Buckle Up for Buckling: Poacher Fish Scale Behavior Under Compression

Abstract

Poacher fishes (Agonidae) are a family defined by their bony armor. Unlike the flexible scales possessed by many fishes, poachers are encased in rigid plates that overlap in rows. Uniquely, when viewed in the hoop direction, rows appear as distinct geometric rings which can be dissected out both digitally and physically. These rings vary across species in both number and arrangement of plates, forming octagons, hexagons, and squares. The diversity in row count and overlap across species implies different ring designs may be specialized for different functions. To test this, we 3D-printed models of poacher armor rings as regular polygons with flexible joints at either the center or corners, varying the number sides to match the diversity seen in nature. We used a material testing system to compress the models to a locking position, recording force displacement curves for each model. We found that, across most shapes, models with central joints, which most closely match poacher scale morphology, require more force to reach a locked position than models with joints in the corners. This means the models that closely resemble fish, despite being contradictory to conventional engineering design, resist compression better. Poachers are not fast swimmers and thus hide under rocks, compressing themselves, but due to their scales’ resistance, they can prevent damage to their soft internal structures.