This article is reproduced from the research of the Academia Sinica, and Pan-Science is the implementation unit of publicity and promotion.
- Interview and writing / Lin Chengxun, Jian Kezhi
- Art Design / Lin Xun’an, Cai Wanjie
40-year-old mystery of synchronised coral spawning solved
As early as the 1980s, scientists discovered that corals have a good understanding of each other and will reproduce together in a short period of time, and the nearby waters are filled with large numbers of coral eggs, forming a breathtaking and magnificent picture. For the synchronous spawning of corals, scholars in the past speculated that it was affected by factors such as temperature, tide, and light, but the key reason for triggering spawning has not been confirmed.
After 40 years, with the efforts of the Academia Sinica Biodiversity Research Center team, the secret was finally revealed!Academia Sinica’s “Research” has an exclusive interview with associate researcher Yoko Nozawa and postdoctoral researcher Lin Zhehong. They found that the key to synchronous spawning of corals lies indark hours from sunset to moonrise。
Strange knowledge added: It turns out that corals are a group of animals with quite different morphologies!
Because corals can only attach to a fixed location and cannot move, they were once mistaken for plants. Moreover, the appearance of coral is easy to mislead the public, and it is intuitively believed that a large coral is an organism. But in fact, most corals are actually colonies of a group of polyps; only a few species, like some of the Fungiidae, are a giant polyp as an individual coral.
Taking reef-building corals as an example, the coral polyp colony can be divided into two parts: non-living and living: the coral stone composed of calcium carbonate is the protective shell and residence; the coral stone is covered with countless live polyps.
Corals are classified in the phylum Cnidaria. They are similar in appearance to sea anemones in the same phylum. They have a cylindrical body, an opening, and several tentacles are distributed around the opening. The tentacles are densely covered with cnidaria, which can catch plankton to eat. Another food source for coral polyps is provided by the mutually beneficial symbiotic zooxanthellae, which photosynthesize to produce nutrients and oxygen, and also bring colorful colors to white coral stones and transparent polyps.
When you see large and small corals in the sea, they are originally formed by a tiny polyp that splits and divides once more. The polyps continue to reproduce asexually and split into a large number of individuals over the years. The calcium carbonate secreted by numerous coral polyps gradually accumulates from generation to generation until it becomes as huge as a castle, forming the so-called “coral reef”. Coral reefs are considered by scientists to be tropical rainforests in the sea, providing habitats for fish, crustaceans and other creatures, as well as abundant food and energy.
Nozawa Yoko, an associate researcher at the Chinese Academy of Sciences, believes that coral is a very magical creature. From an individual polyp that was originally so small that it might not be recognized by the naked eye, it can continue to divide and reproduce. Life on Earth as seen from space: The Great Barrier Reef.
It’s just that the polyps use fission reproduction to add individuals with exactly the same genes and appearance as the previous generation. This type of asexual reproduction cannot increase genetic diversity, and it will also make the group lose the ability to adapt to environmental changes. Therefore, corals must spend additional time and energy to fertilize eggs and reproduce sexually to produce offspring with new genes.
Coral also understands investment?Egg bundling and desperate spawning options
Unlike fish, which can find a mate and then spawn and fertilize, immobile corals can only release sperm and eggs directly into the sea. In order to overcome the disadvantage of being immobile, they will adopt a synchronization strategy, making an appointment to release a surprising number of sperm and eggs together in a short period of time. In this way, the concentration of sperm and eggs can be greatly increased to increase the success rate of fertilization. Even if there are predators who want to take the opportunity to eat a meal, they will be dazzled and lose sight of one another.
The beauty of coral spawning in the eyes of people is also the efforts of creatures to overcome the difficulties of nature in order to reproduce.
Simultaneous coral spawning can be further subdivided into two modes. Yoko Nozawa pointed out that coral spawns only once a year, and some species prefer to spread the risk. In the next few days; other corals are desperate and make an appointment to live “all” together. Relatively speaking, the probability of fertilization of the latter is of course higher, but if the weather factors such as heavy rain and typhoon disrupt the situation that day, there may be almost no offspring that will survive successfully that year.
「It seems that the risk is very high, but since different methods will evolve, it means that both sides have their own advantages.“Nozawa Yoko explained. But no matter it is a cautious or a strong species, what method do corals that cannot move and cannot communicate with each other make an appointment to spawn together? Since the discovery of synchronous spawning in 1980, this mystery has puzzled the world for 40 years.
Seven years of fieldwork data show that key factors are hidden in the monthly cycle
Since 2010, Nozawa Yogeng’s research team has come to Green Island for diving surveys every year during the coral breeding season (usually April, May and June in southern Taiwan). During the investigation, the team went into the water every night to record the species, quantity and ovulation time of corals. After accumulating survey data for seven years, postdoctoral researcher Lin Zhehong found that each coral has a distinct reproductive pattern.
