Stingray spine diversity reflects performance trade-offs linked to puncture and breakability

Stingrays (order Myliobatiformes) possess defensive spines that inflict intense pain through mechanical damage and envenomation. These spines exhibit striking variation among species, yet the functional underpinnings of their morphological diversity remain poorly understood. We analysed spine functional morphology across 30 species from five families to characterize variation along phylogenetic and ecological gradients. Spines ranged from slender, sharp-tipped forms with low-profile serrations to robust and wide-tipped with more prominently projecting serrations. By integrating morphological data with finite element modelling and puncture and removal tests, we uncovered biomechanical trade-offs shaping stingray spine evolution. At one extreme, slender, sharp-tipped spines puncture deeply but are prone to breakage, while robust spines resist fracture and anchor more effectively, though with reduced puncture performance. These contrasting morphologies likely represent divergent defensive strategies-spines that readily fracture maximize damage through breakage and retention in predator tissues, facilitating escape. Conversely, more durable spines may be reused across multiple defensive bouts. The latter are often in freshwater stingrays, potentially linked to habitat-specific features like refuge availability. Our findings suggest that trade-offs between puncture, anchoring and durability have been central to stingray spine evolution. Similar patterns across taxa suggest shared mechanical constraints shaping the evolution of biological weapons.