Morphologically complex biological armor acts as drag reducing agents in benthic fishes

Biological armor is a morphologically diverse feature found across vertebrate taxa from armadillos to crocodiles to several groups of bony fishes. While armor is typically considered defensive, it is a multifunctional trait with many plausible alternative functions. In fishes, armor can act as a shield against abiotic and biotic assaults, be a signal of a quality mate, provide storage for minerals, and act as a system to alter flow around the organism. Here, we explore the hydrodynamic function of armor in twenty species of poacher fishes (Agonidae) by visualizing flow to assess hydrodynamic impacts of different shaped plates. We used micro-computed tomography scans to compare the rugosity across species and geometric morphometrics to capture morphological variation. We then used Digital Particle Image Velocimetry along with 3D printed models of the whole body and scaled up rows of plates to assess overall hydrodynamics as well as the effect of small surface features. Our morphometric analysis revealed that much of the variation in armor morphology is driven by plate shape, spine size and spine prominence. We show that spinier armor reduces drag by generating vortices that keep the boundary layer attached further along the length of the fish, decreasing drag and allowing poachers to expend less energy to stay in place on the benthos.