According to the existing records of the research team, corals belonging to the Coral family (Merulinidae) adopt a risk-spreading strategy, with different groups spawning simultaneously in batches. Although the spawning days between groups are staggered, the timing is very fixed, and they are all five to eight days following the “full moon”; there is another large coral in Green Island, which is some species under the genus Acropora, They are “all” scheduled to lay their eggs on the same day, but the exact date varies from year to year.
“The Jomon family is fixed to lay eggs five to eight days following the full moon; Axophora is also following the full moon, but there is no rule at all.” Lin Zhehong said. Even so, both spawn following the full moon, and the research team then pinned down the factors of the lunar cycle:moonlightto check.
Repeated indoor and outdoor results show that nighttime light sources inhibit coral spawning
Since Dipsastraea speciosa of the Cordaceae family is very common on Green Island, it is easy to observe and obtain samples, and there are traces of reproductive time, so the team chose this species for the experiment. “When the moonlight is blocked, the daisy coral will spawn earlier.” Nozawa Yoko said that the preliminary experimental results mean thatThe darkness following the full moon is the environmental signal that the coral is ready to spawn。
In order to avoid the interference of other environmental factors, the experiment was first carried out in the water tank of the research laboratory; then the team came to the mansion on the north side of Green Island to confirm that whether the coral is in an artificial environment or a natural habitat, it will be shrouded in darkness. spawning. “We go into the water every day and cover the coral with opaque aluminum foil or transparent cloth three days before the full moon, the day before, and the day following the full moon,” said Lin Zhehong. The results were as expected: The sooner the corals were covered with black cloth, the sooner they would spawn, with regular large numbers five to eight days following receiving the dark signal.
Light sources with different spectra will have the same suppression effect
In addition to lighting or not, Lin Zhehong also added experiments on the spectrum and density of light sources. Because a 2006 paper in the journal Science pointed out that corals may detect moonlight. Nozawa Yoko mentioned that the paper shows that as long as the coral is exposed to moonlight, the cry gene in the body will be expressed, and the cry gene is particularly responsive to blue light.
So the team went back to the research room, used artificial light sources to simulate the intensity of moonlight, and gave three different colors of red, blue, and green light to confirm whether it was really as described in the literature that different spectral light sources would bring different degrees of stimulation to corals. However, experiments have confirmed that under the three colors of light, the corals do not lay eggs. In other words, the clues collected so far all point to:Darkness is key to coral spawning。
40 years of coral mystery, the answer is the dark period between sun rise and moon set
After a series of threading and cocooning, it was finally confirmed that the light at night would inhibit coral spawning. However, the team wanted to know more regarding whether corals would be disturbed if they were exposed to light for a brief moment during the long night, or how long the exposure would need to be to achieve the suppression effect. Therefore, in the laboratory environment, the team individually discussed four situations, including darkness all night, lighting all night, lighting in the first half of the night (sunset to midnight), and lighting in the second half of the night (midnight to sunrise).
The results showed that when the group was exposed to light in the second half of the night, as in the group that was kept dark all night, the corals synchronously ovulated following five days. The effect of light in the first half of the night was the same as that of the whole night, causing the corals to delay production and reduce the synchronization rate of spawning. “Seeing this phenomenon, we speculate that the receptors of corals that sense light should have ‘business hours’.” Lin Zhehong said with a smile, the business hours of the receptors are probably from following sunset to midnight, but there are still some differences between individual corals.
The answer is finally revealed:In the case of fenugreek corals, the requirements for simultaneous spawning are achieved as long as there are two consecutive nights with an hour or so of darkness following sunset.This also explains why corals breed following the full moon. Lin Zhehong pointed out that because the earth rotates and the moon revolves around the earth, the time of the moon rising will be delayed by regarding 30-70 minutes every day.[Note 1]. Compared with the monthly cycle of April in the breeding season, the moon will rise at more than 2:00 pm at the beginning of the month. After that, the moon will rise following sunset every day until the full moon. The dark period in the middle is to tell the corals that they can prepare for production .
It has its advantages to choose to produce following the full moon. Yoko Nozawa reminded that the spawning of daisy is in the dark and neap tide. The dim environment can slightly blind the predators. In addition, the waves are not so strong during the neap tide, and the sperm eggs are not so strong. Not to be washed away immediately.
Coral eggs take five days to ripen following receiving a “hint”
As for the microscopic mechanism of the Coronal family to lay eggs five to eight days following the full moon, the research team is still working hard to study it, which may be related to the maturation mechanism of sperm and eggs. The following is the research team’s speculation regarding the observed phenomenon.
Coronal corals are hermaphrodites. The polyps first produce sperm and immature eggs. When the coral receives two consecutive days of dark stimulation, the nucleus of the egg will gradually move to the edge of the egg cell. The entire process, called germinal vesicle migration (GVM), takes regarding five days.
