Reproductive ability of corals decreases with water depth – Eurasia Review
A new study from Tel Aviv University, in collaboration with the Eilat Interuniversity Institute of Marine Sciences, found that coral spawning events in the Gulf of Aqaba and Eilat, Red Sea, The deep end of the focal species’ depth range (~30–45 m) occur at much lower intensities than those in shallow waters (0–30 m). The study shows that while in shallow waters around half of the corals participate in each spawning event, this proportion drops to just 10-20% in the deepest part of the reef. According to the researchers, the significance of this finding is that there is insufficient basis for the widespread hope that deep reefs can serve as a “lifeline” for degraded shallow reefs. In fact, the researchers suggest that for some species of coral, the opposite is true: to survive over time, deeper coral populations may more often depend on coral from shallow reefs than the reverse. The study also demonstrates that large increases in water temperature within a day or two affected the onset of reproductive events in the species examined.
The study was led by PhD candidate Ronen Liberman of Tel Aviv University’s School of Zoology and Dr. Tom Shlesinger of the Florida Institute of Technology; and supervised by Professor Yehuda Benayahu of Tel Aviv University’s School of Zoology and the Steinhardt Museum of Natural History. Professor Yossi Loya, also from the TAU School of Zoology and the Steinhardt Museum, also participated in the study. This research was recently published in the prestigious journal Ecology. The study was partially funded by a grant from the European Commission under its Horizon 2020 programme.
The study was conducted over five years to include five breeding seasons. He looked at the reproduction of soft corals, also called “Octocorallia”, some of which live in a wide range of depths in the Gulf of Aqaba and Eilat. The uniqueness of the study lies in the intensive and long-term examination of coral reproduction over a broad depth gradient extending from 0 to 50 m. The researchers focused on one species of soft coral, called Rhytisma fulvum, which reproduces by “surface brooding” – a mode of reproduction by which the coral brood, or hatch, its bright yellow larvae glued externally to the surface of the coral for several days. This unique mode of reproduction helps scientists overcome many of the difficulties encountered in examining and monitoring coral reproductive events, especially at the most difficult depths to work with.
Ronen Liberman explains: “Most species of coral are hermaphroditic, which means that each individual functions as both a male and a female, and that they reproduce by brief and synchronous spawnings, which generally occur once a year during the summer months. During this synchronized event, many corals simultaneously release a huge amount of sperm and ova which meet externally in the water, where they undergo fertilization and form embryos. In other species, the male corals release sperm into the water, and these cells migrate into the female corals and fertilize the eggs internally, so that fertilization and embryonic development occur within the coral. Either way, the event only lasts a few minutes, mostly at night, so it’s very difficult for researchers to “capture the moment”, especially at great depths where divers can’t stay long. . Consequently, very little is known about coral reproduction at depths greater than about 15 m.
In the present study, the researchers focused on soft coral Rhytisma fulvum which lives in the Gulf of Eilat and Aqaba over a wide depth range: reef flats close to the sea surface down to 50 m. A particular reason for choosing this species is its unique reproductive strategy, called “surface incubation”. This reproductive process begins when the male colonies release sperm in a synchronized fashion, which then reach the female colonies where internal fertilization occurs. Unlike other coral species, however, in this species the embryos do not develop inside the coral. Instead, the fertilized eggs are released and cling to the colony via mucus for six days, where they hatch into larvae. “The developing embryos have such a vibrant yellow color that it is a very colorful event, lasting several days. depth range throughout five annual breeding seasons,” says Ronen.
The researchers dived to different depths, positioned temperature sensors and examined several characteristics of breeding events – the timing, duration and intensity of the events. In particular, they sought to understand which environmental factors influence the onset of reproductive events. The study showed that the timing and timing of spawning events, at any given depth, is associated with a sharp and rapid increase in water temperature of 1 to 1.5 degrees Celsius in 24 to 48 hours – a kind of typical “heat wave” in the waters of the Gulf of Aqaba and Eilat in early summer. In shallow water (about 5–15 m), spawning events always occurred days or weeks before being observed at greater depths. The researchers attributed this phenomenon to short-term “heat waves” in deep waters that usually did not occur until days or weeks after they appeared in shallow waters.
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 depths greater than 30 meters,” Ronen adds. “While at shallow depths around half of the colonies participated in each spawning event, in deeper waters the participation rate fell to just 10-20 percent.” In light of these findings, the researchers believe that deep-sea coral populations are less likely to thrive on their own and are to some degree dependent on shallower reef populations. Due to their lower reproductive intensity, it appears that the population of deep-sea corals requires the contribution of larvae from corals found in shallower waters. The researchers suggest that this “weakness” among deep-sea corals may be related to the much lower intensity of sunlight that reaches their habitat. Sunlight is necessary for photosynthesis in which the symbiotic algae present in the coral tissue convert light energy to supply the host coral with the chemical energy it needs.
The researchers conclude: “Today, as coral reefs around the world are severely damaged by climate change and other human impacts, many are pinning their hopes on the deeper reefs to provide a ‘lifeline’ of support. coral reefs in shallow waters, which may be more exposed to certain hazards. Although we do not wish to diminish optimism, our research suggests that this hope may have been overstated. Rather, it seems that coral populations are more which need shallow populations to persist more than the reverse, so these hidden deep reefs need full attention and protection, perhaps even more so than shallow reefs.