Researchers modeling the tough exteriors of two types of mollusk have finally figured out how seashells keep those soft, squishy insides safe. Thanks to the shape, stress is sent to the parts of the shell where the animal occupants are least likely to be. These mechanical principles, described in Science Advances this week, may inspire better designs for useful items like emergency shelters during natural disasters. 

In the wild, form and function are often closely related. "Nature keeps on making things that look beautiful, but we don't really pay attention to why the shapes are what they are," Rice University's Chandra Tiwary says in a news release. Mollusk shells, for example, range from the wide, fan-shaped shells of bivalves like clams (pictured above) to the long, spired, screw-shaped shells of predators called terebridae (pictured below). They’re all engineered to protect their innards from harsh conditions and the tremendous pressures at the bottom of the ocean. We know that many are made of nacre (also known as mother of pearl), a strong, resilient matrix of calcium carbonate. But what drove these tough exoskeletons to evolve the shapes that they did? 

To investigate, Tiwary and a team of engineers analyzed the microstructures of screw-shaped shells and fan-shaped shells collected from beaches in India. In order to run stress tests, they also created computer simulations and printed simple, 3D-polymer variants of the two shell types: fan shells without their converging ribs and screw shells without their complexities. 

As it turns out, the more complex natural shells can sustain loads of nearly twice the weight managed by their simpler spherical and cylindrical synthetic counterparts. Furthermore, the shells redirect the stress so that fractures occur where they’re least likely to hurt the soft creature inside. The shapes evolved in a way that distributes external pressure toward the outer regions of the shell, preventing cracks and safeguarding the core cavity. 

With the clamshells, the semicircular shape and curved ribs force stress towards the hinge. The 3D-printed, rib-less fans, on the other hand, spread the stress to various regions across the shell. When cracks eventually appeared, they occurred near the base in both the real and printed shells. 

Meanwhile, screw-shaped shells direct the load toward the center and then to the wide top: the tough inner core took the most punishment, relieving stress from the surface and diverting it toward the highest ring. In the stress map to the right, you can see how stress is redirected to the strongest areas, the center and widest end. 

Seashells are a fantastic example of evolutionary optimization in handling a variety of mechanical loads. The shapes of the shells also help with movement, eating, and sensory functions. The researchers say that these mechanical principles may be adapted for human-scale structures, ranging from boxes for fragile objects to vehicles and buildings that can dampen impacts and protect human inhabitants. 

Images: Indian Institute of Science/Rice University