Tokyo Rivers and Jellyfish

Sumida Aquarium's Aqua Base"Labo" is where the jellyfish on display within the aquarium are raised (see previous column). Here, animal care staff are always on hand to care for the jellyfish, so if you have any questions about the jellyfish in front of you or their mysterious ecology, feel free to ask. Through this daily communication with visitors, we had the opportunity to look at the lives of jellyfish in their surrounding environment, so we decided to investigate.
(Takao Momosaki, Exhibit and Breeding Team)

■ jellyfish in Tokyo's rivers?
Every spring, while I'm working in the Labo, customers often ask me, "I saw jellyfish in a river in Tokyo. Is that normal? What's the name of the jellyfish?" One of animal care staff, who loves fishing, also told us, "There were Japanese sea nettle and Moon jelly in the Sumida River in spring," so as the person in charge of jellyfish, I couldn't help but check it out! So, over the two spring seasons of 2023-2024, we conducted a jellyfish survey of the Sumida River, which has the same name as Sumida Aquarium.

■Conditions for jellyfish to appear in rivers <br />Although some jellyfish live in freshwater, most jellyfish live in the sea. However, jellyfish are not very good swimmers and drift underwater, constantly being swept away by the current. Therefore, it is thought that jellyfish that appear in rivers are individuals that have swum upstream with the tides. Therefore, even in places where there is information about jellyfish sightings, it is not necessarily the case that you will be able to see them whenever you go. With this in mind, we have considered what conditions are necessary for you to see jellyfish in rivers.
First, there's the tide. I predicted that the probability of jellyfish being washed away would be high during spring tides, when a lot of seawater flows into the river. Weather was also an important factor. If there had been heavy rain in the river basin the day before, more water would flow, and even with a high tide, jellyfish wouldn't be washed into the river. Furthermore, if it rained on the day of observation, the jellyfish would be more likely to sink, and the water surface would become turbulent, making observations difficult, so I predicted that it would be best to time the observation so that there was as little rain as possible. And the most important factor was the observation location. Naturally, if there was a discrepancy between the observation location and where the animals were, observations would be impossible. Therefore, using map information as a reference, I identified locations along the Sumida River that would be easy to observe, spanning a 3km stretch.

■The jellyfish survey begins<br />With all this in mind, I headed out on the day to observe the jellyfish in the Sumida River. Was my hypothesis correct? Would I see jellyfish? I was excited and nervous. When I arrived at the site and looked at the water's surface...there they were! Moon jelly! The vague shape of a Moon jelly floating in the murky water! As I watched the jellyfish floating here and there for a while, I was immersed in the joy that my various hypotheses had turned out to be correct, and the thrill of seeing a jellyfish.

Moon Moon jelly swimming upstream

However, observing over a 3km stretch, I spotted jellyfish everywhere, and I was grateful to be able to see so many. No, really, I think it was because I carefully considered the conditions and followed my hypothesis. As I observed, I saw jellyfish in so many different places. They were floating gently in areas with a slow current, and some washed up on land in shallower areas. What surprised me most was the jellyfish jellyfish carried upstream in the main stream of the river at a speed faster than I had imagined. Watching them jellyfish upstream at about 50cm per second really brought home to me just how much ocean water rises in the Sumida River. Incidentally, I also spotted jellyfish near jellyfish Bridge, near Tokyo Skytree®, but their pulsation (the opening and closing of their umbrellas) had completely stopped. I think this was due to the shock of the salinity dropping too low. Examining the water in that area revealed that the salinity was about one-third that of seawater, suggesting a harsher environment for jellyfish. If you want to observe lively pulsating jellyfish, I think it would be better to go downstream from Ryogoku Bridge.

■ jellyfish in the Sumida River
Over two years of observing the jellyfish in the river, I was able to see three types of jellyfish. Moon jelly were by far the most numerous, and if you were lucky, jellyfish were dense enough to be in your field of vision at all times. The next most common species was the Japanese sea nettle. Compared to the Moon jelly, they floated up and down faster in the water and seemed heavier. Incidentally, Japanese sea nettle feed on Moon jelly. Even during my observations, I spotted a Japanese sea nettle feeding. Perhaps it was sinking because it was full. The least common jellyfish were the spider jellyfish. Perhaps because I was observing late in the season, I was only able to spot three. As an aside, spider jellyfish are known for their cucumber-like smell, and the odorous components are said to be the same as those in cucumbers. If a large number of spider jellyfish were to be washed down the Sumida River, it would probably smell like cucumber.
So, the jellyfish that flow into the Sumida River in spring are the Moon jelly, the Japanese sea nettle, and the spider jellyfish. The Moon jelly was what caught my eye the most, so I think that jellyfish Moon jelly are the ones that customers tend to see when they go for a walk.

(Left) Moon jelly (center) Japanese sea nettle (right) Spiral jellyfish

A Japanese sea nettle swimming in the Sumida River and a moon Moon jelly being preyed upon

■ Moon Moon jelly and red Japanese sea nettle at Sumida Aquarium
Visitors to Sumida Aquarium have likely seen Moon jelly and Japanese sea nettle swimming in the tanks. In fact, in 2024 and 2025, the aquarium exhibited the descendants of jellyfish found in the Sumida River. The Moon jelly eggs were collected on-site, while the red Japanese sea nettle were taken back to the aquarium (for more on the life cycle of jellyfish, please see the column "About the jellyfish Exhibits at Sumida Aquarium"). Behind the visitors looking at the jellyfish on display, I secretly thought to myself, "No one would guess that the beautifully displayed jellyfish were the descendants of jellyfish that originally drifted into the Sumida River."

(Left) Collecting eggs from a female moon Moon jelly(Right) Attempting to collect eggs from a Japanese sea nettle

More than half of these jellyfish are descendants of moon Moon jelly from the Sumida River.

Descendants of the Japanese sea nettle from the Sumida River

■There are rare jellyfish in Tokyo's rivers!
In the summer of 2024, a newspaper headline appeared in a different Tokyo river than the Sumida River we investigated this time, shocking to me as a jellyfish specialist. It read, "An extremely rare 'Mattagei jellyfish' found in the Edogawa Floodway!" The name Mattagei jellyfish may be unfamiliar to many of you. Until then, this jellyfish had only been spotted in Nagasaki Prefecture. It was only recently described in 2013, with its scientific name, Malagazzia hirsutissima Akiyama, Horinouti & Kubota, 2013. Even in the 21st century, new jellyfish are still being discovered in surprisingly familiar places. I once traveled all the way to Nagasaki Prefecture just to see this jellyfish. The news that the Mattagei jellyfish had been spotted in Tokyo was a shock to me. I couldn't wait any longer, so I headed straight to the Edogawa Floodway. Although I set out with great enthusiasm, the Edogawa Floodway is over 4km long, and finding a jellyfish around 1cm in size wasn't easy, so I ended up making four trips to see the jellyfish. But when I did finally see a cottontail jellyfish, I was especially thrilled. On the day I was able to observe, the tiny jellyfish that occasionally appeared and disappeared on the surface of the murky Edogawa Floodway looked like shining pearls to me. This habitat struck me as being "similar" to the one in Nagasaki. A small amount of freshwater flows in, ensuring a stable salinity, and it's an environment that's home to Japanese gobies, sea lice, and sakura crabs. It was a moment of renewed awe, realizing that such an environment still exists in Tokyo.

Cotton jellyfish from Tokyo

It's true that the grass is always greener on the other side. I never expected to encounter such a wide variety of jellyfish at the foot of Tokyo Skytree. I hope to continue to keep an eye on the jellyfish that can be observed in Tokyo's rivers.