In the Deep Blue Sea

Former MS Marine Biology students (Mercer Brugler and Eric Pante), and former professor Scot France, were featured in Science Magazine this week. Below is the full article, written by Elizabeth Pennisi which can be found here.

The deep sea may not seem like a crucible of evolution. But, to the surprise of biologists, a new construction of the coral family tree suggests that evolution proceeds at full bore in waters well below where sunlight penetrates. Moreover, some coral diversity may have bloomed there first, before spreading coastward—the reverse of what has long been thought. “As people look in the deep sea, they are finding much more diversity than they expected,” says Clifford Cunningham, an evolutionary biologist at Duke University in Durham, North Carolina. “We’re just at the very beginning of understanding deep-sea evolution.”

At a meeting* in Seattle earlier this month, a dozen experts celebrated progress in assembling an improved family tree showing the relatedness of jellyfish, corals, anemones, and hydra, which are collectively known as cnidarians. Relatively simple creatures characterized by their production of specialized stinging cells, cnidarians are important to the marine ecosystem. Some live in deep water; others in the shallows. Some, like reef-forming corals, live in large colonies and partner with algae; others go it alone.

By comparing a variety of genes among cnidarians, Cunningham and the others gathered in Seattle have pieced together large swaths of their family tree. This Cnidarian Tree of Life project and other studies have unveiled where some of the species arose and where traits such as coloniality came from. The most paradigmshaking result so far concerns certain corals. Researchers have long thought that deep-sea corals were derived from a few species of shallow-water reef dwellers that migrated out to sea over time. Those newcomers didn’t diversify very much in their new environs, it was thought.

But 40 years ago, marine biologists discovered quite a diversity of worms and other softbodied creatures in deep-sea sediments, with up to 100 species per shovelful of mud. Marveling over this surprising variety, Scott France of the University of Louisiana, Lafayette, wondered whether hard, rocky sea floors were also rich in coral species. Since 1992, he and his colleagues have been collecting specimens and data from submersible expeditions and from museum collections, focusing on black corals—named for their dark, flexible, treelike skeletons—and octocorals, all of which have eight tentacles as opposed to the black coral’s six.

To figure out who lived where, Eric Pante, one of his graduate students, charted the data on depths at which 3100 museum specimens were collected and incorporated specimens that he and France collected at sea. They found, for example, that most of 420 metalliccolored chrysogorgiid corals documented lived in the deep sea, but a few were confined to shallow water.

France and his colleagues also sequenced parts of several genes from representatives of three families of octocorals, those commonly known as sea fans and sea whips. Comparisons of the DNA sequences allowed them to make a family tree, upon which they could overlay the depths of each species’home. The deep-sea species in the three octocoral families “all seem to be coming from a single common ancestor” that lived in deep water, France concluded at the meeting. Mercer Brugler, another of France’s graduate students, looked at three families of black corals, which are valued for jewelry, and concluded that they too diversified in the deep ocean.

This emerging story isn’t limited to corals. Marymegan Daly, a systematist at Ohio State University, Columbus, and co-coordinator of the Cnidarian Tree of Life project, has found a similar pattern among the sea anemones she’s analyzed. “We see lots of radiations of deepsea forms,” she reported. In short, concludes France, “we can’t say that the deep sea is a boring environment in terms of evolution.”

And although the Cnidarian Tree of Life project has conf irmed that other types of corals have shallow origins, those lineages may find refuge in the deep sea during tough times, says Marcos Barbeitos, an evolutionary biologist at the University of Kansas (KU), Lawrence. Barbeitos studies solitary corals, which are individual polyps, and some of their reef-forming counterparts, which are large colonies of polyps that tend to live in shallow water. Taxonomists have tended to group reef corals with each other, putting deep-water solitary corals in a separate group. They have also assumed that the colonial corals arose from an ancestor that was a solitary coral. But Barbeitos’s study, based on DNA samples from 97 coral species, suggests that some solitary corals are closer kin to shallow-reef colonial species than to other deep-water solitary corals and that the evolutionary path to coloniality may equally well go the other way.

A statistical analysis revealed that at least two of the solitary lineages from the deep sea evolved from colonial forms rather than the other way around. These colonial forms apparently “diversify in shallow water and give rise to [solitary] species that live in deeper water,” says Barbeitos. Furthermore, his data indicate that colonial forms are about as likely to arise from solitary forms as the other way around, suggesting that coloniality can be lost and gained over evolutionary time.

Barbeitos suggests that as reefs waxed and waned over the planet’s history, corals have managed to survive in part because deepwater lineages persist. They then expand and diversify into shallow-water, reef-forming corals when conditions are again favorable. As with the black and octocorals, deep water is critical to Cnidarian evolution, says KU’s Paulyn Cartwright, co-coordinator of the Cnidarian Tree of Life project. Corals “may be more robust because of the connection to deep-water relatives.”

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