Kurage Wonder Research Team Member's Observation Journal / Amakusa Jellyfish Edition

Introducing the animals

Kurage Wonder was established in 2020. Here you can encounter around 30 different types of jellyfish.
Jellyfish have a wide variety of characteristics. By learning about the highlights of each species, you may be able to find your favorite jellyfish.

A jellyfish eating jellyfish?!

The Amakusa jellyfish has a brown, white, or pale pink umbrella with long, frill-like mouth arms and tentacles. The umbrella measures about 15-20cm in diameter and is often found in the Kyushu region, particularly in the Amakusa region of Kumamoto Prefecture, hence the name. There are also records of it being collected in the sea off Kyoto in the past.

The Amakusa jellyfish is a type of jellyfish that is known for eating jellyfish.

Although they do eat jellyfish, they do not eat the same kind of jellyfish, but other kinds of jellyfish. At Kyoto Aquarium, they are mainly fed Moon jelly and small jellyfish of the class Hydrozoa (such as horn jellyfish and white jellyfish), which they quickly catch using their long tentacles when placed in the tank.

The Amakusa jellyfish loves jellyfish, but I find it strange that they don't cannibalize each other.

▶Amakusa jellyfish eating jellyfish

Also, compared to other jellyfish, they seem to be very voracious eaters. They can swallow a moon Moon jelly, which is about the same diameter as their umbrella, whole and digest it in about three hours, so it seems like the more food you give them, the more they'll eat.

A highly developed and powerful poison

To eat other jellyfish, it would need to have stronger venom than its prey. The Amakusa jellyfish is a type of jellyfish that has strong nematocyst venom, and its thick, long tentacles are so poisonous that touching them causes instant, intense pain.

Many jellyfish belong to the Cnidaria class, and have a venomous stinger system called a nematocyst. When a sensor called a stinger detects a stimulus in the cell containing the venomous stinger, the stinger inside pops out. When we say we are "stung" by a jellyfish, we are actually being pricked by a venomous stinger.

▶ Small Amakusa jellyfish

No poking umbrellas!

You may hear customers talking to each other saying, "Jellyfish have poison in their tentacles, so it's okay to touch the umbrella part without getting stung." Unfortunately, that's only half true.

If you look closely at the surface of the Amakusa jellyfish's cap, you can see small, granular protrusions. These are called cnidocytes, which are clusters of nematocysts, or poisonous stingers, that have formed a lump-like structure. What would happen if you touched an Amakusa jellyfish's cap? Of course, you would get stung by a poisonous stinger.

The nematocysts on the outside of the umbrella like this are called external umbrella nematocysts. If you look closely at an adult Amakusa jellyfish, you can see these nematocyst clusters.