Investigating Coral Speciation And Adaptation
February 7, 2013

Biologists Investigate How Coral Species Adapt To Different Ocean Environments, Depths

Brett Smith for — Your Universe Online

When Darwin first formed his theory of evolution, he noted how physical isolation due to geographic barriers could result in the development of specialized adaptations. These novel adaptations were then selected and passed on to future generations. Eventually a new species would evolve.

While many researchers are focused on how physical barriers and isolation can lead to new species on land, a pair of LSU biologists are more interested in the speciation of animals in the ocean, where physical barriers are much less prevalent.

“Marine plants and animals can drift around in the ocean extremely long distances,” said Michael E. Hellberg, associate professor in the Department of Biological Sciences at LSU. “So how do they specialize?”

According to a new report in the Proceedings of the National Academy of Sciences, Hellberg and his graduate assistant Carlos Prada were able to describe a new mechanism for speciation in sea fan corals.

Based on their research in the Caribbean, the two Louisiana biologists found the key to coral speciation appears to be habitat depth in the ocean. Instead of being isolated on a small island in the Galapagos, the different evolutionary pressures of the various depths within the Caribbean are responsible for how a coral species survives and passes on its genetic material.

The researchers focused their study on candelabrum corals, known as “sea fans,” because their sister species are segregated by ocean depth. One sister species proliferates in shallow waters, while another is better adapted to deep waters.

In the study, Prada performed ocean dives near the Bahamas, Panama, Puerto Rico and Curaçao to sample the various sea fan coral colonies. Back at LSU, he performed a genetic analysis on coral samples to determine how shallow and deep corals become genetically different.

Because sea fan corals don´t reach reproductive age until they are 15 to 30 years old, the initial, larval stage of the coral can travel over large distances before landing and beginning to grow in either shallow or deep waters.

“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 has a different morphology than its deep water relative, fanning out into a wide array of branches. Deep water sea fan corals tend to grow tall and spindly. The researchers found transplanting the shallow species into deep water environments, and vice versa, can result in the coral adapting morphology closer to its sister species.

“Their morphologies are not super fixed,” Prada said. “But they can´t change all the way to a different morphology.”

These findings suggest the two corals likely had a common ancestor that adapted to the different water depths, resulting in the formation of two separate species. Hellberg theorized these differences in morphology were genetic and said future research could investigate the genetic mechanisms of these different morphologies.