A new study led by scientists at the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science found that shallow-reef corals are more closely related to their shallow-water counterparts over a thousand miles away than they are to deep-water corals on the same reef.

The researchers studied the genetic connectivity of a common Caribbean coral species, the mustard hill coral Porites astreoides, at sites in Florida - in the Upper Keys, Lower Keys and Dry Tortugas - and in Bermuda and the U.S. Virgin Islands (USVI) to understand if reefs in shallow water might be able to recover from disturbances by recruiting new coral larvae from deeper, more intact populations of corals.

Corals at these sites, and throughout the Caribbean, are in decline due to a variety of local and global stressors, including climate change, disease, pollution, and overfishing.

They found that coral colonies on reefs less than 35-feet (10-meters) deep in the Florida Keys were more closely related to corals at the same depths in the USVI, over a thousand miles away. They also found that corals of the same species found at depths greater than 80 feet (25 meters) in the Florida Keys were very different from their shallow-water neighbors, despite their close proximity.

"These results are interesting because they are the opposite of what we might expect," said study lead author Xaymara Serrano, a postdoctoral researcher at the UM-based Cooperative Institute of Marine and Atmospheric Sciences (CIMAS) and UM alumna.

"We had expected nearby deep corals to be closely related to their shallow water counterparts, but instead we found that this is generally not the case in Florida."

The findings showed that most shallow corals across the region were very closely related to each other, indicating that they were well-connected and shared larvae. The exception was Bermuda, whose shallow corals formed a separate population, which, according to the researchers, reflects its isolated location in the western Atlantic.

In the Florida Keys, deeper corals formed a separate genetic population that was distinct from the shallow corals of Florida, the USVI and Bermuda.

Results from the study agree with earlier findings, published by the same team in 2014 for another widespread coral species, the great star coral Montastraea cavernosa, despite the fact that two coral species reproduce in different ways.

The researchers also noted that the deep-shallow differences did not exist in Bermuda, and were not as strong in the USVI, suggesting that differences in local current patterns and water clarity might be driving the isolation of Florida's deeper corals.

"This study highlights the need to do all we can to protect shallow water corals in the Florida Keys," said study co-author Andrew Baker, UM Rosenstiel School associate professor of marine biology and ecology. "We can't rely on deeper reefs to help out our shallow reefs in this time of crisis."

The study, titled "Long distance dispersal and vertical gene flow in the Caribbean brooding coral Porites astreoides" was published in the Feb. 22 issue of the Nature Publishing Group journal Scientific Reports. The study's authors include: Xaymara Serrano and Andrew Baker from the UM Rosenstiel School, Iliana Baums from Pennsylvania State University, Tyler Smith from the University of Virgin Islands, Ross Jones from the Australian Institute of Marine Science, and Tonya Shearer from the Georgia Institute of Technology.