Exploring fish reproductive biology and implications for future food

Japan is an island nation surrounded by the ocean. Fish are deeply familiar to us, yet they hold many mysteries and possibilities—from their ability to change sex depending on environmental conditions to their remarkable vitality in passing life to the next generation. Fascinated by fish traits, Professor Kohei Ohta researches and explores the world of fish with endless curiosity. His research spans from fundamental studies on the origins of life to applied technologies that support Japan’s future fisheries. In this feature, he shares insights into the fascinating aspects of fish reproduction, the potential of aquaculture technologies, and the outlook for sustainable food production.

Fascinated by fish: how the environment shapes their diverse sexual systems

- Could you tell us about your research?

I study fish reproductive biology. Simply put, it is the study of how fish reproduce and pass on their genetic information to the next generation. This process is often referred to as reproduction. My research focuses on how fish reproduce and the biological mechanisms that enable them to pass life on to the next generation.

One fascinating aspect of fish is their remarkable diversity in reproductive mechanisms, which differ significantly from those of mammals. While fish do have male and female, their sex differentiation does not follow a simple, uniform pattern. In some species, sex is determined genetically. In others, it is influenced by environmental factors such as water temperature, population size, or density. Even more surprisingly, some species can change sex during their lifetime—from male to female or vice versa.

Building on fundamental research into fish reproduction, I also pursue applied studies that aim to benefit society. For example, I am working to develop aquaculture technologies that improve reproductive efficiency in fish. These efforts include developing new fish species using cutting-edge genome editing, culturing specialized reproductive stem cells in vitro—cells that give rise to eggs and sperm—and freezing and storing them to create future generations as needed. Through this research, I aim to explore a broad spectrum, from basic studies that seek to uncover the fundamental questions of life to applied technologies that directly support the future of the fisheries industry.

- When did you begin researching as you do now?

I began studying sex determination and reproduction in fish during my time as a student at Kyushu University. Back then, I spent countless days conducting experiments at the Fisheries Research Laboratory of Kyushu University in Fukutsu City and on the Hakozaki Campus in Fukuoka City, both located in Fukuoka Prefecture. After earning my doctoral degree, I continued my research as a postdoctoral fellow at the National Institute for Basic Biology in Aichi Prefecture. In an effort to broaden my perspective, I became increasingly interested in the sex determination system in mammals. This interest led me to spend time at the John A. Burns School of Medicine at the University of Hawai‘i at Mānoa, where I had the opportunity to further my research.

A major turning point in my research career came when I was appointed associate professor at Ehime University—a position that also marked my return to Japan. I joined the South Ehime Fisheries Research Center. At the time, the lack of young researchers in the fisheries field led me to take on a role in fostering regional collaboration. Nearly every day, I had opportunities to speak directly with people on-site, including fishermen and aquaculture operators—experiences I truly valued.

I returned to Kyushu University in 2016, just as chub mackerel aquaculture was becoming well established in Karatsu City, Saga Prefecture. Fortunately, I’ve been able to continue my research on sex determination and reproduction in fish wh while remaining closely involved in the aquaculture industry.

- Given the wide variety of fish species available, how do you decide which ones to focus on?

Although classifications vary among researchers, more than 400 fish species are known to undergo sex change. Since it is not feasible to study all fish species, I focus on those with traits that make them excellent experimental models and align well with our research objectives.

For example, the wrasse is a fascinating species because it can change sex in response to social cues. A single male establishes a territory to attract multiple females for spawning. When the male disappears, the largest female undergoes a sex change and becomes male to take over the territory. During this process, the entire individual—including the brain and gonads—changes sex. This transformation process provides a valuable model for analyzing the sexual systems at the genetic, cellular, and hormonal levels.

In the applied research, I have been studying chub mackerel, inspired by my connection with researchers and community members in Karatsu who have long been dedicated to this species. Chub mackerel offers broad research potential, ranging from basic science to the development of aquaculture technologies. I am also conducting research on Japanese anchovy in Karatsu. The species has a short reproductive cycle—it matures quickly, spawns, and produces the next generation in a short time. Therefore, it is well-suited for repeated observations of reproductive mechanisms and for conducting experiments such as genetic manipulation when needed.

Discoveries in the lab and memorable words from the field

- Could you share any memorable stories from your research so far?

