Katherine A. Galloway and Marianne E. Porter
The red lionfish, Pterois volitans, an invasive species, has 18 venomous spines: 13 dorsal, 3 anal, and one on each pelvic fin. Fish spines can have several purposes such as defense, intimidation, and for anchoring into crevices. Instead of having hollow spines, lionfish have a tri-lobed cross-sectional shape with grooves that deliver the venom, tapering towards the tip. We aim to quantify the impacts of shape (Second moment of area) and tapering on the mechanical properties of the spine. We performed two-point bending at several positions along the spines of P. volitans to determine mechanical properties (Young’s modulus, Elastic energy storage, and Flexural stiffness). The short and recurved anal and pelvic spines are stiffer and resist bending more effectively than the long, dorsal spines. In addition, mechanical properties differ along the length of the spines, most likely because they are tapered. We hypothesize that the highly bendable dorsal spines are used for intimidation, making the fish look larger. The stiffer and energy absorbing anal and pelvic spines are smaller and less numerous, but they may be used for protection since they are located near important internal structures such as the swim bladder. Lastly, spine second moment of area varies across the Pterois genus. These data suggest there may be morphological and mechanical trade-offs among defense, protection, and intimidation of lionfish spines. Overall, the red lionfish venomous spine shape and mechanics may offer protection and intimidate potential predators, significantly contributing to their invasion success.