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

Abstract Armour has been present in vertebrates for millions of years and has evolved independently several times in the ray finned fishes. While armour is typically considered defensive, it is a multifunctional trait with many plausible alternative functions. We explore the hydrodynamic function of armour in nine species of poachers (Agonidae) by visualizing flow to assess drag reduction. We used microcomputed tomography scans to categorize armour morphology into morphotypes, and compared the rugosity (surface roughness) across these morphotypes. We then used digital particle image velocimetry to visualize boundary layer separation along 3D-printed whole-body models made using the scans. Poacher armour comprises eight rows that, some distance behind the second dorsal fin, merge into six. We found four morphotypes, with higher rugosity observed for the prominent spine morphotype and the lowest rugosity in the no-spine morphotype. Principal component analysis revealed that much of the variation in armour morphology is driven by plate shape, spine size, and protrusion. The boundary layer was retained longer in species with larger spines. Overall, the presence of a spine increased boundary layer retention, decreasing the drag, which is advantageous for these benthic fish.