Sexual reproduction of corals: a synchronized nocturnal dance and its threats
Coral reproduction is a phenomenon that intrigues a large number of scientists, and it can be either by asexual or sexual means.
In asexual reproduction, new individuals are generated from a single parent, without any production of gametes (sperm or eggs) or fertilization. It can occur by different means, in particular through the budding of polyps, where the division of a polyp results in two identical polyps, or by fragmentation of coral colonies. Regardless of the mechanism, individuals resulting from asexual reproduction are genetically equal to each other.
On the other hand, sexual reproduction requires the encounter of female and male gametes and fertilization. The new organisms will be genetically different from the parental colonies. Therefore, this type of reproduction contributes to increasing the genetic diversity of the new generation of corals, making the coral population more resilient to environmental changes. This type of reproduction involves the synchronized release of gametes (“coral spawning”) by the coral colonies to allow them to meet and fertilize.
The importance of synchrony
Coral colonies of the same species often respond in a similar way to environmental factors in terms of spawning. When nighttime conditions are favorable for reproduction, once or twice a year, coral colonies synchronize to release the bundles of eggs and sperm, or only sperm. In this way, a liberation of millions of gametes takes place synchronously, forming a cloud of pink particles floating on the water surface. The larvae will then be formed from the eggs’ fertilization by the sperm. The synchrony for spawning is essential as corals are sessile animals, and it is the only way to increase the probability of fertilization and avoid the dilution of gametes in the water column.
Figure 1 : Coral bundles being released by a colony during a spawning event (Source : Auscape International Pty Ltd).
The lunar cycle and the position of the sun play a major role in the synchrony of coral spawning, particularly through the luminosity on the night of the full moon that defines the right time for reproduction. In addition to determining temporal factors, low nighttime light would also reduce the effect of predation and thus better fertilization .
Nevertheless, there is increasing evidence that the synchrony of coral reproduction is threatened by environmental factors in recent years, which several authors mention as a “reproductive struggle” . Climate change, light pollution and endocrine disruptors are suspected to interfere with the environmental signals necessary for coral reproductive cycles. The consequences of this include a lower number of juveniles, as well as a limited genetic diversity in coral populations .
Some environmental factors related to gamete release
The precise timing of coral spawning (month, day, hour and minute) is regulated by several environmental parameters . For a long time, researchers focused on the mean sea surface temperature and the lunar cycle as the main factors determining the month of spawning . It is only recently that solar radiation, precipitation periods, tidal cycles, atmospheric pressure and wind have been identified as important in dating spawning .
A scientific paper published in the scientific journal of Biological Letters in 2020 by a team of japanese researchers, shows the results of a study conducted for more than 5 years analyzing the environmental conditions before and after the days of massive spawning. The species studied are of the genus Acropora spp. found on Australian and Japanese reefs (Figure 2) .
Figure 2 : Location of coral reefs included in the spawning date study. Source: 
The first factor analyzed was the average seawater temperature. The data obtained showed that the higher the temperature at sea, the sooner the maturation of the gametes and the initiation of spawning happen (Figure 3) . The study of wind speed showed that a strong wind before spawning, would delay this phenomenon. Indeed, the gametes would be dispersed and this would reduce the probability of fertilization contrary to a weak wind .
The study of surface water temperature and wind speed therefore suggests the ability of Acropora spp. corals to adjust their development and physiology in response to these environmental factors . However, since the environment is not controllable, if the environmental factors favorable to reproduction as well as the lunar cycle do not match each other from one year to the next, a mismatch in the spawning period may be observed, threatening the viability of coral populations.
Figure 3 : Correlation between the spawning day and the mean water temperature (Source: ).
The study of environmental factors influencing sexual reproduction in Acropora spp. corals offers a better understanding of this event. Surface temperature, wind and especially night time light play an important role in determining the day and time of coral spawning. Nevertheless, this synchrony is greatly disturbed by the human pressures, which threaten the adaptive capacity of corals to new environmental conditions linked to climate change.
 Fogarty, Nicole & Marhaver, Kristen. (2019). Coral spawning, unsynchronized. Science. 365. 987-988. 10.1126/science.aay7457.
 Sakai Y, Hatta M, Furukawa S, Kawata M, Ueno N, Maruyama S. 2020 Environmental factors explain spawning day deviation from full moon in the scleractinian coral Acropora. Biol. Lett.
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