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Starfish-like creatures reveal surprise about biodiversity in the deep ocean

Most of us are vaguely familiar with the brittle star -- the slender, five-armed cousin of the starfish. But perhaps you were not aware of how amazingly ubiquitous they are throughout all the oceans and seas on the planet. 

This diverse group of animals can be found across the globe, from the poles to the equator, and they live at nearly every depth.

Near the shore they are small and secretive, tucking themselves under rocks, but in the deep sea they come into their own and can cover the seafloor in the millions.  

Now, thanks to the humble brittle star, scientists are starting to understand what parts of the cold, dark and mysterious deepest ocean are most important to protect. 

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In a new study published Wednesday in Nature, researchers assembled and analyzed an immense data set that included 165,000 brittle star species’ distribution records from more than 1,600 research expeditions. The researchers figured that the areas that exhibited the highest species diversity for brittle stars would have the highest species diversity for other life forms as well. 

“Our research included material from 1872 to 2015,” said Timothy O’Hara, deputy head of marine science at Museum Victoria in Melbourne, Australia, who worked on the study. “And it was deposited in tens of museums across the globe.”

After analyzing the data set, the science team, led by Skipton Woolley, a PhD candidate at the University of Melbourne, found that the most diverse parts of the deep sea do not match up with the most diverse areas in more shallow waters and on land. 

In more shallow waters like the continental shelf (65-650 feet), and the upper continental slope (650-6,500 feet) the highest brittle star diversity was seen in warm tropical waters near the equator. However, for deep-sea species -- those that live between 6,500 feet and 21,000 feet beneath the ocean surface -- the highest diversity was seen in the mid- to high latitudes like the North Atlantic, and off the coasts of South Africa, Japan and southern Australia and New Zealand.

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Brittle star (Macrophiothrix spongicola), from southern Australia.
Brittle star (Macrophiothrix spongicola), from southern Australia.
(J. Finn )

But that’s not the end of the story. Upon further investigation, the scientists found that the patterns that govern the diversity of the different ocean strata may not be so different after all. 

In shallower waters, diversity trends follow energy trends -- where there is more light and warmth, you’ll find more species of brittle stars. In the deep sea, where sunlight does not penetrate, the diversity patterns also follow energy trends, except in this case the energy comes from chemical energy -- also known as food.

“Deep-sea animals ultimately depend on dead microscopic plants and animals known as plankton floating down from the surface waters,” O’Hara said.

Because there are higher concentrations of plankton found in mid-latitudes than at the equator, it follows that the brittle star population would be more diverse in mid-latitudes.

“The deep sea has a different pattern from shallow water and land, but ultimately they all seem to be regulated by energy,” O’Hara said.

The authors say their research can help policy makers decide what parts of the deep ocean should be protected. Going forward, they plan to compile maps of important conservation areas. They also hope to learn more about the origins of life in the deep ocean.

“For example, how old are deep sea animals, and when did the animals in the Atlantic start to differ form those in the Pacific? And what does it require for animals to become adapted to the deep sea?” O’Hara said.

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One mystery of the deep may be solved, but many more remain.

Do you love science? I do! Follow me @DeborahNetburn on Twitter and “like” Los Angeles Times Science & Health onFacebook.

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