The pattern on the umbrella is also fascinating - the Casario one-jellyfish

In January of this year (2026), a rare jellyfish was exhibited in the "Plankton" section of the "Tokyo Sea" area. The name of the jellyfish is...

This is a "Casario One Jellyfish".

This jellyfish is rarely collected, and this is its first exhibit at Tokyo Sea Life Park. As its name suggests, the entire outer surface of its bell is covered in radiating, wavy stripes that resemble "decorations." These stripes increase towards the edge of the bell, reaching 70 to 150 stripes.

It is rarely seen in coastal areas and is often observed after typhoons, so it is thought to be a jellyfish that is distributed in the open ocean. This particular specimen was collected at a fishing port in Numazu City, Shizuoka Prefecture, on a windy day.

As I had never kept this type of jellyfish before, I was excited to observe it in the tank, wondering what kind of jellyfish it was. I discovered that it had many other interesting features besides its pattern, such as tentacles that stretch out to over 70cm and its food preferences.

The Casario One jellyfish is a jellyfish that eats other jellyfish. As a test, we gave it a misshapen Moon Jellyfish, which it caught but let go halfway through. Next, we gave it a shrunken cucumber jellyfish, which completely changed course, entangling it with its tentacles and pulling it in with great force to eat. This time, we were able to observe it not only bending the edge of its umbrella, but also tilting its drooping mouth to skillfully eat.

A jellyfish (Cardiophora japonica) eating a melon jellyfish.

Additionally, members of the Aequorea victoria family are known to emit light. When exposed to black light, the edges of their caps appear to glow green due to the green fluorescent protein (GFP).

When a black light is shone on the Kazario One Jellyfish, the edge of its umbrella glows.

Since there was very little information on how to properly care for them, we first adjusted the strength and direction of the water flow to prevent contact with the walls and entanglement of their tentacles. Unfortunately, however, they gradually weakened, and on January 23rd, we ended the exhibit after 23 days of captivity.

This experience has revealed some challenges, such as the type of food and the shape of the tank, so if we are able to collect them again, we will try to raise them in a way that allows us to keep them healthy for a longer period of time.

[Kotaro Sakamoto, Animal Care and Exhibition Staff, Tokyo Sea Life Park]

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(February 13, 2026)