Image caption The spines do not break cleanly, suggesting they are not made solely of single crystals

The precise structure of sea urchins' strong spines has been unravelled - and the find may contribute to stronger concrete in the future.

The tough spines are known to be made of calcium carbonate, which has a number of naturally occurring forms, some more brittle than others.

X-ray studies now show they are built from "bricks" of the crystal calcite, with a non-crystalline "mortar".

The results are reported in Proceedings of the National Academy of Sciences.

The spines serve as a defence against predators, hard and at the same time shock-absorbing. As a result of these properties, the spines are among the most-studied biomaterials.

Image caption The urchins' strong spines serve as a defence against predators

But efforts to understand exactly how they are put together have yielded confusing results.

"Some people were arguing that the spine is a single crystal, and others who were looking at the mechanical properties were arguing that it's more like a glassy material," said senior author on the research Helmut Coelfen, from the University of Konstanz in Germany.

He told BBC News: "It still hasn't been resolved."

If the tough spines were single crystals, they should break cleanly along planes, as does mica or slate - but instead they break roughly, as glass or ceramic might.

To investigate further, the team started with sea urchin samples gathered in Beijing, looking at them with increasingly powerful imaging techniques.

Basic recipe

Along the way they gathered up expertise and collaborators from seven other institutions, starting with a standard light microscope, moving on to electron microscopes and then on to X-ray crystallography at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.

"We started using more high-power techniques to go further down in the structure, and the further we go down, the more different modes of architecture and different organisations we find," said first author of the work Jong Seto, also from Konstanz.

Image caption X-ray scattering tests showed patterns indicating 200 nanometre-sized "bricks"

The team discovered the bricks-and-mortar structure was made up of 92% calcite crystals (the bricks) bound together with 8% of calcium carbonate that has no crystal structure (the mortar).

This basic recipe builds up a range of structures that become apparent at different levels of magnification, making it what is known as a mesocrystal.

"With the help of these different techniques we were able to understand from the nanometre scale all the way to the millimetre scale how everything is arranged," Dr Seto told BBC News.

Mimicking nature's solutions to the material challenges that sea urchins face could be helpful also for us on land, Prof Coelfen said.

"The most obvious application... is building materials, to get fracture-rsesistant materials by just copying or trying to copy that building principle," he explained.

"We are already working with two major international companies trying to improve the properties of concrete by trying to order the little nanoparticles in concrete to make it tougher and more fracture-resistant."