One of his main focuses is on scientific clarification of the actual interactions of between jellyfish and its symbionts.
 
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Professor Ohtsuka specializes in plankton. Notably, he made his debut in this specialized field as an expert in research on planktonic copepods, a group of minute crustaceans. Originally he belonged to the same laboratory as Professor Shin-ichi Uye, who is world-renowned as the authority in giant jellyfish research.

Subsequently, Dr. Ohtsuka started to work on research themes from a different perspective to that of Professor Uye. One of them was “scientific research into commercially harvested jellyfish in Asian waters,” which later became the first pillar of Professor Ohtsuka’s research.

In Japan, Bizen Karuga (Rhopilema esculentum) is considered to be the best edible jellyfish species, and the largest one can weigh more than 30 kg. In recent years, the population of this jellyfish has been on the rise in the Sea of Ariake. Professor Ohtsuka therefore goes aboard the Hiroshima University Training and Research Vessel “Toyoshio-maru” to study the jellyfish there. He also conducts similar research in Thailand and other Southeast Asian countries. 
 
As for edible jellyfish, since only limited scientific data is available, we do not yet fully understand which species are harvested, and when, where, in what ways and in what quantity, and whether edible jellyfish is, in fact, imported to Japan.

To investigate these matters, his research group has conducted surveys in Southeast Asian countries such as Thailand, Indonesia and Malaysia, including interviews with local people, for the last 17 or 18 years.

Professor Ohtsuka hopes to speak on behalf of the jellyfish, which is haunted by a negative image, from a biological perspective. “Since the jellyfish has survived for hundreds of millions of years, it must hold the keys to survival,” said the professor.

 
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He points out that one of these keys is the fact that jellyfish are relied on by other various organisms. For example, the Japanese butterfish (Psenopsis anomala) lives in symbiosis with the Japanese sea nettle (Chrysaora pacifica), a highly toxic jellyfish species. By snuggling to the jellyfish, the Japanese butterfish can protect itself from other carnivorous fish, and also may eat the jellyfish, according to the professor.

“Their relationship resembles that of clownfish and sea anemone. In the aforementioned case, however, there is no benefit for the jellyfish. For the jellyfish, just being relied on would be a nuisance,” Professor Ohtsuka says with a laugh.

 
Exhaustive investigation of a wide variety of jellyfish from the viewpoint of “symbiosis” is one of the most pioneering research projects.
 
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The second pillar of his research is to study the jellyfish’s interspecific relationships. He explains “This research project aims to study how the jellyfish lives with what kind of organisms. We have found that jellyfish are related with various other organisms, and that other organisms use jellyfish to protect themselves with jellyfish poisons, and other organisms make use of jellyfish as food and as a vehicle to broaden their range of distribution, among other things.”

“That’s why it can be said that essentially, the jellyfish plays an indispensable role in the aquatic ecosystem. As such, if we simply regard the jellyfish as a villain and remove it, certain fish will be in great trouble.” says Professor Ohtsuka.

There are only a few researchers around the world who study jellyfish by focusing on “symbiosis” like he does, and Professor Ohtsuka is literally the world’s front-runner in this field.
 
The third pillar of his research is the study of the effect of parasites on puffers. In western Japan, there was a year when cultivated tiger puffers were annihilated by a parasitic copepod of the genus Caligus.

“This parasite eats away the skin of the puffer and bacteria can enter the fish’s insides through the wound, destroying the fish. Thus fishermen cried for help. Since then, I have worked with them to conduct this research, which is still one of my priorities,” Professor Ohtsuka said.

According to Professor Ohtsuka, aquafarms are like a paradise for parasites, and explosive growth of parasites can be seen there. If it is unable to overcome this problem, the aquafarming industry will become unsustainable. Therefore the outcome of the parasite research is expected to bring about significant benefits directly linked with our everyday lives.
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Professor Ohtsuka states: “There are parasites that are specific to puffers. We are very aggressively conducting joint research as to the reason why these parasites opt to live parasitically on toxic puffers.”
 
“Symbiosis” is a huge research theme. Without symbiosis, the world of living things could not survive.
 
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劣化度測定装置「レッカミール」
 
The main theme of the professor’s research is symbiotic biology. “Without symbiosis, the world of living things could not survive. This also holds true for human beings. The science fiction movie "Avatar" conveys the same message as well. We human beings impose burdens on other life forms and on the global environment. I believe that we must be mindful that we could be retaliated against some day.”

 

Although he understands that his research is endless, he hopes that he will be able to complete his present work before he retires from his professorship. Professor Ohtsuka says, “By the time of my retirement, I wish to clarify what kind of organisms the jellyfish has established symbiotic relationships with, at least in Asia. This is one of my goals. Other goals are to sufficiently investigate edible jellyfish in Thailand and Malaysia, and to physically identify the reason why tiger puffer-specific parasites choose the puffers as their host. I want to achieve these three goals.”


Moreover, he expressed his hope to have as many students as possible study under him. “It is encouraging to see the steady progress being made in the research on jellyfish, because our laboratory was able to obtain many students. In addition, I’m now instructing a female student from Malaysia, who is conducting research on the aquafarms in her homeland. It is my hope that many more students will join our research activities,” said Professor Ohtsuka.

 
Susumu Ohtsuka

Professor
Laboratory of Aquatic Field Science
Graduate School of Biosphere Science

Hiroshima University

Posted on Sep 2, 2013