The Denticle Multiverse: Morphological Diversity of Placoid Scales across Ontogeny in the Portuguese Dogfish, Centroscymnus coelolepis, and Its Systematic Implications

Abstract

Centroscymnus coelolepis is a deep-water sleeper shark and, like most sharks, it is covered in placoid scales, or dermal denticles. The morphological diversity of the dermal denticles in this species, however, has not been described in detail, and ontogenetic changes in denticle morphology are poorly understood in sharks. Combining scanning-electron microscopy and micro-CT imaging, we demonstrate the presence of eleven dermal-denticle morphotypes across the ontogeny and different regions of the body of Centroscymnus coelolepis. The snout, interspiracular, and trunk/tail regions have similar changes in denticle morphotype during development. For example, on the trunks and tails of juveniles (~350 mm TL), denticle crowns have two to three longitudinal ridges and three posterior cusps that are gradually replaced by ridgeless and cuspless crowns in adults (>800 mm TL). Sixteen measurements were obtained from the 3D models generated. A principal component analysis demonstrated that the eleven distinct dermal-denticle morphotypes observed were located in different regions of the morphospace. The denticle volume and surface area showed negative allometry with respect to body length throughout the ontogeny. The results reflect the considerable diversity within the denticle multiverse (ontogenetic and intraspecific variations), and much of this diversity remains to be explored to fully understand the role of dermal denticles in shark taxonomy, ecology, and biomechanics.