Unravelling the skin of the nurse shark: A morphological description of the placoid scales of Ginglymostoma cirratum

Ginglymostoma cirratum, commonly known as the nurse shark, is a nocturnally active benthic shark, often found in western and eastern Atlantic waters. Although this shark has been well explored in many biological aspects, few studies have thoroughly examined the morphology of its dermal denticles (or placoid scales). The dermal denticles are teeth-like structures present in the skin of sharks and play an important role in the interaction of these fishes with their environment. In the present study, we describe the morphological variation of dermal denticles throughout the body of the nurse shark and discuss the putative functional roles of the morphologies uncovered. Twenty-four skin samples were extracted from different regions of the body and were analysed using a microtomography and a stereoscope. The terminology was standardized based on previous morphological descriptions of denticles. A total of 10 variations were identified, ranging from denticles with few or no ridges with a lozenge-shaped crown to denticles with multiple ridges bearing diamond- or leaf-shaped crowns. The morphology of the scales supports a primary function associated with abrasion strength, as previously suggested by other authors. Also, the location and distribution of ridged denticles may indicate a secondary function associated with hydrodynamics. The diagnosis of a second species within the genus, Ginglymostoma unami, based on differences in dermal denticle morphology was also discussed. The Unami nurse shark was described as having denticles with a distinct morphology and a greater number of ridges than in G. cirratum. However, we suggest that G. cirratum and G. unami have denticles with a similar morphology and ridge counts.