Sand Paper Shark Skin

Sharks may look smooth and sleek, but up-close their skin is anything but.. In fact, in the past shark skin was used as sandpaper! Shark skin is made up of millions of tiny scales, called "dermal denticles", which are actually modified teeth. Their hard coating of smooth enamel and orientation from nose to tail, reduce drag for faster and easier swimming. They are so perfect that their structure is used by engineers to study hydrodynamics! Today, Olympic swimming teams wear suits designed based upon the structure of shark skin!

"dermal denticle" (Latin): dərmɪs = skin, denticulus = tooth

Shark skin under magnification (Image source: www.sharksinfo.com)

In addition to improving hydrodynamics, in many species of small sharks, it it thought that dermal denticles provide abrasion strength and defence from parasites. Many "benthic" (bottom-dwelling) species of sharks live in rocky environments, so it thought the dermal denticles protect them when then squirm amongst sharp rocks or coral when hunting. Comparatively, in "pelagic" (living in the open ocean) species, it is thought that dermal denticles impede parasites from adhering to the sharks' skin (Feld et al, 2019).

In order to determine the function of dermal denticles, a team of researchers recently used microscopy to study the structure and shape of dermal denticles in several species of slow-moving sharks. They also used Particle Image Velocimetry (PIV) to measure how the denticles affected the flow of fluid over the skin. This method involves taking two consecutive photographs and then comparing the flow of particles between them. This makes it possible to visualise how fluids flow; where they flow rapidly versus where they flow more slowly, and where they are perturbed versus where they flow smoothly. This allowed them to determine where the structure of dermal denticles differed over different regions of the sharks body (Feld et al, 2019).

Small spotted catshark dermal denticles (Feld et al, 2019)

The scientists studied three species of slowly swimming sharks, to see how the dermal denticles contrasted:

1. The small spotted catshark (Scyliorhinus canicula) is a small, benthic species, which hunts in rocky areas and can often be seen resting on the substrate,

2. The spiny dogfish (Squalus acanthias), which is also a small benthic species, but has a relatively active lifestyle; never stopping swimming (even to rest) and hunting aggressively in large packs,

3. The Greenland shark (Somniosus microcephalus) which is a large, pelagic shark living in very cold waters (even in the Arctic). These sharks swim very slowly, grow very slowly and can live to be hundreds of years old (Compagno, 1984).

Spiny dogfish dermal denticles (Feld et al, 2019)

Greenland shark dermal denticles (Feld et al, 2019)

The researchers found that in all three species, the dermal denticles were smallest and most densely distributed in the skin around the fins and gills, and the largest dermal denticles were found on the sharks' backs.

In the spiny dogfish, the denticles differed in that they were smaller compared to the other species and in areas with the smallest dermal denticles, they were found in lower densities. Fluid flowed smoothly and uniformly over the denticles.

Comparatively, dermal denticles on the Greenland shark were especially large compared to the overall size of the shark and were more sparsely distributed compared to the other species. Their shape was also different; with rivulets on their surface and a very pointed shape. These dermal denticles created bubbles behind them as fluid moved over them and the flow was more random and unstable (Feld et al, 2019).

The researchers concluded that, in comparatively slow-swimming shark species, the dermal denticles are important to avoid bacterial invasion and to impede parasite attachment. This is called "antifouling". The perturbations in the flow of water over the sharks' skin, means bacteria and other microscopic pathogens cannot settle and attach, and larger parasites, like lampreys (Order Petromyzontiformes), are swept away and cannot gain a hold. This means that the strucuture of shark skin is critical for their health and fitness (Feld et al, 2019).

Lamprey parasite attached to Greenland shark skin (Gallant et al, 2009).

References

Compagno LJV (1984). Sharks of the world: An annotated and illustrated catalogue of shark species known to date. FAO Fisheries Synopsis.

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.

By Sophie A. Maycock for SharkSpeak.