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Since the annotations made by English naturalist Charles Darwin on the Galapagos Islands, researchers have been fascinated in how physical barriers, such as seclusion on a particular island, can lead to the formation of new species through the process of natural selection. Natural selection is a process whereby genetic traits that augment endurance become more widespread in consecutive generations, while adverse genetic traits become less frequent. Animals and plants that have morphologies or other traits that improve their aptness to a particular upbringing become more universal and more personalized to that particular environment. Michael E. Hellberg, associate professor in the Department of Biological Sciences at LSU, however, is interested in a more ambiguous form of speciation: the speciation of animals in the ocean.
“Marine plants and animals can drift around in the ocean extremely long distances,” Hellberg said. “So how do they specialize?”
In a recent publication in the Proceedings of the National Academy of Sciences, or PNAS, Hellberg and his graduate student Carlos Prada examined how corals focus to picky environments in the ocean. Coral larvae can scatter to huge expanse in open water. Various coral species share related geographical locations, with different species often growing only yards apart. As Prada and Hellberg put forward in their recent publication, the large dispersion potential of coral larvae in open water and the nearness of different species on the ocean floor create inscrutability for researchers who study speciation. Hellberg and Prada ask, “How can new marine species emerge without obvious geographic isolation?”
When it comes to corals within the reasonably small boundaries of the Caribbean, which distances around 3 million square kilometers, the solution to the mystery seems to be habitation depth in the ocean. In others words, natural selection has led to the formation of different coral species according to how deep in the ocean these different corals grow.
Prada and Hellberg study candelabrum corals of the genus Eunicea, generally known as “sea fans,” for which sister species have been shown to be separated by ocean depth. One sister species survives better in shallow waters, whereas the other does well in deep waters. These corals, just like other corals, are very slow-growing animals. In fact, sea fan corals don’t reach reproduction age until they are 15-30 years old, and can continue reproducing until they are 60 or more years old. So while candelabrum coral larvae can disperse large distances from their parents, landing and starting to grow in either shallow or deep water habitats, small variations in survival rates at different depths between the two species and long generation times can unite to fabricate isolation.
“When these coral larvae first settle out after dispersal, they are all mixed up,” Hellberg said. “But long larvae-to-reproduction times can compound small differences in survival at different depths. By the time these corals get to reproduction age, a lot has changed.”
The shallow water sea fan coral even has an unusual morphology than its deep water sister. The shallow water coral fans out into a broad network of branches, while the deep water coral grows tall and spindly. According to Hellberg, these differences in morphology might well be hereditary, with the different corals having special protein structures and levels of expression that are better modified to their specific water depth environment. Hellberg hopes in future research to investigate the genetic basis of these different morphologies
In other interesting results, Prada explained how transplanting the shallow coral species to deep water environments, and vice versa, can cause the coral to take on a morphology more like that of its sister species.
“Their morphologies are not super fixed,” Prada said. “But they can’t change all the way to a different morphology.”
Prada did ocean dives in the Bahamas, Panama, Puerto Rico and Curaçao to sample candelabrum coral colonies. Back in the lab, he performed tests on the coral samples’ genes to resolve how shallow and deep corals become genetically different.
“Normally, organisms are differentiated by geography,” Prada said. “But these corals are differentiated by depth.” Prada and Hellberg’s research provides new insights into how new species form in the ocean, a topic of relatively limited research as opposed to speciation of terrestrial organisms.
