Longing for the heart pattern

In this column, we'll introduce you to the heart-shaped jellyfish, the Aoba moon Moon jelly! We'll tell you about the challenges the staff faced as the Aoba moon Moon jelly, born at Sumida Aquarium, grew to maturity. (Kanami Suzuki, Exhibit and Breeding Team)

■From "Ogasawara Moon jelly" to "Antarctic moon Moon jelly" <br />Sumida Aquarium breeds jellyfish, and the aquarium's Aqua Base"Labo" houses many jellyfish polyps (small sea anemone-like structures that form before they take on the shape of a jellyfish ) ( see previous column ). Since 2013, the aquarium has also been raising polyps from "moon Moon jelly collected from the waters of Ogasawara ," from which small jellyfish are hatched.

The Ogasawara moon Moon jelly and the moon Moon jelly commonly found in the waters off Honshu have different shapes of four gonads in the middle of their umbrellas, but in the past, when jellyfish were raised to 5cm, no difference was apparent. As time passed, research progressed, leading to the question of whether the commonly observed Honshu moon Moon jelly and the Ogasawara Moon jelly might be different species, and in 2023 it was discovered that they are genetically different species. This jellyfish has now been given a new name: the "Aoun Moon jelly."

Taking advantage of this opportunity to introduce the Ogasawara Moon jelly as the "African moon Moon jelly" once again in the spotlight, we decided to try raising them again to see if there were any differences in their shape, and we have now begun our efforts to raise them!

■What is the Afghan moon Moon jelly?
As of 2025, three species of Moon jelly have been confirmed to inhabit the waters near Japan: the Moon jelly Aurelia coerulea, the northern moon Moon jelly Aurelia limbata, and the Auriga moon Moon jelly Aurelia malayensis. However, jellyfish classification is still in its infancy, as even the most popular Moon jelly has seen its scientific name change from Aurelia aurita to Aurelia coerulea in the last 10 years. Among these Moon jelly, the Auriga moon Moon jelly is a relatively new jellyfish, having only been proposed in 2024.

The Aoba moon Moon jelly lives in warm waters such as Palawan Island (Malay Archipelago) in the Philippines and Hawaii, and has been spotted in Japan in Okinawa and the Ogasawara Islands. Its appearance differs from that of the Moon jelly in that it has a jagged, heart-shaped gonad, a deeper slit in the cap, thinner gelatin in the cap, and simpler shaped oral arms and brood pouches. What particularly surprised me was the shape of its gonad! While the Moon jelly's gonad is rounded and horseshoe-shaped, the Aoba moon Moon jelly's gonad is heart-shaped! Incidentally, the gonad is the clover-shaped area in the middle of the jellyfish's cap, where eggs and sperm are produced. Moon Moon jelly are often exhibited in many aquariums and seen in the ocean, but who knew there was a relative of the Moon jelly with a gonad shaped like this?! I was surprised when I first learned about it, and it really piqued my interest in raising one!

■ Experiments before rearing <br />There are several hurdles to overcome before the jellyfish can grow into the adult jellyfish that are on display. The first hurdle is obtaining ephyrae, or baby jellyfish , which are the baby jellyfish . Depending on the type of jellyfish, the conditions for the polyps to turn into ephyrae or baby jellyfish are often unknown, so it can be difficult to obtain jellyfish even if you want to raise them. However, the Afro-moon Moon jelly kept at Sumida Aquarium have ephyrae hatching from the polyps, so we were able to start raising them straight away.

The next hurdles are water temperature and food. As for water temperature, the Aoba moon Moon jelly lives in warm seas, so of course it prefers higher temperatures, but I wondered whether it would thrive in the same water temperature as the Moon jelly off the coast of Honshu, so I tried raising it at two different temperatures. As for food, I fed it the zooplankton Artemia, and tried feeding it once, the same as the moon Moon jelly, and twice, which allows it to eat more food. These rearing conditions can be summarized as follows:

Condition 1: Water temperature 23°C, food once a day (same conditions as the moon Moon jelly kept at Sumida Aquarium)
Condition 2: Water temperature 28°C, rice once a day. Condition 3: Water temperature 28°C, rice twice a day.

