Taking the Rough with the Smooth

You might imagine that sharks look so sleek, that their skin must be very smooth, maybe even slimy to the touch. But this could not be further from the case! Shark skin is actually very abrasive and can even cause burns if you run your hand the wrong way over their bodies. This is why shark skin was once used as sand paper and to improve the grip on swords and knives. So what does this skin actually look like up close? Why is their skin so rough? What could possibly be the benefit?

Viewing under a microscopes exposes how rough a lemon shark's skin actually is (image Credit: Pascal Deynat/Odontobase / WikimediaCommons)

Not To Scale

The reason a shark's skin is so rough in one direction, is because it is made up of millions of tiny scales called "dermal denticles". Bizarrely, these are actually modified teeth, so they have a hardened coating of enamel, which makes a shark's skin very tough. As they are arranged as interlocking plates, they can slide over each other, so they also mean a shark's skin is very flexible (Feld et al, 2019).

The spiny dogfish's dermal denticles are especially small, to make them hydrodynamic, but they are different across varying regions of the shark's body (Feld et al, 2019).

Depending where you look on a shark's body, the dermal denticles vary in size and density, to accomodate for how the skin in that areas needs to function. For instance, in many slow-swimming species denticles are smallest and most densely distributed in the skin around the fins and gills, but the largest dermal denticles can found on the sharks' backs (Feld et al, 2019)

What a Drag!

Dermal denticles are oriented from nose to tail. This is consistent across all species of sharks because it reduces drag and makes swimming easier. If you take a very magnified look at the skin of the spiny dogfish (Squalus acanthias) for instance, you can see they are covered in very small, interlocking scales, with the tips pointing towards the tail (Feld et al, 2019). In the fastest species of sharks - the shortfin makos (Isurus oxyrinchus) - the denticles are rounded at the leading edge to make them especially hydrodynamic (Patricia et al, 2019).

Dermal denticles are strong, but also allow the skin to be flexible (Image Credit: John Michael Bullock / Flickr)

Roughed Up

When you look at the skin of different species of sharks under a powerful microscope, it becomes clear that the shape, size and density of the dermal denticles can very enormously between species. This has lead scientists to conclude that dermal denticles also have other functions depending on the species (Southall & Sims, 2003; Feld et al, 2019).

Each species of shark, like this Small spotted catshark, has uniquely sized and shaped dermal denticles (Feld et al, 2019)

For instance, in addition to improving hydrodynamics, in many species of small "benthic" sharks, like the small spotted catshark (Scyliorhinus canicula), it is thought that dermal denticles provide abrasion strength. In these sharks the dermal denticles help to protect them from injuries as they are squirming amongst sharp rocks or corals when hunting (Feld et al, 2019).

Completely unique to lesser spotted catsharks is how they also use their denticles for feeding. Scientists have observed juvenile catsharks also use the denticles on their tails to secure their food, to make it easier to chew into bite-sized pieces (Southall & Sims, 2003).

Scientists think that dermal denticles make it harder for parasites (like this lamprey on a Greenland shark) to hold on (Gallant et al, 2009).

Get Off!

Comparatively, in "pelagic" species (that live out in the open ocean), it is thought that dermal denticles might be "antifouling"; stopping parasites, bacteria and other microscopic pathogens from attaching to the shark. This means that the strucuture of the denticles are also critical for the shark's health and fitness (Southall & Sims, 2003; Feld et al, 2019).

The rivulets on their surface and sparsity of dermal denticles on the Greenland sharks' skin create eddies in the water, to stop parasites from latching on (Feld et al, 2019)

For example, in slow-swimming species like the Greenland shark (Somniosus microcephalus), the denticles are sparse, and shaped in such a way that they create bubbles and eddies as the water flows over the skin. These perturbations mean bacteria and other microscopic pathogens cannot settle and attach, and larger parasites, like lampreys, are swept away and cannot gain a hold (Feld et al, 2019).

Perfectly Designed

Over hundreds of millions of years, shark skin has evolved to become absolutely perfect; flexible yet strong, hydrodynamic to allow easier swimming, but also rough enough to protect from parasites. It is a miracle of evolution! In fact, shark skin is such a marvel that it's structure has been used by engineers to study hydrodynamics and Olympic swimming teams wear suits specifically designed to mimick shark skin! Nothing that we could design could ever be as effective as that created by nature (Crawford, 2007).

The shape, size and density of the dermal denticles is unique to each shark species (Image Credit: Viana STFL / WikimediaCommons / Viana & Carvalho, 2020)

References

Crawford D (2007) Shark. Reaktion Books.

Feld K, Kolborg AN, Nyborg CM, Salewski M, Steffensen JF & Berg-Sørensen K (2019). Dermal Denticles of Three Slowly Swimming Shark Species: Microscopy and Flow Visualization. Biomimetics, 4:2, 10.3390/biomimetics4020038. Access online.

Gallant J, Harvey-Clark C, Myers R & Stokesbury M (2009). Sea Lamprey Attached to a Greenland Shark in the St. Lawrence Estuary, Canada. Northeastern Naturalist. 13. 35-38. Access online.

Patricia FW, Guzman D, Iñigo B, Urtzi I, Maria BJ & Manu S (2019). Morphological characterization and hydrodynamic behavior of shortfin mako shark (Isurus oxyrinchus) dorsal fin denticles. Journal of Bionic Engineering, 16, pp.730-741. Access online.

Southall EJ & Sims, DW (2003). Shark skin: a function in feeding. Proceedings of the Royal Society of London. Series B: Biological Sciences, 270, 47-49.Access online.

Viana STFL & Carvalho MR (2020). Squalus shiraii sp. nov. (Squaliformes, Squalidae), a new species of dogfish shark from Japan with regional nominal species revisited. Zoosystematics and Evolution, 96:2, 275-311. Access online.

By Sophie A. Maycock for SharkSpeak.