The mysterious and intriguing jellyfish, the upside-down jellyfish

When we take care of the jellyfish in the " Labo " at Aqua Base , we get a lot of questions from customers.

Among these, the most frequently asked question is about upside-down jellyfish.

This jellyfish is so beautiful that you'll want to ask the animal care staff a few questions. Today, we'll be talking about the upside-down jellyfish.

(Author: Momozaki, Exhibition and Breeding Team)

■What is upside-down jellyfish?

The upside-down jellyfish sits on the seabed with its umbrella facing the bottom and barely jellyfish.

However, they do not live deep on the ocean floor, but in shallow, sandy, muddy areas where sunlight reaches.

This is because they have tiny symbiotic algae called zooxanthellae living inside them, which require sunlight to survive.

Perhaps because they rely on photosynthesis for most of their nutrition, they do not drift like other jellyfish.

Sumida Aquarium has created a special breeding environment for upside-down jellyfish that is suited to their ecology.

Since they do not require a water current to keep them afloat, they are kept in square tanks, and lighting is also installed for photosynthesis (although the lighting is quite blue, so photographing them may be difficult).

Although they rely on photosynthesis for most of their nutrition, they also eat food, and like other jellyfish, they are also fed zooplankton.

■Questions from customers and my decision

This upside-down jellyfish has distinctive appearance.

"You don't swim? You can't swim?"

"Umbrellas are normally facing downwards, but why don't they turn upside down?"

"Are they gathering at the beach too?"

"Is it disguised as seaweed?"

We receive many questions, such as:

However, until recently, none of the animal care staff at Sumida Aquarium had ever observed an upside-down jellyfish "in the ocean."

Every time I received a question, I felt a strong desire to show customers what the wild upside-down jellyfish looked like with my own eyes, and I decided to conduct a field survey.

However, it is often difficult to see wild jellyfish if the time or weather is bad, and that was the case for me with the upside-down jellyfish.

The first time, the survey was abandoned due to a typhoon.

The second and third times I went, there were no jellyfish, so I couldn't see any.

The fourth attempt took place in Kagoshima Prefecture just before a typhoon hit.

With the help of the animal care staff at Io World Kagoshima Aquarium, I went snorkeling in the ocean and was finally able to observe a wild upside-down jellyfish on my fourth try.

"Now we can show our customers what wild upside-down jellyfish look like..." I will never forget how excited I felt.

Below, I would like to answer some frequently asked questions from customers, based on my own experience of seeing upside-down jellyfish in the wild.

Q: Don't you swim? Can't you swim?

A: I can swim, and I swim occasionally.

In the sea off Kagoshima, we observed three individuals with umbrella diameters of approximately 2 to 6 cm swimming!

All of the individuals swimming were heading sideways, and since there were almost no waves and only a few individuals swimming, we believe they were not being swept away by the waves but were swimming of their own accord.

However, this time we did not see any large individuals swimming around that were larger than the size of a palm.

Q: Will it sink if it's upside down with the umbrella facing up?

A: I didn't see any upside down.

We have an organ called an equilibrium stone that senses gravity and body movement and controls our sense of balance, so I think it helps us maintain balance, know which is up and which is down, and move so that the umbrella is facing downwards.

Q: Are you meeting outdoors?

A: The upside-down jellyfish at Sumida Aquarium tend to gather together in the tank. This is thought to occur because the jellyfish's pulsation creates a water current that attracts them together.

When observed outdoors, they did not appear to be crowded together, but rather scattered about.

However, at the location observed this time, depending on the day, the fish tended to be relatively closer to either the south or north side of the 1.6 km water area.

In other areas, they have been spotted nestling side by side in closely separated habitats.

For this reason, although there are cases where people gather outdoors, there does not seem to be any reason why they must gather together.

Q: Is it disguised as seaweed?

A: I don't know about this.

However, I think there is no or very little dependence on seaweed.

This is because, even though there was no seaweed growing on the seabed at the location we observed this time, large upside-down jellyfish were growing there.

This suggests that the jellyfish are able to grow without the use of seaweed.

On the other hand, there were areas where eelgrass was beginning to grow, and I did think that the appendages (see photo) and seaweed did indeed look similar.

There seems to be some connection, but I look forward to future research.

■ Impressions of seeing wild upside-down jellyfish

Well, in addition to being able to answer the questions with confidence, I was also very impressed.

One of them is that we were able to see multiple variations in patterns and colors.

Zooxanthellae live symbiotically with the upside-down jellyfish and perform photosynthesis.

The amount of zooxanthellae seems to determine the depth of brown.

It is also said to contain pigments that protect against ultraviolet rays, which may also be a factor.

Also, the body shapes of bred and raised individuals are often different from those of wild individuals, but through this observation, we discovered that the oral arms (the fluffy parts) of wild upside-down jellyfish are so large that they extend wider than the umbrella! The wild ones are incredibly fluffy!

Another memorable experience was the tingling sensation on my skin when swimming in waters where upside-down jellyfish are present.

Many people have experienced this since ancient times, and in recent years it has been discovered that they release nematocysts into the ocean.

Nematocysts are cells containing poisonous stingers, and the reason we get stung by jellyfish is because they have these jellyfish.

The sea is filled with these nematocysts. When I was snorkeling in Kagoshima, I applied Vaseline to protect my skin, but towards the end of the dive, I felt a tingling sensation where my face met my underwater mask and around my mouth.

"So this is the pain caused by the nematocysts released into the sea by the famous upside-down jellyfish!" I thought, grinning even though I was being stung by a jellyfish...

If you are raising jellyfish, this is an experience you'll want to have at least once, and it's very satisfying.

When we stimulated the upside-down jellyfish, it tingled particularly strongly, making us realize that it was releasing its nematocysts into the sea.

Once again, I was able to personally experience the importance of seeing, feeling, and experiencing the wild.

In this column, we introduced the mysterious jellyfish, the upside-down jellyfish. What did you think?

We live in an age where a lot of information is readily available, but there is still much we don't know about the creatures that live in aquariums, and there is a lot of excitement in learning new things.

For example, the same goes for questions about animals other than jellyfish , such as, "What role do those long fins play?" or "What effect do polka dots have?"

If you come across such a "random question" while at the aquarium, please feel free to ask the animal care staff.

I would be happy if we could face this mystery together and share the excitement.