After rearing them under each of these three conditions for the same period of time, condition 3 showed the best growth! In condition 2, they grew, but slower than condition 3. In condition 1, we were unable to confirm any signs of the ephyra growing. Based on these results, we decided to start rearing them under a water temperature of 28°C and feeding twice a day. However, even the ephyra under condition 3, which were growing well, seemed a little small and their growth rate seemed slow... Since the polyps were collected 10 years ago, I was a little worried that something might be wrong with them... but then we heard some good news!

Sumida Aquarium sometimes travels to Ogasawara for green sea turtle ( see previous column ) and beach cleanup activities ( see previous column ), and we thought that this might be a good opportunity to ask them to collect Aotu Moon jelly eggs! However, the staff traveling there had no experience in jellyfish egg collection, so we had them practice egg collection on the moon Moon jelly in Sumida Aquarium's large Big Schale before sending them off to Ogasawara. After a week of waiting, they successfully brought back new Aotu Moon jelly eggs, newly hatched larvae (planulae)! From there, we obtained the 2024 version of Aotu Moon jelly polyps and began full-scale cultivation.

■Start training right away!
Ephyra are obtained when polyps change shape and detach (this is called liberation).
The ephyra that had become free from the petri dish where the polyps were being grown were then moved to the aquarium and cultivation began. The newly released ephyra of the Aurelia Moon jelly were different in that they were lighter in color than those of the Moon jelly.

After 14 days of release, the diameter of the umbrella had grown to about 2cm. From this point on, the number of feedings was increased from two to three times a day, and by the 30th day after release, the diameter of the umbrella had grown to 8cm. At this point, the shape of the gonads had changed to be slightly squarer than that of Moon jelly!

We continued raising it like this, and around 45 days after it was released, it was clearly different in appearance from a normal Moon jelly! We continued raising it for 60 days, with the hope that its gonads might become heart-shaped if we continued raising it like this.

After 60 days of breeding, the diameter of the umbrella had grown to 17cm!

■Introducing our grown-up children!
It's big! It's cute! I'm so happy that it's grown so well!! But on the other hand, the shape of its gonad, which I was hoping to see, was different from that of the moon Moon jelly, but it wasn't heart-shaped, and the gonads were smaller than those of wild Aotuo moon Moon jelly. On the other hand, I was able to clearly see another difference, that the Aotuo moon Moon jelly has a deeper notch on the edge of its cap.

Furthermore, these observations revealed some slight differences from individuals observed in the wild. When comparing the Moon jelly and A. aurita Moon jelly raised in aquariums, both of which had umbrella diameters of approximately 17 cm, the umbrella of the A. aurita Moon jelly appeared to be three-dimensional and whitish. This may be due to differences in the environments in which the Moon jelly and A. aurita Moon jelly were raised (individual density and amount of food), as well as the ages at which they were compared, and these differences may have been reflected in the characteristics of the jellyfish. Raising them under identical conditions may have produced different results. Alternatively, the comparison of wild characteristics was based on large, sexually mature jellyfish, and no comparisons of jellyfish of intermediate sizes were made. Perhaps comparing the characteristics of jellyfish in the wild during development could lead to new discoveries.

■Longing to see a wild Aoba moon Moon jelly <br />When I tried raising Aoba moon Moon jelly this time, I could clearly see that they have different characteristics from Moon jelly , but I still wanted to raise them to have that distinctive gonad shape so that I could show them to customers, and as I have yet to see one in the wild, I would like to go and see it! I would like to try again by coming up with some ideas about the breeding environment and food!

If you're lucky, you might be able to see the Aoba moon Moon jelly being cultivated in the museum's " Labo ." If so, we'd be thrilled if you could compare it to the jellyfish Moon jelly and come back again and again to see how it grows! Thank you for watching until the end!