Comb jellies fuse into a single entity when their wounds are put together

Comb jellies (Mnemiopsis leidyi) have translucent, jelly-like bodies and look like jellyfish. However, they are completely different organisms. Comb jellies are often exhibited at aquariums and other places because their small bodies, about 5 centimeters long, glow in rainbow colors when exposed to light. Taxonomically, comb jellies are among the earliest-branching animal groups from the common ancestor of all extant animals on Earth.

When a cut was made to the bodies of two comb jellies and the injured parts were kept together, the two individuals were found to fuse and behave like a single individual. The U.S. Grass Foundation's Grass Fellowship is a program in which 30 young researchers from around the world are selected and spend three months together at the Marine Biological Laboratory, where they conduct research based on their own interests. This study was led by Kei Jokura (University of Exeter in the U.K. as an overseas research fellow of the Japan Society for the Promotion of Science at the time of the study; currently a researcher of the Exploratory Research Center on Life and Living Systems and the National Institute for Basic Biology (NIBB) at the National Institutes of Natural Sciences). The results were obtained in his collaboration with Dr. Tommi Anttonen from the University of Southern Denmark, Dr. Mariana Rodriguez-Santiago from Colorado State University in the USA, and Dr. Oscar Arenas from the University of California, Berkeley in the USA.

Jokura said, "We were a group of people from various backgrounds doing research on what we liked. I had always been interested in comb jellies, so I collected comb jellies every morning and put them in tanks for my research. One morning, I found an oddly shaped comb jelly that looked like two comb jellies stuck together. So, I decided to reproduce it in the laboratory. I made a cut to the bodies of two comb jellies, put their injured surfaces together, and left them that way overnight. By doing so, I was able to reproduce the fusion of two comb jellies. At that time, I thought, 'This is going to be a Nature paper,' but when I looked into it, I found that the fusion phenomenon itself had been reported in 1937."

Since the previous paper had reported only the phenomenon and described no details, Jokura and his colleagues analyzed it in detail. Time-lapse photography of the fusion process under a microscope revealed that the excised sites of the two individuals began to gradually fuse together about 30 minutes after the start of the experiment, and the boundary between the two individuals was almost indistinguishable after 1 hour. Thereafter, their muscle contractions gradually began to synchronize, and the synchronization rate reached 90% after 2 hours. When the fused individuals were examined under a microscope, their digestive tracts appeared to be connected.

Therefore, the research group gave food (brine shrimp) colored with a fluorescent substance from the mouth of one of the fused comb jellies. They found that the food digested in one individual was transferred to the digestive tract of the other individual through the fused digestive tract, and the feces were finally discharged from the anus of the individual that was not given the food.

Jokura said, "The high probability of fusion observed in this experiment suggests the possibility that comb jellies do not have a mechanism to distinguish between self and non-self. The observation of synchronized muscle contractions also suggests that neurons in comb jellies are fused together and share electrical signals. Moving forward, I would like to investigate the molecular mechanism of fusion by examining how genetically distant the fused individuals are from each other and by visualizing the activity of the actual neurons of the comb jellies to observe how physiological functions are coordinated." The findings were published in Current Biology.

Researchers of universities and research institutes in Japan claim that they can no longer conduct research based on their own free ideas due to the decreases in research hours and basic expenses. A similar trend is also seen in some universities in Europe and the United States, and the Grass Foundation fellowship program provides a valuable opportunity to break through a sense of stagnation. There are still many such opportunities in the world, and it would be the responsibility of seniors to support them. As a side note, the research paper that reported this fusion phenomenon >90 years ago was also inspired by research at the Marine Biological Laboratory.

Journal Information
Publication: Current Biology
Title: Rapid physiological integration of fused ctenophores
DOI: 10.1016/j.cub.2024.07.084