X-ray Microscopic Comparison of Shark Denticles

Figure 1: Photograph of 3D printed shark denticles extracted from micro-CT imaging data 

Biomimicry of Shark Denticles

Although it is commonly known that most fish species have scales, it may be less widely known that shark skin goes one step further with the presence of denticles more like teeth than traditional scales. While sharks may appear slick to the naked eye, a microscopic view reveals that sharks are almost completely covered in these dermal denticles. Olympian swimsuit designers have used biomimicry to create a fabric that mimics the exact proportion of the shark’s denticles, hugely improving a swimmer’s speed.  Also inspired by shark skin, scientists have developed products to reduce algae adhesion on smooth surfaces and to aid in reducing the spread of bacteria with medical devices and hospital surfaces.

It is thought that denticles provide several evolutionary advantages, ranging from reduced drag and turbulence while swimming to increased speed and efficiency, as well as increased protection from injury in hunting within dense coral reefs. Interestingly, while most shark species share the common presence of denticles, the physical size and shape of these features vary widely among different species. 1,2

X-Ray Microscopy Imaging of Shark Denticles

For our article this month, we chose to compare the denticles present on two different species of Atlantic sharks using the SkyScan 1272 high-resolution desktop micro-CT. As can be seen from the view in Figure 1 above, obvious physical differences are visible between the two species, as demonstrated by  the enlarged 3D printed representations of the denticles obtained from our X-ray microscopic imaging of isolated skin samples. To prepare for imaging, small (~75mm2) sections of skin were extracted from each sample and dehydrated in ethanol. After dehydration, the samples were chemically dried using hexamethyldisilazine (HMDS) prior to imaging. 

As shown in Figure 2, both species of shark are commonly located on the Atlantic coast of North America and are of similar size. However, as we see from the micro-CT imaging, each evolved to develop a quite different denticle architecture. 

Figure 3: Planar 2D and 3D printed views of an Atlantic sharpnose shark denticle

When examining the Atlantic shapnose shark denticles, a three-pointed top scale is observed (Figure 3). These scales are relatively blunt at the face and extend horizontally along the direction of flow when a shark is swimming (nose to tail).  

Figure 4: Planar 2D and 3D printed views of a spiny dogfish shark denticle

When examining the spiny dogfish shark denticles, a triangular scale is observed with one large central peak curving from front to back of the shark (Figure 3). These scales are larger than those observed for the Atlantic Sharpnose shark and have more pronounced peaks.   

Figure 5: Rendered 3D high resolution views of Atlantic sharpnose (top) and spiny dogfish (bottom) denticles

Comparing the rendered 3D views of each skin sample, the local density of the denticles appears higher in the Atlantic sharpnose shark than the spiny dogfish shark (Figure 5). For both samples, the alignment of the denticles with the direction of flow while swimming is also evident. 

Figure 6: Interactive 3D models of Atlantic sharpnose (left) and spiny dogfish (right) denticles

Conclusion

The SkyScan 1272 allowed us to compare shark skin samples from which we observed large differences between the denticles present for each species. From the micro-CT data, we isolated a denticle from each species and utilized additive manufacturing to produce a copy at 1000% scale. We hope you found this Image of the Month informative and encourage you to subscribe to our newsletter and social media channels in preparation for the continuation of our image of the month series next month. 

Scan Specifications

SampleShark Denticle
Voltage (kV)50
Current (µA)120
Pixel Size (µm)2.2
Rotation Step0.15
Scan Time (HH:MM:SS)02:42:49

These scans were completed on our desktop SkyScan 1272 system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon and visualization of 2D and 3D results were completed using DataViewer, CTVox, and CTAn. 

Would you like your work to be featured in our monthly newsletter? If so, please contact us by calling Seth Hogg at 610-366-7103 or e-mailing seth.hogg@microphotonics.com

References

  1. https://oceanconservancy.org/blog/2019/08/02/wonder-shark-scales/
  2. https://ocean.si.edu/ocean-life/sharks-rays/biomimicry-shark-denticles
  3. https://www.fisheries.noaa.gov/species/atlantic-sharpnose-shark
  4. https://www.fisheries.noaa.gov/species/atlantic-spiny-dogfish

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