Slippery and Smooth Shark Skin: How Mucus Transforms Surface Texture.

Shark skin is covered in denticles that provide texture important for hydrodynamic function. In bony fishes, both skin texture and function are modified by mucus that covers the outermost layer of the skin and scales. Despite the similar potential for mucus to change skin texture and function in shark skin, little is known about the occurrence and effect of external mucus in sharks. Specifically, we do not know where mucus is present along the shark body or how mucus alters surface texture, which could alter denticle function. To fill these gaps, we obtained individuals of Mustelus canis (dusky smooth-hound shark) and used gel-based profilometry to quantify the texture of the three-dimensional surface at eight body regions under two conditions: (1) a live anesthetized condition with mucus and (2) a condition after mucus was removed during preservation. We discovered that mucus covers and obscures the denticles on the dorsal fin and tail trailing edge tips; as a result, these regions were smoother and had a different surface texture than the preserved condition at the same region. Specifically, five parameters were significantly changed by mucus in these regions: roughness, skew, kurtosis, developed interfacial area ratio, and exposed area of the denticles. Notably, mucus did not change surface texture at any of the other body regions. Both the tips of the dorsal fin and tail are regions where flow separates and vortices are shed, so these results could indicate that mucus is modifying the boundary layer flow. Our results demonstrate that shark skin mucus is secreted selectively in particular body regions and that it can drastically change the surface texture when present. These findings suggest a need to both explore the morphology and properties of shark mucus and to consider mucus in studies of shark skin hydrodynamics.