【Why we choose this article】
Corals are fantastic and inexplicable creatures. Since the Triassic period, it has survived thousands of catastrophe in the ocean and has become an important indicator of the modern marine ecological environment. Scientists have long studied why corals spawn in large numbers at the same time. Let’s follow scientists from Academia Sinica to get to know the mysterious animal coral! (Editor in charge: Chen Yijun)
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 “What’s in Research” interviewed Associate Researcher Yoko Nozawa and Postdoctoral Researcher Lin Zhehong,They found that the key to synchronizing coral spawning is the dark time from sunset to moonrise。
Yogeng Nozawa’s team conducted long-term observation and research on Green Island, Taiwan, and finally discovered the key factors for synchronous spawning of corals. After the full moon during the coral breeding season (around April-June in southern Taiwan), the dark period between sunset and moonrise triggers the conditions for coral spawning. Pictured is a daisy coral spawning. (Image source: Lin Zhehong)
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 a colony of coral polyps; only a few species like Fungiidae are a giant polyp as an individual coral。
Taking reef-building corals as an example, coral polyp colonies can be divided into two parts: abiotic and biotic:The coral rock composed of calcium carbonate is the protective shell and shelter; the coral rock is covered with countless live polyps.
Polyps are classified in the phylum Cnidaria,They are similar in appearance to sea anemones in the same phylum. They have a cylindrical body and an opening. Several tentacles are distributed around the opening. The tentacles are densely covered with cnidaria, which can fish for 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.
The polyp colonies of reef-building corals (also known as stony corals) initially start with a single polyp, and through a slow and continuous growth process, gradually grow into the coral reefs we see. (Image source: Wikipedia)
Seeing corals big and small in the sea,At first, it was formed by a tiny polyp, which split and then split again. The polyps continued 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.
Yoko Nozawa, an associate researcher at the Academia Sinica, believes that,Coral is a very magical creature. From a single coral polyp that was originally too small to be recognized by the naked eye, it can continue to divide and reproduce, and finally hundreds of millions of polyps swarmed into the only life on earth that can be observed from outer space: large barrier reef.
only,The polyps newly added by fission reproduction are individuals with exactly the same genes and appearance as the previous generation. This type of asexual reproduction cannot increase genetic diversity, it will also make the ethnic group lose the ability to adapt to environmental changes.thereforeCorals must spend additional time and energy to fertilize eggs and reproduce sexually to produce offspring with new genes.
Corals can reproduce individuals through both asexual and sexual reproduction. (Image source: Research and Development, data source Global Foundation for Ocean Exploration)
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 adopt a synchrony 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. a few days later;Other corals are desperate, promising 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 survey, 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 Zhehong Lin found that each coral species had distinct reproductive patterns.
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 after 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 after the full moon; Axophora is also after the full moon, but there is no rule at all.” Lin Zhehong said. even so,Both spawn after the full moon, so the research team locked in on the lunar cycle factor: moonlight, to test.
Cordaceae vs Axopora. (Image source: Research and Development, Source: Wikipedia, iStock)
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,Preliminary experimental results suggest that the darkness after the full moon is an environmental signal for corals to prepare to spawn.
The chrysanthemum coral belongs to the Coronal family, and large spawning between groups usually occurs five to eight days after the full moon. (Image source: Lin Zhehong)
In order to avoid the interference of other environmental factors, the experiment was first carried out in the water tank of the 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 after 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 after receiving the dark signal.
The research team set up an experiment on the Green Island to observe the spawning of the coral chrysanthemum, and artificially controlled the moonlight for three days before the full moon, one day before and one day after the full moon. (Image credit: PNAS)
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,The artificial light source is used to simulate the intensity of moonlight, and three different colors of red, blue and green are given respectively. I want to confirm whether it is really as described in the literature. Different spectral light sources will 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 for 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 aboutCorals will be disturbed as long as they are exposed to light for a moment in the long night, or how long should the exposure 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 after 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 in corals that sense light should have “business hours”.“Lin Zhehong said with a smile, the receiver’s business hours are probably from after sunset to midnight, but there are still some differences between different coral individuals.
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 after sunset.This also explains why corals tend to breed after 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 about 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 after sunset every day until the full moon. The dark period in the middle is to tell the corals that they can prepare for production .
Choosing to produce after the full moon has its advantages, Yoko Nozawa reminded,The spawning of the daisy corals coincides with 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 and eggs will not be washed away immediately.
After years of natural observation and control of experimental conditions, the research team finally found out the secret of synchronous spawning of corals. The key lies in the dark period after the full moon day of the breeding season. Figure 1 shows that before the full moon day, moonlight inhibits coral spawning, and Figure 2 shows that after the full moon day, the dark period between sunset and moonrise triggers the conditions for coral spawning. (Image credit: PNAS)
Coral eggs take 5 days to ripen after receiving a hint
As for the microscopic mechanism of the Coronal family to lay eggs five to eight days after the full moon, the research team is still working hard.It may be related to the maturation mechanism of sperm and eggs,The following is the research team’s speculation about 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 about five days.
After the nucleus has migrated, the egg nucleus will begin to disintegrate, which takes about 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. Yoko Nozawa mentioned,After the sperm-egg bundle is excreted by the coral, it will float all the way to the water surface. After all, the probability of sperm-egg encounter 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 your chances of being fertilized will only get slimmer.”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.
Associate researcher Yoko Nozawa explains the microscopic mechanism of coral spawning currently under study. (Image source: Research has something)
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 when we were 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 again 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 secrets 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 about coral spawning can get more attention.
Coral spawning research requires long-term investment. The research results of associate researcher Yoko Nozawa (middle) and postdoctoral researcher Lin Zhehong (right) have finally been published in the Proceedings of the National Academy of Sciences (PNAS). (Image source: Research has something)
Difficult ecological research will be dark and bright, forming a multinational team and starting again
Recalling that he chose coral as a research topic because of his interest in diving, after 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 daisy corals, and they also wanted to know whether other species of the Cordaceae family were also stimulated by darkness to spawn synchronously; Are there any more secrets behind the irregular spawning pattern of axolotl corals after the full moon, and the lack of light that leads to no spawning?
It is also worth mentioning that after 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 Yoko Nozawa, Lin Zhehong and others have paid off, andA coral rescue adventure across the Red Sea, Green Island and Ryukyuwaiting for them just ahead.
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 time of the moon phase (new moon/full moon) and the season, and the delay time varies from about 30-70 minutes .
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(This article is reproduced with permission from our partner, Researched by Youwu, and agrees to CitiOrange to write the introduction and revise the title. The original title is “Why do corals spawn in large numbers at the same time? The key is the dark period from sunset to moonrise!”. Source of the first image: Wiki encyclopedia)
