How rugose can you go? Spiny Agonidae armour decreases boundary layer separation.

This article investigates the hydrodynamic function of bony armour in nine species of poacher fishes (family Agonidae), focusing on how armour morphology and surface rugosity affect boundary layer separation and drag reduction. Using microcomputed tomography scans, the researchers categorized armour into four morphotypes based on spine prominence—no spine, small spine, depressed spine, and prominent spine—and quantified three-dimensional rugosity. Digital particle image velocimetry on 3D-printed models revealed that species with larger, more prominent spines exhibited delayed boundary layer separation, maintaining attached flow longer and thereby reducing drag, which is advantageous for these benthic fishes. The study highlights that beyond defensive roles, poacher armour morphology influences swimming performance by modulating fluid dynamics, with spine size playing a key role in hydrodynamic efficiency.