Active defense mechanisms of thorny catfish

The thorny catfish (Doradidae; order Siluriforme), has characteristic barbed pectoral fin spines and mid-lateral scutes that work in concert to provide an active mechanical defense mechanism. The pectoral spines can be locked into several fixed positions at angles between 0° and 90° to the longitudinal axis. Upon deployment they provide a formidable barrier to deglutition by a predator. Additionally, each spine contains two rows of sharp serrations which are inclined to its axis and can slice through tissue. We characterize the internal structure of the rotation joint of the pectoral spines and unravel the locking mechanism which is ensured by dual sources of friction generated from the dorsal and anterior components on the spine base. In addition to the lockable spines, thorny catfish also possess two arrays of mid-lateral dermal scutes with a sharp hook shape and gradient inner structure, which also have potential cutting capability and are part of the fish’s active defense, since they can injure the predator through rapid motions of the catfish body. The structural design of these two weapons is very impressive, including a hollow structure, porous components, and gradient transitions, leading to outstanding performance by maintaining strength, toughness and light weight synergistically. These designs may provide inspiration for the development of novel structural materials or new armor materials.