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What will be the lifeline of the coral reefs in the deep and shallow waters?

A new study estimates that there is little basis for the hope that coral reefs at great depth will help the survival of corals in shallow waters

Coral reef in the Red Sea. Image: depositphotos.com
Coral reef in the Red Sea. Image: depositphotos.com

"Most coral species in the world reproduce through momentary reproductive events, which occur once a year, usually in the summer. In that synchronized event, very many corals are released into the environment at the same time, a huge amount of sperm cells and eggs, which meet in the water, undergo fertilization and create embryos." Doctoral student Ronan Lieberman explains From the Faculty of Life Sciences named after George S. Wise. "In other species, male colonies of corals release sperm cells into the water, and these migrate into female colonies and fertilize the eggs there, so that fertilization and embryonic development take place within the colony. In both cases, it is an event that lasts only a few minutes, usually at night, so it is very difficult for researchers to 'seize the moment' - especially when it comes to great depth, where researchers are not allowed to stay for a long time. Therefore, there is today all over the world extensive documentation of coral reproduction in shallow waters, but very little is known about coral reproduction at depth."

A new study by Tel Aviv University, in collaboration with the Interuniversity Institute of Marine Sciences in Eilat, found that coral breeding events in the Gulf of Eilat at a depth of about 45 m are characterized by a much lower intensity than those that occur in relatively shallow waters, up to 30 m. In the study it was found that while in shallow water about half of the corals participated in each event, the proportion of corals that reproduce decreased to only 20-10% in deep water. According to the researchers, this means that there is no sufficient scientific basis for the widespread hope that deep reefs will serve as a lifeline for the shallow reefs, which unfortunately are severely damaged. In practice, the situation is completely different, since the deep coral reefs need the support of the shallow ones in order to survive and thrive over time. The study also revealed that a major factor that schedules the breeding events of the studied coral species is a sharp increase in water temperature for 48-24 hours, which occurs at different times during the summer months.

The research was conducted under the leadership of the doctoral student Ronen Lieberman and under the guidance of Prof. Hodi Baniho from the School of Zoology. Also participating: Dr. Tom Schlesinger from the Florida Institute of Technology, USA, and Prof. Yossi Loya, also from the Zoology School of Tel Aviv University. The study was recently published in the important journal Ecology. The research was partially supported by a grant from the European Community within the Horizon 2020 program.

Heat waves during the breeding season

The study was carried out over five years, and included five breeding seasons. He examined the reproduction of soft corals, also called 'shamonaim', living at different depths in the Gulf of Eilat. The uniqueness of the research is that it examined the reproduction of corals along the water column, i.e. at different depths. The researchers focused on the species called 'Yellow Carpet', which is characterized by a unique breeding process, which allows accessibility and monitoring through relatively convenient means.

In the current study, the researchers examined the reproduction of corals in the depths of the reef, in order to bridge the knowledge gaps that exist today in the field. For this purpose, they chose the soft coral called Marbadan Tsoud, which lives in the Gulf of Eilat from shallow waters to a depth of about 50 m, and is characterized by a unique reproduction process. In this process, the male colonies release sperm cells in a coordinated manner, and the sperm cells reach the female colonies and fertilize them. However, unlike other species, here the development of the embryos does not take place within the colony. Instead, the fertilized eggs are released and adhere by means of mucus to the surface of the colony for 6 days, during which the embryo develops into an embryo (larve). Ronan Lieberman adds: "This is a very colorful event that lasted several days, so we were able to track a large number of colonies in a large depth range of about 45-0 m."

The researchers dived to the various depths, placed temperature sensors, and carried out observations and surveys, which examined several characteristics of the breeding events: timing of reproduction, synchronization between the different colonies, and the intensity of reproduction, i.e. the number of colonies that participated in each event. The temperature sensors, placed at four different depths - 5, 15, 30, and 45 m, measured the water temperature every 15 minutes for about five years. The findings of the study were based on these data and showed that the timing and synchronization of the breeding events, throughout the depth of the water column, are related to a clear and significant increase of about 1.5-1 degrees in the water temperature within 48-24 hours, a kind of 'heat wave' that characterizes the waters of the Gulf Eilat at the beginning of summer. In relatively shallow water, of 5 and 15 m, breeding events were observed in perfect accordance with the shallow heat waves, while at greater depth, there was a certain delay in the timing of the breeding events. This is because the heat waves in the deeper waters often occurred several days to weeks later than the warming of the shallow waters.

Contribution of prematures from the shallow water

The measure of reproductive intensity was the number of colonies that reproduced and released embryos in each event. Ronan Lieberman points out: "We found that the number of colonies that release embryos is significantly smaller at a depth of over 30 m. While in shallow water about half of the colonies participate in each breeding event, in deep water the rate of female participants drops to only 20-10%." Following the findings, the researchers believe that deep water populations of corals, which are found throughout this range of depths, are unable to thrive on their own and are somewhat dependent on populations from the shallower environment. Since the corals in the deep reefs reproduce at such a low intensity, it seems that the deep reefs need a contribution to the mats (preemies) that reach them from corals found in shallower water. According to the researchers, the reason for this 'weakness' of the deep corals may be the much lower intensity of sunlight in their habitat. The sunlight resource is necessary for the essential process of photosynthesis, which is carried out by cooperative algae, which are found in the coral tissue and provide it with sources of energy, i.e. food.

The researchers conclude: "Today, when many coral reefs around the world are severely damaged as a result of human impacts, many are pinning their hopes on deeper reefs, which might be a 'lifeline' and support their sisters in shallow water, which are more exposed to environmental hazards. Our research indicates that this hope is without a solid scientific basis. In practice, it is precisely the deep reefs that need the shallow ones in order to survive, and therefore they deserve protection and preservation just as much, and maybe even more, than shallow reefs."

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