Deep corals may be a lifeline for shallow reefs

Coral reefs, known as the rainforests of the sea, have been declining worldwide and the situation is grim in the Florida Keys National Marine Sanctuary (FKNMS).

These vibrant underwater ecosystems have seen a staggering 50% drop since the late 90’s. However, amidst this environmental crisis, a ray of hope emerges from the unlikeliest of places – the mesophotic zone.

The mesophotic mystery

This zone, ranging from 100 to 500 feet deep, has been relatively untouched by human exploration.

These deep coral ecosystems possess the potential for resilience amidst anthropogenic stressors due to their isolated existence and depth.

Moreover, they often act as refuges for depth-generalist species by providing viable larvae to shallow reefs affected by various disturbances.

In the Florida Keys National Marine Sanctuary, mesophotic corals have been spared the destruction caused by coral bleaching and stony coral tissue loss disease. Hence, they might be the primary source of genetic diversity for their shallow counterparts in the region.

Connectivity of deep and shallow corals

Researchers from Florida Atlantic University’s Harbor Branch Oceanographic Institute used the blushing coral star (Stephanocoenia intersepta), a widespread species in the Western Atlantic, to explore the relationship or “connectivity” among coral populations at varied depths and locations.

Through genomic DNA analyses, the experts probed into the genetic diversity, differences, and connectivity among blushing coral star populations and their symbiotic algal partners.

“Although blushing star corals are less common on shallow reefs throughout their geographic range, they become more abundant and cover more area in the mesophotic zone, making them ideal for studying how populations at various depths and locations are linked,” explained Ryan Eckert, a PhD candidate at FAU Harbor Branch.

Deep corals as reproductive sources

The study, published in the journal Heredity, revealed four distinct genetic lineages of the blushing star coral.

Two main genetic lineages showcased robust genetic connectivity between shallow and mesophotic populations.

Two shallow-specific lineages displayed lower genetic diversity and increased inbreeding compared to the two depth-generalist lineages. The analysis also highlights the significance of mesophotic reefs as crucial reproductive sources.

“Corals from deeper, mesophotic zones have a significant potential to help replenish and restore the declining shallow reefs in the FKNMS,” said Dr. Joshua Voss, the principal investigator of the study.

Genetic diversity across coral lineages

The lower diversity and higher inbreeding levels across shallow specific lineages can present potential risks for conservation and restoration efforts.

However, these deeper corals could still be valuable, either as a “seed bank” or for rescue and breeding programs in land-based nurseries.

The unique lineages of blushing star coral, with minimal overlap despite residing in the same areas, suggest some level of reproductive isolation. This could be due to differing spawning times or high inbreeding rates from localized fertilization.

“Recognizing and characterizing genetic biodiversity across different coral lineages assists us to better manage and protect coral populations,” said Dr. Voss.

“This approach ensures that coral conservation and restoration efforts are informed by a comprehensive understanding of genetic diversity, essential for maintaining the resilience and health of our coral reef ecosystems.”

Coral conservation and deeper reefs

As global reefs continue to face challenges from climate change, pollution, and diseases, conservation efforts may increasingly turn to the potential of deeper mesophotic reefs as a lifeline for struggling shallow populations.

These deeper reefs, with their relative isolation and protection from extreme surface temperatures, could serve as a vital genetic reservoir for restoring degraded shallow ecosystems.

By providing viable larvae and maintaining genetic diversity, mesophotic corals could play a critical role in ensuring the long-term health and recovery of shallow reefs.

Conservation strategies that incorporate the protection of these deeper ecosystems, such as those outlined in the FKNMS Restoration Blueprint, are crucial for sustaining coral populations in the face of ongoing environmental changes.

Additionally, the research emphasizes the importance of understanding the unique dynamics between shallow and deep coral ecosystems.

While deeper corals might not directly repopulate shallow reefs, their genetic contributions offer an invaluable resource for breeding and restoration programs.

The study is published in the journal Heredity.

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