University of the Ryukyus study reveals coral near Sesoko Island, Okinawa shows increased hybridization following large-scale bleaching

In the waters around Sesoko Island off the northern part of Okinawa Island, a research group from the University of the Ryukyus and other institutions has revealed that corals of the genus Acropora show increased hybridization between different species following large-scale coral bleaching events. Bleaching occurs primarily due to rising seawater temperatures, which causes corals to lose their symbiotic zooxanthellae. The finding that interspecies hybridization occurs as an adaptive response to major environmental changes could provide valuable insights for coral recovery and conservation efforts in the face of future climate change.

The Tropical Biosphere Research Center (TBRC) at the University of the Ryukyus estimated bleaching risk using seawater temperature data observed by the Japan Meteorological Agency in waters off Okinawa Island since 1980. The method accumulates seawater temperatures exceeding the warmest monthly average temperature (threshold), with 4 degrees above the threshold indicating "bleaching risk present" and 8 degrees above indicating "high bleaching risk." Since 1998, when large-scale bleaching occurred worldwide, bleaching risk from rising seawater temperatures has been increasing.

Coral identification is challenging, and the same names sometimes refer to different species in different countries, creating international problems. Associate Professor Masaya Morita, who studies biology at the Sesoko Station of the TBRC, had long been considering solutions to this problem for corals in the Indian Ocean, Pacific Ocean, and East China Sea.

Thinking "I want to at least name the species of Acropora around Sesoko Island," he began detailed observations of Acropora corals. As a result, he noticed that specimens living together had characteristics very similar to three species within the genus but differed in size and branch tip morphology.

Specifically, among species all belonging to the genus Acropora, specimens in nature showed characteristics similar to three species-Oyayubi-midoriishi (Acropora cf. gemmifera), Tsutsuyubi-midoriishi (Acropora cf. humilis), and Sankaku-midoriishi (Acropora cf. monticulosa)-and were identified as hybrids. These hybrids differed from parent species in the size of surface corallites and branch dimensions. When hybrids were crossed with parent species, normal fertilization occurred, suggesting a high likelihood of hybridization.

Genome analysis was conducted on the "colonies" of three types of Acropora hybrids to confirm their hybrid nature, revealing genetic exchange between different species. Since corals are organisms composed of aggregated units called polyps, what appears to be a single individual is called a colony.

Morita had discovered in previous research that reducing coral sperm concentration below normal levels led to hybridization with other species. However, this was a laboratory result, and it was unclear whether it applied to natural phenomena. This study revealed that hybridization progresses in nature during what is called bleaching periods when sperm concentration becomes low.

Mathematical modeling was then used to estimate when hybridization occurred. Gene mixing had occurred within 25 years of the 1998 bleaching event. As one generation takes about 4-7 years, this confirmed the process had occurred over five generations. Tsutsuyubi-midoriishi showed particularly frequent hybridization with Oyayubi-midoriishi. Oyayubi-midoriishi also showed some hybridization with Sankaku-midoriishi.

These results collectively show that even during bleaching, a critical situation for Acropora, interspecies hybridization occurs as a survival strategy. Morita reflected: "Tsutsuyubi-midoriishi leaves genes even in relatively distant species, but we don't yet understand the reasons. A limitation of this research is that we cannot examine the state of corals before bleaching occurred. In this study, we struggled to determine what parameters should be investigated."

With high seawater temperatures continuing in recent years, research will continue to examine changes in marine environments and gain insights into coral genetic regions.

The research was conducted with funding from the Japan Society for the Promotion of Science's Grants-in-Aid for Scientific Research program, and the results were published in the electronic edition of the American scientific journal Current Biology on July 7.