A jellyfish + a jellyfish = a jellyfish?

Produced by: Science Popularization China

Author: Su Chengyu

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life" to interpret life phenomena and reveal biological mysteries from a unique perspective. Let us delve into the world of life and explore infinite possibilities.

In 2023, at Kei Shirokura's Marine Biological Laboratory in the United States, scientists noticed a unique comb jellyfish in the tank.

Figure 1 A unique comb jellyfish individual

(Image credit: Mariana Rodriguez-Santiago)

It is larger than other comb jellies, and it has two mouths ! Kei Shirokura speculates that it may be formed by the inadvertent fusion of two damaged comb jellies.

The leidian comb jellyfish ( Mnemiopsis leidyi ), also known as the sea walnut, is a species of plankton widely distributed in the Atlantic and Black Seas, known for its transparent body and luminous comb plates. Their bodies are oval in shape, about the size of a palm, transparent throughout, with a slightly iridescent sheen.

Figure 2 Freshwater comb jellyfish

(Image source: ocean)

The comb jellyfish has eight rows of comb plates on both sides of its body, which are composed of many cilia. When these cilia move, they refract beautiful rainbow light. They have two sensory organs at the top, called apical organs, which are used to sense water flow and gravity. They swim by moving their comb plates, which looks like they are glowing in the water.

To verify the idea of ​​fusion, the researchers collected ten freshwater comb jellies from different locations and at different times, and divided these individuals into multiple groups for experiments. Each group consisted of two individuals, and the scientists cut off part of their leaf-like structures, docked these cut leaves together, and then fixed them with a dissecting dish overnight.

The next day, when the scientists checked these individuals, they were surprised to find that nine of them had successfully fused, their incision boundaries had gradually disappeared, and the epithelium and the middle gelatin layer were perfectly connected together, as if they had never been separated. The entire fusion process lasted about 12 to 18 hours.

The nervous systems are also fused together? !

After the fusion, scientists further observed the comb jellies. To verify whether they were truly fused into one, the research team mechanically stimulated one of the leaves, and the entire fusion body showed a startled response and trembled slightly, which means that the nervous systems of the two comb jellies were also integrated.

Figure 3 The stimulated combined jellyfish

(Photo credit: Kei Jokura)

This startle response is a typical neural reflex, controlled by the comb jellyfish's neural network. When one blade is stimulated, the signal is transmitted through the neural network to the entire fused body, causing muscle contraction and reaction throughout the body. This shows that the fused nervous system has effectively worked together, allowing the two originally independent individuals to react as a whole. They are no longer two independent individuals, but a whole that shares information .

To better understand the fusion process, the research team performed time-lapse imaging. In the first hour, the blade movements of the two comb jellies were completely out of sync, but surprisingly, after an hour, their movements gradually became synchronized . After two hours, 95% of the blade movements were completely synchronized, as if they were telepathically connected.

Figure 4 shows the changes in leaf synchronization rate during six independent transplant experiments.

The gray line represents the result of a single experiment, and the blue line represents the average. It can be seen that the synchronization rate was low in the first 20-40 minutes after the transplantation, but as time went on, the synchronization rate increased significantly, reaching 95% at 100-120 minutes, indicating that the fused comb jellies gradually achieved physiological and behavioral synchronization.

(Image source: Document 1)

This phenomenon is reminiscent of the neural synchronization settings in some science fiction works, where characters achieve shared consciousness and coordinated combat through neural networks.

The digestive system is also integrated!

Not only the nervous system, the researchers also wanted to see if their digestive systems had also merged.

The research team fed the combined comb jellies with fluorescently labeled Artemia and observed the movement of food particles under a microscope. With the fluorescent markers, scientists can clearly see the movement path of food particles under a microscope and verify whether the two digestive systems have truly achieved functional integration.

