Coral reproduction capacity decreases with water depth

Credit: Tom Shlesinger

A new study from Tel Aviv University, in collaboration with the Interuniversity Institute of Marine Sciences in Eilat, has shown that coral spawning events in the Gulf of Aqaba and Eilat, Red Sea, are at the deep end of the depth range of the focal species (~ 30 –45 m) occur at much lower intensities than those in shallow water (0-30 m). The study shows that while in shallow water about half of the corals were involved in each reproductive event, this proportion dropped to just 10-20% in the deeper part of the reef.

According to the researchers, the significance of this finding is that there is insufficient basis for the prevailing hope that deep reefs could serve as a “lifeline” for degraded shallow reefs. In fact, the researchers suggest that for some coral species, the opposite is true: To survive over time, deeper coral populations may rely on shallow coral reefs more often than vice versa. The study also shows that sharp increases in water temperature within a day or two influenced the onset of breeding events in the species studied.

The study was led by Ph.D. candidate Ronen Liberman of Tel Aviv University’s School of Zoology and Dr. Tom Shlesinger of the Florida Institute of Technology; and supervised by Prof. Yehuda Benayahu of Tel Aviv University’s School of Zoology and the Steinhardt Museum of Natural History. Prof. dr. Yossi Loya, also of TAU’s Zoology School and Steinhardt Museum, also took part in the study. This research was recently published in the leading journal Ecology. The study was funded in part by a grant from the European Commission as part of its Horizon 2020 programme.

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 in a wide depth range in the Gulf of Aqaba and Eilat. The uniqueness of the study lies in the long-term and intensive investigation of coral reproduction over a wide depth gradient of 0-50 m. The researchers focused on a species of soft coral called Rhytisma fulvum, which reproduces by breeding on the surface – a reproductive mode in which the coral incubates, or hatches, and their striking yellow larvae are glued externally to the coral surface for several days. This unique reproductive mode helps scientists overcome many of the difficulties of investigating and tracking coral reproductive events, especially in the more challenging depths.

Coral reproduction capacity decreases with water depth

Credit: Tom Shlesinger

Ronen Liberman explains: “Most coral species are hermaphrodites, meaning each individual functions as both male and female, and they reproduce through short and synchronous spawning events, usually occurring once a year in the summer months. During this synchronized event, many corals simultaneously release a huge amount of sperm and eggs that come together 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 to female corals and fertilize the eggs internally, so that fertilization and embryonic development takes place in the coral In both cases, the event lasts only a few minutes, usually overnight, so it is very difficult for researchers to ‘capture the moment’, especially at great depths where divers don’t spend a while for a long time, which is why very little is known about coral reproduction on that pts of more than about 15 m.”

In the current study, the researchers focused on the soft coral Rhytisma fulvum that lives in the Gulf of Eilat and Aqaba along a large depth range: from reef flats close to the sea surface and up to 50 m. A particular reason for choosing this species is the unique reproductive strategy called surface brooding. This process of reproduction begins when male colonies release sperm cells in a synchronized manner, which later reach female colonies where internal fertilization occurs. Unlike other coral species, however, embryos in this species do not develop internally within the coral. Instead, the fertilized eggs are released and stick to the colony via mucus for six days, where they develop into larvae. “The developing embryos have such a vibrant yellow color that it is a very colorful event lasting several days. Thanks to that fact, we were able to quite easily track a large number of colonies over a wide range of depths over five annual reproductive seasons” , says Ronen.

The researchers dived to different depths, placed temperature sensors and examined different characteristics of the breeding events: timing, duration and intensity of the events. Remarkably, they tried to understand which environmental factors influence the onset of reproductive events. The study showed that the timing and synchronization of reproductive events, at a certain depth, are associated with a marked and rapid rise in water temperature of 1-1.5 degrees Celsius within 24-48 hours – a type of “heat wave” that is typical. for in the waters of the Gulf of Aqaba and Eilat in early summer. In shallow water (about 5-15 m), the reproductive events always occurred for days to weeks before being observed at greater depths. The researchers attributed this phenomenon to the short-lived “heat waves” in the deeper water that usually occurred several days to weeks after they occurred in the shallows.

Coral reproduction capacity decreases with water depth

Credit: TASCMAR Project

Reproduction intensity was measured by the number of colonies that reproduced and released embryos at each event. “We found that the number of colonies that released embryos was significantly smaller at depths greater than 100 feet (30 meters), Ronen adds. “While at a shallow depth about half of the colonies participated in each spawning event, the participation rate in the deeper water dropped to only 10-20 percent.” In light of these findings, the researchers believe that the deepwater coral populations are less likely to thrive on their own and depend to some extent on populations from the shallower reef. Due to their lower breeding intensity, it seems that the deepwater coral population needs the contribution of the larvae of the corals in the shallower water. The researchers suggest that this “weakness” in the deep corals may be related to the much lower intensity of sunlight reaching their habitat. Sunlight is needed for photosynthesis where symbiotic algae found in the coral tissue convert light energy to provide the coral host with the chemical energy it needs.

The researchers conclude: “Today, with coral reefs around the world being severely damaged by climate change and other human influences, many are pinning their hopes on deeper reefs to provide a ‘lifeline’ of support for shallow-water coral reefs, which may we don’t want to diminish the optimism, our research suggests that this hope may have been overestimated Rather it seems that the deeper coral populations need the shallow ones to survive more than the other way around Therefore, these hidden deep reefs need attention and protection in their own right , maybe even more than the shallow reefs.”

Deep reefs probably won’t save shallow coral reefs

More information:
Ronen Liberman et al, Soft coral reproduction phenology along a depth gradient: can “going deeper” provide a viable refuge?, Ecology (2022). DOI: 10.1002/ecy.3760

Provided by Tel Aviv University

Quote: Coral reproductive capacity decreases with water depth (2022, July 29) retrieved September 22, 2022 from

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