RESEARCH ARTICLE Mechanical properties of the venomous spines of Pterois volitans and morphology among lionfish species Katherine A. Galloway* and Marianne E. Porter ABSTRACT The red lionfish, Pterois volitans, an invasive species, has 18 venomous spines: 13 dorsal, three anal and one on each pelvic fin. Fish spines can have several purposes, such as defense, intimidation and anchoring into crevices. Instead of being hollow, lionfish spines have a tri-lobed cross-sectional shape with grooves that deliver the venom, tapering towards the tip. We aimed 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 as 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 for lionfish spines. Overall, the red lionfish venomous spine shape and mechanics may offer protection and intimidate potential predators, significantly contributing to their invasion success. KEY WORDS: Lionfish, Biomechanics, Stiffness, Elastic energy, Flexural stiffness INTRODUCTION Spines are multi-functional biological materials found in nature that can greatly benefit organisms in terms of gripping, injection, damage and defense (Anderson, 2018). For example, cacti use spiny modified leaves that prevent water loss in their dry desert habitat and protect against herbivores (Koch et al., 2009). Hedgehogs use their quills for protection against predators and the quills absorb energy during impact from high falls (Vincent and Owers, 1986). Stonefish have a lachrymal saber that is an elongation of an anterior spine, which they are able to rotate into a locked lateral position possibly for defense (Smith et al., 2018). In addition, triggerfish have a modified anterior dorsal fin spine that has several purposes including self-defense, anchoring into crevices in the coral reef when sleeping and providing protection against a strong ocean surge or waves (Cleveland and Lavalli, 2010). Similar to differences in anatomy, spine material varies, and affects the overall mechanics. Both lionfish and stingray spines are made of mineralized collagen, a combination of hydroxyapatite and collagen (Halstead and Modglin, 1950; Halstead et al., 1955). However, the mechanical properties of lionfish and stingray spines remain unknown. Spines in porcupines, hedgehogs and echidnas are made of keratin (Vincent and Owers, 1986) and have Young’s moduli (E) ranging from 5.56 GPa in porcupines to 11.56 GPa in hedgehogs (McKittrick et al., 2012; Vincent and Owers, 1986). Biomechanical properties have only been examined for stingers (bees, wasps and scorpions), where venom is delivered through the middle of the spine (Zhao et al., 2015; Zhao et al., 2016). Lionfish spines, similar to those of stingrays, have venom glands and grooves that line the sides of the spine, whereas in bees, wasps and scorpions, venom flows through the middle (Halstead and Modglin, 1950; Halstead et al., 1955). Venom delivery morphologies in combination with material composition may affect the properties of the spine under various loading regimes. In several organisms, mechanical properties vary along the length of the structure. In wasp stingers, the elastic modulus and hardness decrease along the length from the base to the tip (Das et al., 2018). In contrast, Young’s modulus increases towards the tip of owl feather shafts (Bachmann et al., 2012). The tapered morphology of porcupine fish spines changes the location of maximum stress to the distal end (tip) of the spine, but does not change spine stiffness or toughness (Su et al., 2017). By focusing spine damage toward the distal ends, porcupine fish may conserve the energy required for regrowth. The red lionfish, Pterois volitans, has 13 dorsal fin spines, three anal fin spines and one spine on each pelvic fin (Fig. 1A). In cross- section, P. volitans spines are solid and have a tri-lobed morphology, thought to be exclusive to lionfish (Halstead et al., 1955; Fig. 2A). This tri-lobed shape is formed by a pair of lateral grooves along the outer two-thirds of the length and these grooves contain glandular tissue that houses venom (Fig. 2B). Both the spines and the glandular tissue are covered by a thin membrane, which ruptures when the spines penetrate an object, releasing the venom (Halstead et al., 1955). The length of the refractory period between venom delivery events and whether the presence of venom in the lateral grooves affects the mechanical properties remain unclear. The tri-lobed cross-section of the lionfish spine is reminiscent of I-beams used in building design and construction. Engineering beam theory demonstrates that the I-beam shape is able to carry both bending and shearing loads because most of the material is distributed away from the neutral axis. As a result, I-beams have a high second moment of area and span-to-depth ratio, meaning that this shape effectively resists bending (Vogel, 2013). The lionfish spine tri-lobed cross-section also has a large portion of the material located away from the center of the structure (Fig. 2A). Received 7 December 2018; Accepted 22 February 2019 Department of Biological Sciences, Florida Atlantic University, Boca Raton, FL 33431, USA. *Author for correspondence (kgalloway2016@fau.edu) K.A.G., 0000-0003-0711-6893 1 © 2019. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2019) 222, jeb197905. doi:10.1242/jeb.197905 Journal of Experimental Biology