Functional aspects of Placoid Scales: A review and update

Abstract

The function of placoid scales (dermal denticles) is considered along three perspectives. Firstly, in a historical context, denticles have been modified along several functional lines, including protection from predators and ectoparasites, reduction of mechanical abrasion, accommodation of bioluminescent and sensory organs, and reduction of frictional drag. Each function selects for a unique morphology described herein. Secondly, denticles characteristic of benthic and/or sedentary sharks are re-examined through an analysis of 15 genera. Generalizations are difficult due to the wide range of variation in crown morphology and overall scale weight, but individual denticle types do reflect combined optimal characteristics for both hydrodynamics and protection, tending toward the latter. Lastly, the relationship between slight variations in regional flow dynamics over the body, and the potential impact of this relationship on the previously defined hydrodynamic scale characteristics, is examined in two pelagic carcharhinids. These findings suggest that along the anterior portion of the body, strong indications of reduced ridge dimensions (heights and spacings) on the dorsum are present and may be related to water velocity.