Sharks don’t have bones. Instead, their skeletons are made from mineralized cartilage, an adaptation that has helped these predators move through the oceans for over 400 million years. A new study takes a deeper look — quite literally — at how this cartilage works. Using a combination of high-resolution 3D imaging and in-situ mechanical testing, a global team of scientists have mapped out the internal structure of shark cartilage and found it to be much more complex than it appears on the surface. The findings not only help explain how sharks maintain their strength and flexibility, but also open the door for developing tough, adaptable materials based on nature’s own engineering.
The research focused on blacktip sharks (Carcharhinus limbatus) and involved a collaboration between the Charles E. Schmidt College of Science, the College of Engineering and Computer Science at Florida Atlantic University, the German Electron Synchrotron (DESY) in Germany, and NOAA Fisheries. Blacktips are medium-sized coastal sharks commonly found in warm, shallow waters around the world, including the Gulf of Mexico, the Caribbean, and parts of the Indian and Pacific Oceans. They typically grow to about 5 feet (1.5 meters) in length, though some individuals can reach up to 8 feet (2.4 meters). Named for the distinctive black markings on the tips of their dorsal, pelvic, and tail fins, blacktip sharks primarily eat small fish, squid, and crustaceans, using quick bursts of speed to chase down prey.
The team zoomed in on their cartilage using synchrotron X-ray nanotomography, a powerful imaging technique that can reveal details down to the nanometer scale. What they found was that the cartilage wasn’t uniform. In fact, it had two distinct regions, each with its own structure and purpose. One is called the “corpus calcareum,” the outer mineralized layer, and the other is the “intermediale,” the inner core. Both are made of densely packed collagen and bioapatite (the same mineral found in human bones). But while their chemical makeup is similar, their physical structures are not. In both regions, the cartilage was found to be full of pores and reinforced with thick struts, which help absorb pressure and strain from multiple directions. That’s especially important for sharks, since they are constantly in motion. Their spines have to bend and flex without breaking as they swim. The cartilage, it turns out, acts almost like a spring. It stores energy as the shark’s tail flexes, then releases that energy to power the next stroke. The scientists also noted the presence of tiny, needle-like crystals of bioapatite aligned with strands of collagen. This alignment increases the material’s ability to resist damage. Researchers also noted helical fiber structures in the cartilage, the twisting patterns of collagen helping prevent cracks from spreading. These structures work together to distribute pressure and protect the skeleton from failure; this kind of layered, directional reinforcement is something human engineers have tried to mimic in synthetic materials, but nature has been perfecting it for hundreds of millions of years.
Dr. Vivian Merk, senior author of the study and an assistant professor in the FAU Department of Chemistry and Biochemistry, the FAU Department of Ocean and Mechanical Engineering, and the FAU Department of Biomedical Engineering, explained in a press release that this is a prime example of biomineralization: “Nature builds remarkably strong materials by combining minerals with biological polymers, such as collagen – a process known as biomineralization. This strategy allows creatures like shrimp, crustaceans and even humans to develop tough, resilient skeletons. Sharks are a striking example. Their mineral-reinforced spines work like springs, flexing and storing energy as they swim.” Merk hopes that understanding how sharks pull this off can help inspire new materials that are both strong and flexible, perfect for medical implants, protective gear, or aerospace design.
To test just how tough this cartilage really is, the team applied pressure to microscopic pieces of the shark’s vertebrae. At first, they saw only slight deformations of less than one micrometer. Only after applying pressure a second time did they observe fractures, and even then, the damage stayed confined to a single mineralized layer, hinting at the material’s built-in resistance to catastrophic failure. “After hundreds of millions of years of evolution, we can now finally see how shark cartilage works at the nanoscale – and learn from them,” said Dr. Marianne Porter, co-author and an associate professor in the FAU Department of Biological Sciences. “We’re discovering how tiny mineral structures and collagen fibers come together to create a material that’s both strong and flexible, perfectly adapted for a shark’s powerful swimming. These insights could help us design better materials by following nature’s blueprint.”
Dr. Stella Batalama, dean of the College of Engineering and Computer Science, agreed: “This research highlights the power of interdisciplinary collaboration. By bringing together engineers, biologists and materials scientists, we’ve uncovered how nature builds strong yet flexible materials. The layered, fiber-reinforced structure of shark cartilage offers a compelling model for high-performance, resilient design, which holds promise for developing advanced materials from medical implants to impact-resistant gear.”
This research was supported by a National Science Foundation grant awarded to Merk; an NSF CAREER Award, awarded to Porter; and seed funding from the FAU College of Engineering and Computer Science and FAU Sensing Institute (I-SENSE). The acquisition of a transmission electron microscope was supported by a United States Department of Defense instrumentation/equipment grant awarded to Merk.
