In the evolutionary battleground of the sea, most of the action is thought to take place in shallower waters. There, the constant struggle between predator and prey has sparked new ways of killing and better means of defense. Those species less equipped for the fight have often taken refuge in deeper water. Yesterday a biologist presented the first strong evidence that some corals have taken the opposite path, rising from the deep to invade shallow water several times.
The corals are called stylasterids, also known as lace or hydrocorals. They first appeared 65 million years ago and live as deep as 2800 meters--and perhaps further down too. Just 10% of stylasterid species inhabit shallow water. Alberto Lindner of the Universidade de São Paulo, Brazil, collected samples of stylasterids from around the world, mainly from fishing trawlers that had snagged the corals and from scientific dredging.
Lindner sequenced and compared their DNA. By constructing a family tree, he and colleagues determined that the shallow-water stylasterids evolved from relatives in deeper water. They have invaded the tropics three times, and the temperate waters once. Two lineages gained enough of a foothold to thrive and diversify. A paper on the findings is in review.
The team doesn't yet know exactly when or how the invasions took place. One idea is that the stylasterids managed to sneak into a shallow-water microhabitat that was similar to their deep-water environment, such as a cave or a dark overhang. Marine ecologist Richard Aronson of the University of South Alabama's Dauphin Island Sea Lab says it's not surprising that some species would move up from deep water, but that this is first solid example.