Sensational shark skin and dazzling dermal denticles

Abstract

Shark skin has a unique structure which has served as a source of inspiration for engineers interested in reducing drag in water or preventing parasites from becoming attached. The skin is covered in tooth-like structures called dermal denticles, made of an enamel-type tissue and dentine on the outermost portion and containing a central cavity filled with pulp. Their general shape is similar to a pointy canine tooth where the base is wider than the edge and is thought to reduce the amount of drag pulling on the animal when moving through water. Interestingly, previous work suggests a wide range of denticle shapes on the skin around the gills of dogfish, where the denticles closer to the head are rounded and smooth compared with the ridged and sharp denticles toward the tail. But does this diversity in denticle shape near the gills also occur in other shark species? And what purpose might these different shapes serve?To study these questions, Molly Gabler-Smith and George Lauder from Harvard University, USA, Dylan Wainwright from Yale University, USA, and colleagues from Harvard University compared the shape and size of the denticles around the gills of 13 shark species with various lifestyles. They aimed to better understand whether the shape of these denticles differs between species and whether these differences might help water to flow over the gills.The researchers collected the sharks from fishing surveys and museums, ranging from the thresher shark, known for its massive tail, enabling it to jump out of water like a dolphin, to the spiny dogfish, which are smaller and known to live in deep sea beds. They then took 3D measurements of denticle length and width and used electron and light microscopy to look at differences in denticle shape across the skin around the gills of individual sharks and between different species.Comparing the structures, it turned out that the denticle shape varied across different areas of the skin surrounding the gills in all of the species, with the short broad denticles arranged toward the front of the shark and longer, pointier denticles occurring toward to the tail of the shark. In fact, the denticles found closer to the tail had pronounced ridges on their surfaces and three spiky spines, in contrast to the smoother almond-shaped denticles found towards the head. Some species, such as the chain catshark, had extreme changes in denticle shape over a distance of a few millimeters. They also found that the shark species that continuously swim to breathe had larger denticles toward the head compared with shark species that breathe by opening and closing their mouths to pump water over the gills.The extreme differences in shape between the denticles found towards the head versus those toward the tail may serve to reduce friction and prevent damage to the skin when the gills flap against each other while breathing. The dramatic shape difference may also reduce drag from water moving through the gills, which could prevent water from flowing in the opposite direction, stopping the shark from getting enough oxygen from the water. This study gives us a foundation for future research on how denticle shape can help sharks to breathe and if there are differences in how water moves over the denticles surrounding the gills of active or passively breathing species. Understanding the great diversity and function of denticles in shark skin can help us to learn how they evolved and even allow us to better imitate their amazing ability to reduce drag in water.