The Palmyra Atoll reef is undergoing a drastic change from structures of stony corals to entire systems of corallimorphs, and researchers fear the process is irreversible.

A new study by marine biology researchers from the University of Hawai'i at Mānoa found that while the invading corallimorphs have been the same species that existed in the Palmyra Atoll reef. However, they report a change in its appearance and noted that it has actually become harmful to the coral reefs.

Researchers presented their findings in the article "A Phylogenomic Examination of Palmyra Atoll's Corallimorpharian Invader," appearing in the July issue of the journal Coral Reefs.

Palmyra Atoll
(Photo : USGS Unmanned Aircraft Systems via Wikimedia Commons)
The USGS worked with the Palmyra Atoll cooperating scientists to evaluate the effectiveness of UAS data collection activities to support science that is related to lagoon flows, marine animal studies, vegetation, bird nesting areas, and underwater areas of interest - November 2016.

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Tracing the Invasive Corallimorphs

Corallimorphs refers to an order of marine invertebrates closely related to reef-building or stony corals of the order Scleractinia, as well as sea anemones. While they have been found in temperate and tropical climates, they are mostly tropical, such as the location of the atoll in the equatorial northern Pacific Ocean.

Researchers led by Kaitlyn Jacobs conducted mitochondrial genomic analysis, examining four samples of Rhodactis howesii from Palmyra Atoll. They found out that these corallimorphs are already outcompeting other corals in the reef. Also, these are not entirely new species, finding similarities with a species from neighboring Okinawa, Japan.

They reported that there is "no identifiable population structure" in Palmyra Atoll, which means that these are indeed non-native. However, researchers also noted that while they found a possible relative from Okinawa, there are no taxonomically confirmed sequences for R. howesii in existing genetic databases, requiring additional taxonomic work to confirm further.

A Dangerous Effect of Phase Shifts

Researchers also explained that the change from stony, scleractinian corals to mostly corallimorphs is characteristic of a phase shift. However, it remains unclear whether the scenario is a case of cryptic speciation or morphological plasticity.

Cryptic speciation is a process that results in a set of species containing morphologically similar individuals that are nevertheless classified as distinct species. Meanwhile, morphological plasticity is a process that leads to various ecotypes for the corallimorph R. howesii.

"These phase shifts are negative to our overall biodiversity," explains Kaitlyn Jacobs, lead author of the study and a graduate student from the School of Ocean and Earth Science and Technology (SOEST) at UH Mānoa, as quoted in a university news release.

The Palmyra Atoll National Wildlife Refuge, which covers the entire island above and the marine ecosystem below, was created in 2001 and has remained protected ever since. According to the United States Geological Survey, coral reefs surrounding the area contain near-pristine coral reef habitat, with its emergent land being home to nesting seabirds. 

However, corallimorphs are exceptionally adaptive species. Aside from their ability to directly cause corals to die out, they can also outcompete other organisms in the area, creating a layer that covers and conquers entire coral reefs.

Jacobs explained that the ongoing phase shift from stony corals to corallimorphs might even be irreversible because there is no sufficient coral to return to once it shifts to an ecosystem of algae, sponges, or corallimorphs.


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