Dermal denticles: why shark skin feels like sandpaper

If you are lucky enough to have touched a shark, you’ll notice that its skin has a very interesting texture. When stroked from head to tail, it generally feels smooth. But do it in the opposite direction, and you’ll find that it feels rough like sandpaper.

Close-up of a Port Jackson Shark‘s skin.

The reason lies in the tiny flat V-shaped scales that cover its skin. Called dermal denticles, these scales are surprisingly more like teeth than fish scales.

Microscopic view of dermal denticles. These denticles are packed tightly together and grow with their tips facing backwards. This is why shark’s skin feels rough if you run your finger from tail to head, but smooth from head to tail. Credit: © Trevor Sewell/Electron Microscope Unit, University of Cape Town. Source

Like our teeth, dermal denticles have an inner core (made up of connective tissues, blood vessels, and nerves) covered by a layer of hard calcareous material called dentine. This is then covered with a hard enamel-like vitrodentine.

Made for speed and protection

One of the main functions of dermal denticles is to protect the shark from predators. At the same time, it has an equally important hydrodynamic function.

Dermal denticles have grooves running down their length in alignment with water flow. These grooves disrupt the formation of turbulent swirls of slower water, hence decreasing drag and turbulence, which allows the shark to swim faster and more stealthily.

Microscopic images of dermal denticles from three shark families: blacknose shark (Carcharhinus acronotus) denticle is shown for family Carcharhinidae, a scalloped hammerhead (Sphyrna lewini) denticle for family Sphyrnidae, and a nurse shark (Ginglymostoma cirratum) denticle for family Ginglymostomatidae. Credit: Erin M. Dillon (Source)

Dermal denticles are so effective that swimsuit companies have designed racing suits that mimic the concept of dermal denticles. These specialty suits have helped Olympic swimmers achieve even faster times in the pool, but have since been banned at major competitions.

Here’s another interesting fact: for centuries, sushi chefs in Japan have been using graters lined with shark skin to grate wasabi root. The rough shark skin enables the wasabi to be grated to a much finer consistency so as to better bring out its flavour.

Wasabi grater made of wood and shark skin. (source)

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