Starfish Evolved at Incredible Speeds

By Chelsea Whyte on July 24, 2012 9:18 PM EDT

starfish
A species of starfish had a superfast evolution, separating from its parent species and becoming distinct in just 6,000 years. (Photo: Creative Commons: Benson Kua)

Starfish, or sea stars, aren't animals known for their speed, but they are quick in one area: evolution. A new study published in the journal Proceedings of the Royal Society B, finds that a type of 'cushion star' branched off from its cousins to form a new species in just 6,000 years.

"That's unbelievably fast compared to most organisms," said study co-author Rick Grosberg, professor of evolution and ecology at the University of California, Davis.

Grosberg and colleagues study how new species arise in the ocean. On land, groups of plants and animals can be physically isolated by mountains or rivers and then diverge and evolve until they can no longer interbreed even if they come in contact again, researchers said, according to UPI.

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So, how does this process play out in open water?

To find out, the team studied two closely related 'cushion stars' - Cryptasperina pentagona and C. hystera - living off the Australian coast in the Coral Sea. The two types of echinoderms are are identical in appearance but live in different regions. Hystera only shows up on a few beaches and islands at the far southern end of the habitat range of pentagona.

And though they may look like twins, there is another big difference between the two: their sex lives. Pentagona starfish can be male or female, and they release sperm and eggs into the water. Those emissions find each other, fertilize, grow into larvae and float around in the plankton for a few months before settling down and developing into adult sea stars.

Hystera starfish are hermaphrodites that grow their young internally and give birth to miniature sea stars ready to grow to adulthood.

"It's as dramatic a difference in life history as in any group of organisms," Grosberg said, according to Science Daily.

Those differences began between 6,000 and 22,000 years ago, when the two species separated, according to their findings. The researchers look at the diversity in DNA sequences from sea stars of both species to estimate the length of time since the species diverged.

What they found indicates that the new species clearly did not diverge slowly through genetic changes over a long period of time, but were isolated quickly.

Over the last 11,000 years, the boundary between cold and warm water in the Coral Sea has fluctuated north and south. The researchers suggest that a small population of the ancestral sea stars, perhaps even one individual, might have colonized a remote area at the southern end of the range then been isolated by one of these changes in ocean currents.

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