I have memorable episodes from both basic and applied research. The first was from my time as a graduate student, when I was conducting experiments on species that undergo sex change. I was exploring how this process could be triggered. One day, I discovered a threshold effect: when more than six females are placed in a rearing tank, the largest female develops male characteristics. I was thrilled by this finding. It made it possible to observe the entire sex change process—from start to finish—at any time in a controlled environment. In fact, there are very few experimental models in the world that enable researchers to purposefully and closely examine the sex change process. This discovery has provided a foundation for my subsequent research.

Another episode comes from the applied research in the field. During collaborative research work at production sites in Ehime, I was once told, “It doesn’t make money, you can’t keep doing it.” That phrase has stayed with me. Pursuing discoveries and scientific truths is both fascinating and important. However, this experience reminded me of the value of working closely with local communities to address real-world needs and of promptly translating research outcomes into practical, socially beneficial applications. It is often challenging to satisfy community members or achieve results that directly lead to profit, even when significant time and effort are invested. For researchers, conducting studies that benefit local communities and support the development of the fishery industry may seem like a natural mindset. However, I had not fully recognized this perspective until recently. This realization prompted me to reflect on and reconsider the direction of my research.

Dive into the unknown: exploring what textbooks don’t teach

- What do you keep in mind when teaching your students?

Our laboratory currently has 18 students. The workload fluctuates and can become quite intense, as experiments are often scheduled around fish spawning. Even so, I make a conscious effort to give each student time for independent thinking and the freedom to pursue their research as much as possible.

In general, researchers should follow the rules and principles, such as logically planning experiments and collecting reproducible data. However, when it comes to the next step—what they choose to explore—I hope students approach it with imagination and curiosity. I believe it is most important that they devote themselves to research that they truly find fascinating and develop it in their own way.

- In addition, could you share what you expect from your students in the future?

I’ve been able to continue my research thanks to the support of many supervisors, staff members, mentors, and junior colleagues. Since becoming a faculty member, I’ve continued conducting my research with many students, and I feel fortunate to have worked with so many self-motivated students. In that sense, I often find myself learning from students or being helped by them. On the other hand, I’d be pleased if students could gain something that connects to their future as they explore a broad range of themes―from basic to applied studies. They may discover something through trial and error during their research or through collaboration with on-site personnel. Students who graduate from my laboratory go on to pursue a variety of careers―some join private companies, while others enter public service. Ideally, I hope that many of them will aspire to pursue careers in research-related departments or become researchers at universities or research institutes.

Today, we have easy access to a wide range of information. When you open a textbook, it may seem as though all knowledge is already written down. However, countless things remain unknown. The world of the unknown stretches endlessly, and exploring it is truly fascinating. There is still ample room to advance applied technologies in fisheries. I sincerely hope to see more young researchers take on the challenge of exploring basic research and fisheries research through diverse roles and approaches.

Shaping the future of food in Japan through basic and applied research

-Could you share your future goals or vision as a researcher?

I want to pursue both basic research and applied research in depth over the long term. Fortunately, Kyushu University has fully equipped experimental facilities that provide an excellent environment for this. I want to dig deep into basic research using species that can change sex to explore and unravel the sexual and reproductive systems —deep-rooted mysteries of life. These days, technological advancement is remarkable. As newer analytical technologies emerge, more detailed observations of cells or biomolecules become possible. This will turn what was once invisible into something visible.

On the other hand, I’m exploring applied research themes that aim to contribute to aquaculture, which helps sustain our food supply. Currently, I’m working to develop an aquaculture system for chub mackerel that enables rapid, stable growth. Furthermore, developing fish species that are resilient to global warming or that grow exceptionally fast could help stabilize food production across society. I believe I’ll continue to rely on the support of those working on-site. I couldn’t be happier to help establish a system through this research that enables people to work on-site more securely and sustainably.

Visit Ohta’s lab for more information about his research.

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About Kyushu University 
Founded in 1911, Kyushu University is one of Japan's leading research-oriented institutions of higher education, consistently ranking as one of the top ten Japanese universities in the Times Higher Education World University Rankings and the QS World Rankings. Located in Fukuoka, on the island of Kyushu—the most southwestern of Japan’s four main islands—Kyushu U sits in a coastal metropolis frequently ranked among the world’s most livable cities and historically known as Japan’s gateway to Asia. Its multiple campuses are home to around 19,000 students and 8,000 faculty and staff. Through its VISION 2030, Kyushu U will “drive social change with integrative knowledge.” By fusing the spectrum of knowledge, from the humanities and arts to engineering and medical sciences, Kyushu U will strengthen its research in the key areas of decarbonization, medicine and health, and environment and food, to tackle society’s most pressing issues.