we can suck up any debris in a simple water change. We find this diet does help keep the aquariums clean, and keep the jellyfish healthy.


Moon jellyfish are incredibly hardy, and great for the beginner jellyfish keeper. We love them dearly, and we hope you can as well. By keeping their aquarium water clean and feeding them a healthy diet, your jellyfish will live a healthy life. It may seem challenging at first, but as you learn more about these fantastic animals, you will find they’re quite easy to keep. 

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The Regal ‘Crown Jellyfish’

Learning more about the elusive ‘Cephea cephea’

Your Majesty…

The crown jellyfish is a gorgeous jellyfish indeed! It pulses along majestically through the ocean waters, stunning all in its wake! Such a beautiful jellyfish is truly a sight to behold. The jellyfish sports a royal purple bell, surrounded by a superb frilled bell. At the top of their bell, they have 30 appendages that stick out, but scientists are still baffled by what these could be for.  Due to the rare nature of these jellyfish, very little is known about them. What we do know though is valuable, precious information.

The jellyfish is of an average size, as far as scientists can tell. Their tentacles have been recorded to reach lengths of 80 centimeters, or 31.5 inches in length, but it is unknown if this is the maximum length. However, the stunning bell of these jellyfish can reach  a whopping diameter of 60 centimeters! That’s 23.6 inches, which is well over a foot!

The Monarch’s Life

The crown jellyfish rules the deeper indo-pacific and atlantic ocean’s waters, living at a depth of 200m-1000m (in the Twilight Zone) below the surface. They are most often found in the depths at an average of 700m deep. The waters are dark, and cold, meaning these jellyfish typically live at a temperature of around 7° Celcius. Because they live in almost total darkness, this jellyfish has actually adapted to shine bioluminescent lights! They use these bioluminescent lights to lure in prey, and distract predators alike.

Like most jellyfish, they feast daily on zooplankton. In the Monterey Bay aquariums, they’ve also been known to take readily to brine shrimp. They easily catch their prey with some of the most powerful venom known to jellyfish. Ironically though, they seem to have no impact on most living life. They are completely harmless to humans. In fact, in east asian countries, they are seen as a delicacy. The sting of these jellyfish is so weak that they are unable to do so much as spook a small fish. In fact, they’ve even been found to host various species of fish in the safety of their bells. Below is an example of such a phenomena.

The life cycle of this jellyfish mimics most other jellyfish. However, what is fascinating is the speed at which they grow! The Monterey Bay aquarium has stated, “our aquarists found the species to grow at an extremely fast rate — from just a few millimeters to the size of a dime in two to three weeks” ( The speed at which these jellyfish grow could also play a large part as to why they have such short life cycles. In fact, these jellyfish are known to live only up to six months in the wild. It is unknown for how long these beautiful jellyfish live in captivity.

Can You Keep a Crown Jellyfish?

As of yet, the answer is no. Not in our small aquariums that we sell. Jellyfish Aquarium currently does not have enough information on this species to be able to confidently stock these jellyfish for the public. Of course, as much as I would love to keep one, it’s necessary to develop bigger and better aquariums for these jellyfish. Not only that, but finding cheaper and more available water cooling systems to regulate the temperature is a must.

One day, in the future, it may be possible to keep a crown jellyfish in our own homes. But for now, we can admire this jellyfish from a distance.


If you’d like to learn more about the crown jellyfish, please take a look at some of the links below that were used to write this article!