Figure 5 shows the movement of fluorescently labeled Artemia particles in the digestive system of a comb jellyfish

(Image source: Document 1)

Artemia is a small crustacean that is usually only a few millimeters long, about the size of a grain of rice. It is related to shrimp that humans eat. They belong to the same large class of crustaceans, but are smaller and are not usually eaten directly by humans. Ctenophores feed on plankton, and Artemia is one of their typical food sources.

The scientists were surprised to find that food particles were able to pass between the two digestive systems, from one individual's digestive tract to the other, suggesting that their digestive systems are also functionally integrated.

Figure 6 By observing how fluorescently labeled Artemia flowed between these tubes, the researchers proved that the digestive systems of the two comb jellyfish individuals were functionally integrated after the fusion.

(Image source: Document 1)

However, the researchers also found that the two anuses' excretion times were not synchronized, suggesting that some functions were still independent. Perhaps this is a sign of the early stages of fusion, or that they have retained some of their independence to cope with uncertain environmental changes.

Freshwater comb jellies may not be able to recognize alloantibodies

Kei Shirokura's research team realized that the freshwater comb jellyfish may lack a mechanism called "foreign recognition." For most animals, foreign recognition is a protective mechanism that they can use to identify and reject "non-self" tissues to prevent them from being invaded by foreign individuals.

In the process of human organ transplantation, it is precisely because of this strong allogeneic recognition mechanism that the human immune system will regard the transplanted organ as a "foreign invader", thus producing an immune rejection reaction. This is why humans need a very well-matched donor when undergoing organ transplantation, and need to take immunosuppressive drugs for a long time after the operation to prevent the occurrence of rejection reactions.

But comb jellies don't seem to have this ability, perhaps because they are unlikely to be in close contact with other individuals for a long time in their natural environment. Free-living comb jellies are more concerned with how to survive alone in the vast ocean, so they have evolved simpler "cooperation" mechanisms rather than rejection mechanisms.

Not only comb jellies, other organisms can also "merge"

In fact, not only comb jellies, but many other organisms in nature also exhibit similar fusion phenomena.

The world's largest siphonophore ( Praya dubia ) can reach a length of about 46 meters, which is longer than a blue whale .

Figure 6 Siphonophore

(Image source: Smithsonian Magazine)

It is not actually a single individual, but a group of countless small organisms called "polyps". These polyps have different functions and together form a complete organism. Each polyp has a specific function, such as hunting, digestion, reproduction or movement. These different polyps cooperate with each other to maintain the survival of the entire group.

Another example is the sponge we are familiar with. Sponges are a very ancient class of organisms that also have the ability to fuse. In a 2016 study, scientists found that when two genetically similar calcareous sponges ( Clathrina aurea ) come into contact, their cells can fuse with each other and eventually form a larger individual.

Figure 7 Calcium sponge

(Image source: oeco)

Studies have found that sponges have a primitive immune system that can be used to identify "self" and "non-self" (whether the genes are the same) to decide whether to fuse. This foreign body recognition mechanism is similar to human immune rejection. In this way, sponges can expand their living space while maintaining genetic homogeneity. This fusion strategy helps them better adapt to the environment.

Back to our comb jellies, they are the earliest branching group of extant metazoans. Their neural networks are completely different from those of other animals, but they can achieve complex behavioral integration, which provides us with valuable clues about how the nervous system evolved .

In nature, fusion is not only an ingenious collaboration between organisms, but also one of the miracles of life evolution. It shows how life can break through individual limitations and gain greater survival advantages through cooperation rather than competition.

(Note: Latin parts in the text should be italicized)

References:

1. Jokura, Kei, et al. "Rapid physiological integration of fused ctenophores." Current Biology 34.19 (2024): R889-R890.

2. Padua, André, et al. "Fragmentation, fusion, and genetic homogeneity in a calcareous sponge (Porifera, Calcarea)." Journal of Experimental Zoology Part A: Ecological Genetics and Physiology 325.5 (2016): 294-303.