After the nucleus has migrated, the egg nucleus will begin to disintegrate, which takes regarding three to four hours, called germinal vesicle breakdown (GVBD), and the egg cell is almost ready for fertilization. The mature egg and sperm are then packaged together into a structure called a sperm-egg bundle. Nozawa Yogeng mentioned that following the sperm-egg bundles are excreted by the coral, they will float all the way to the surface. After all, the probability of sperm-egg encounters in the two-dimensional sea surface is higher than that in the three-dimensional underwater space.
The sperm-egg bundle ruptures on the water’s surface, releasing the egg in the final step: squeezing out the polar body inside the cell, which can then be combined with the sperm. Interestingly, young eggs will preferentially mate with sperm from different corals; but over time, even sperm from the same coral will accept it. “Otherwise, if you wait any longer, you will either be scattered or eaten, and the chances of fertilization will only become more and more slim.” Lin Zhehong added.
After successful fertilization, the fertilized egg sinks into the water and develops into a ciliated, free-moving solid-cystic larva. The cyst larvae spend several days searching on the bottom of the sea, and when they find a suitable spot, they attach and metamorphose into polyps that can no longer move at will. Then the polyps will keep dividing, secreting calcium carbonate, and growing into individual corals.
Wonderful opportunity to make years of research effort to be published in international journals
“It’s really lucky to say that the original manuscript was submitted to other journals.” Lin Zhehong, the first author of the paper, said with a smile. A while ago, the Japanese scholar Takahashi Jun came to Taiwan to visit and exchange, and this new discovery of coral spawning was accidentally published in Proceedings of the National Academy of Sciences (PNAS).
When Shunichi Takahashi, a professor at the University of the Ryukyus, stopped at the Academia Sinica, he stopped by the research room of fellow countryman Yoko Nozawa. After chatting, he discovered that the two were actually college classmates. “We were just nodding acquaintances in college, and we haven’t heard from each other since graduation.” Nozawa Yoko said that Takahashi Shunichi later conducted research in the field of tropical biological genes and molecules at the University of the Ryukyu; , Doing coral ecology and behavior surveys, I didn’t expect old classmates to meet once more in the academic circle by chance.
Under Takahashi Shunichi’s suggestion, the two sides cooperated to expand the experiment to be more complete. Lin Zhehong mentioned that Gaoqiao provided some tips for arranging experiments and submitting journals, such as repeating the same results in the laboratory and in the natural environment to increase the persuasiveness of the results; when writing papers, try to be conservative and only write what has been determined. Don’t make extravagances; you also spend your time modifying the arrangement of text and paragraphs, maintaining patience to communicate with the reviewers, and so on.
The ingenious fate has led to the cross-border cooperation between the Taiwan and Japan research teams, and also allowed the results of years of diligent research by Nozawa Yoko, Lin Zhehong and others to be published in heavyweight journals, so that the truth regarding coral spawning can get more attention.
Difficult ecological research will be dark and bright, forming a multinational team and starting once more
Recalling that he chose coral as a research topic because of his interest in diving, following more than 20 years, Nozawa Yoko slowly began to look forward to his research, which can bring some contribution to the declining coral population. Yoko Nozawa mentioned: “I am very happy to be able to study here. The support of the Academia Sinica has given me no worries.”
After solving the mystery of the simultaneous spawning of the daisy coral, Lin Zhehong will go to the current boss’s old classmate: Takahashi Junichi’s laboratory at the University of the Ryukyus to start a new coral research project. Yoko Nozawa said that he would continue to assist Lin Zhehong’s postdoctoral research, because this time the main focus was on the chrysanthemum coral, and they also wanted to know whether other species of the Coronal family were also stimulated by darkness to spawn synchronously; Are there any more secrets behind the irregular spawning pattern of axolotl corals following the full moon, and the lack of light that leads to no spawning?
It is also worth mentioning that following the results of coral spawning were published, Yoko Nozawa received a letter from Levy Oren, a scholar at Bar-Ilan University in Israel. Levy Oren is studying the impact of light pollution on local coral populations in the Red Sea. He is very interested in the research published this time, and looks forward to the opportunity to collaborate. The original theme of coral spawning, because there is only one chance to observe spawning once a year, and diving every day and night to investigate, the high risk and the laborious process have made many scholars discouraged. Now, the years of perseverance by Nozawa Yoko, Lin Zhehong and others have paid off, and the great adventure of rescuing coral across the Red Sea, Green Island, and Ryukyu is waiting for them ahead.
annotation
Note 1: Because the orbit of the moon around the earth is not a perfect circle, the delay time of the moon rising every day will vary according to the moon phase time (new moon/full moon) and the season, and the delay time varies from regarding 30 to 70 minutes. .
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