Coral reproductive capacity decreases with depth

A new study led by Tel Aviv University has found that, contrary to popular belief, the reproductive capacity of soft coral is much weaker for deep water reefs than for those living in shallow waters. While many scientists hoped that deep water coral populations may help replenish degraded shallow ones – which endure significant damage from climate change and human activities – the experts discovered that coral spawning events at the deep end of the species’ depth range (30 to 45 meters) occur in fact at much lower intensities than those in shallower water (0 to 30 meters). Thus, in order to survive, deeper coral populations may more often rely on shallow-reef coral than vice-versa.

The researchers examined five breeding seasons of a species of soft coral called Rhytisma fulvum from the Gulf of Aqaba and Eilat, which lives along a large depth range. This type of coral reproduces by “surface-brooding” – a process that begins when male colonies release sperm cells in a synchronized manner, which then reach female colonies where internal fertilization occurs. Unlike in most other coral species, where embryos develop internally within the coral, in this species the fertilized eggs are released and cling to the colony via mucus for about six days before developing into larvae.

“The developing embryos have such a vibrant yellow color that makes it a very colorful event, lasting for several days. Thanks to that fact, we were able to monitor rather easily a large number of colonies along a large depth range throughout five annual reproductive seasons,” said study lead author Ronen Liberman, a doctoral student in Zoology at the Tel Aviv University.

By diving to various depths and positioning temperature sensors, the scientists examined the characteristics of coral breeding events, such as their timing, duration, and intensity. The analysis revealed that the timing and synchronization of reproduction events are associated with fast increases in water temperatures – a type of “heat wave” common in the Gulf of Aqaba and Eilat in early summer. In shallow water, the reproductive events occurred days to weeks earlier than at greater depths, most likely due to the fact that these “heat waves” occurred later on in deeper water.

Moreover, the researchers found that the number of colonies releasing embryos was significantly smaller at depths greater than 30 meters. “Whereas at a shallow depth, about half of the colonies participated in each spawning event, in the deeper water the participation rate dropped to only 10–20 percent,” said Liberman.

These findings suggest that, due to lower breeding intensity, deep-water coral populations are less likely to thrive on their own, and are reliant on corals from shallower reefs. “Therefore, these hidden deep reefs require attention and protection on their own right, perhaps even more than the shallow reefs,” the authors concluded.

The study is published in the journal Ecology.

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By Andrei Ionescu, Earth.com Staff Writer