Reproductive capacity of corals decreases with water depth, TAU study finds

Photo: TASCMAR Project.

Some deep reef coral populations actually depend on shallow-water populations to thrive

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A new Tel Aviv University (TAU) study has found that coral spawning events in the Gulf of Aqaba and Eilat, Red Sea, at the deep end of the focal species’ depth range (~30–45 meters or 100-150 feet) occur at much lower intensities than those at shallow water (0–30 meters or 0-100 feet). About half of the corals in shallow water engaged in each reproductive event, but this proportion dropped to only 10–20 percent in the deeper part of the reef.

The researchers say that there is an insufficient basis for the prevalent hope that deep reefs can serve as a “lifeline” for degraded shallow reefs. In fact, the researchers suggest that for some coral species, the opposite is true: Deeper coral populations over time may rely more on shallow-reef coral. The study also demonstrates that sharp increases in water temperature within a day or two affected the onset of the breeding events in the examined species.

The study, conducted in collaboration with the Interuniversity Institute for Marine Sciences in Eilat, was led by PhD candidate Ronen Liberman from TAU’s School of Zoology and Dr. Tom Shlesinger from Florida Institute of Technology; and supervised by Professor Yehuda Benayahu of TAU’s School of Zoology and Steinhardt Museum of Natural History. Professor Yossi Loya, also of TAU’s Zoology School and Steinhardt Museum, participated in the study as well. The research was published on May 18, 2022, in Ecology.

The study was conducted over the course of five years to include five breeding seasons. It examined the reproduction of soft corals, also called “Octocorallia,” some of which live throughout a wide depth range in the Gulf of Aqaba and Eilat. The uniqueness of the study lies within the long-term and intensive examination of coral reproduction throughout a wide depth gradient spanning 0–50 meters. The researchers focused on a species of a soft coral, called Rhytisma fulvum, which reproduces by “surface-brooding,” by which the coral brood, or hatch, their strikingly yellow larvae glued externally to the coral surface for several days. This unique reproductive mode helps scientists overcome many of the difficulties in examining and monitoring coral reproductive events, especially in the more challenging-to-work depths.

“Most coral species are hermaphrodites, meaning that each individual functions as both male and female, and they reproduce by brief and synchronous spawning events, which usually occur once a year in the summer months,” Liberman explains. “During this synchronized event, many corals simultaneously release a huge amount of sperm and eggs which meet externally in the water, where they undergo fertilization and form embryos. In other species, male corals release sperm into the water, and these cells migrate into female corals and fertilize the eggs internally, so that fertilization and embryonic development occurs within the coral. In both cases, the event lasts only a few minutes, mostly at night, so it is very difficult for researchers to ‘capture the moment,’ especially at great depths where divers cannot remain for a long time. Therefore, very little is known about coral reproduction at depths greater than approximately 15 meters [50 feet].”

The researchers dove to various depths, positioned temperature sensors, and examined several characteristics of the breeding events, among them the events’ timing, duration, and intensity. They sought particularly to examine and understand which environmental factors influence the onset of reproductive events. The study showed that the timing and synchronization of reproduction events at any given depth are associated with a clear and fast increase in water temperature of 1–1.5 degrees Celsius within 24-48 hours – a kind of a “heat wave” typical in the waters of the Gulf of Aqaba and Eilat in early summer. In shallow water (about 5-15 meters or 15-50 feet), the reproductive events always occurred days to weeks before they were observed at the greater depths. The researchers attributed this phenomenon to the short-term “heat waves” in the deeper water that usually occurred only several days to weeks after they occurred in the shallow water.

The reproductive intensity was measured by the number of colonies that reproduced and released embryos at each event. “We found that the number of colonies releasing embryos was significantly smaller at a depth greater than 30 meters,” Liberman adds. “At a shallow depth, about half of the colonies participated in each spawning event, but in the deeper water the participation rate dropped to only 10–20 percent.”

In light of these findings, the researchers believe that the deep-water coral populations are less likely to thrive on their own and are reliant to some extent on populations from the shallower reef. Because of their lower breeding intensity, it appears that the deep-water coral population requires the contribution of the larvae from the corals found in the shallower water. The researchers suggest that this “weakness” among the deep corals may be linked to the much lower intensity of sunlight that reaches their habitat. Sunlight is necessary for photosynthesis in which symbiotic algae found within the coral tissue convert light energy to provide the coral host with the chemical energy it needs.

“Today, when coral reefs around the world are being severely damaged by climate change and other human impacts, many are pinning their hopes on deeper reefs to provide a ’lifeline’ of support for shallow-water coral reefs, which may be more exposed to some hazards,” the researchers conclude. “While we do not wish to diminish the optimism, our research suggests that this hope might have been overestimated. Rather, it looks like it is the deeper coral populations that need the shallow ones to persist more than vice versa. Therefore, these hidden deep reefs require attention and protection on their own right, perhaps even more than the shallow reefs.”

The study was partially funded by a grant from the European Commission as part of its Horizon 2020 program.

"These hidden deep reefs require attention and protection on their own right, perhaps even more than the shallow reefs."