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Letter tection against self- and alli- Offering Toughness and Protection, gators, as we stated earlier.5 There is Arapaima Scales Provide Effective a cost, in mobility and speed, for hav- ing an armor. Thus, we are unanimous Defense against Predation to agree that the survival of the ara- paima has been ensured by its scales. Marc A. Meyers1 and Robert O. Ritchie2,* Were the arapaima river dwellers, they would not be such an easy prey. Piranhas are not active in running We thank Huskey, Westneat, and Gru- cartilage removed by piranha bites. waterandaremostaggressiveinsea- bich1 for the detailed account on the We recognize that the arapaima shape sonal lakes, when the food supply is arapaima and piranha. Their article is is not highly conducive to be attacked diminished, toward the end of the dry not only informational but adds to by piranha. However, this might also season. our understanding of the complex rela- be an aspect of evolution. The aruana, tionship between evolution, environ- related to the arapaima, is similarly It is true that juvenile arapaima do not ment, and predator-prey dynamics. covered by large scales. have the well-developed scales, but We are by no means evolutionary biol- they are protected by both parents in ogists, but materials scientists, and the In seasonal lakes and river channels, the nest—for the first three months of sole goal of our contributions has been diurnal fish such as peacock bass are their life. The arapaima female prepares to clearly demonstrate that the ara- oftendevouredwhencaughtonthe a nest and lays the in it. These are paima scales are a formidable biolog- line and pulled away from protective later kept in the mouth of the father. ical structure with great toughness, dead underwater trees and branches. The hatching coincides with the begin- among the highest among biological However, pacu are not and their scales ning of the rainy season; when the for- materials, with an impressive JIc frac- provideprotectionevenwhenhooked. À est becomes flooded, the hatchlings ture toughness of ~200 kJm 2.Weun- Since piranhas are diurnal, nocturnal enter it, finding plenty of food and pro- knowingly narrowed Currey’s conclu- fish (primarily siluroidei, scaleless cat- tection. The mother fiercely protects sion about the protection offered by fish) are protected because of their the offspring from predators in the sea- the arapaima scale to piranhas. He habits. The arapaima is an obligate sonal lakes. When the waters recede states:2 ‘‘The huge teleost freshwater air-breather and has to periodically again, the juveniles are already grown fish , which can exceed (every 10 to 20 min) breach the surface. (and have a covering of tough scales); 3 m in length, has somewhat similarly Thus, it would provide an ideal prey if it they reach a length of 800 mm within layered scales, which probably make did not have these outstanding elas- one year.6 This is consistent with the it effectively immune to attack (Torres moid scales. The tongue is also highly growth rate provided by Huskey et al.1 et al., 2008).’’ Torres et al.3 preceded mineralized, and we venture to state We agree with the conclusions pre- us in the publication of the results that this is also a protection from sented by Huskey et al.1 Indeed, we and were the first to analyze the struc- piranha attacks. Huskey et al.1 imply have personally measured the biting ture of the scale and to suggest their that the South American alligator force of piranhas from Paraguay and outstanding toughness. Thus, these (cayman) is the primary predator of the Roosevelt rivers and present the scales are a superb defense against arapaima, but we can hardly reconcile detailed results as well as experimental predators. this with the highly mineralized tongue. procedures in a separate paper.7 This The indigenous inhabitants attribute bite force is surprisingly low and we One of us (MAM) has a 50-year experi- the protection from piranha to the ence in the rivers and scales. has witnessed unprovoked piranha at- tacks against persons and dogs. It is Indeed, elasmoid scales have ap- 1Materials Science & Engineering Program, Department of Mechanical & Aerospace true that appendages such as tails (in peared several times during evolution Engineering, and Department of dogs) and fingers and toes (in humans) in response to predator attacks.4 NanoEngineering, University of California San are favored biting sites because of their They should not be confused with Diego, La Jolla, CA 92093, USA 2 shape. Huskey et al.1 explain this well ganoid scales, which diverged evolu- Department of Materials Science & Engineering, University of California Berkeley, Berkeley, CA by measuring the gape and angle of tionary from them at a very early stage. 94720, USA various piranhas. It is also true that The ganoid scales of the *Correspondence: [email protected] one often finds tails of fish with some of the Mississippi Basin serve as pro- https://doi.org/10.1016/j.matt.2020.10.029

Matter 3, 1979–1980, December 2, 2020 ª 2020 Elsevier Inc. 1979 ll Letter show, using allometry, that it increases 2. Currey, J.D. (2010). Mechanical properties and 6. Arantes, C.C., Castello, L., Stewart, D.J., Cetra, adaptations of some less familiar bony tissues. M., and Queiroz, H.L. (2010). with the square of the head length. J. Mech. Behav. Biomed. Mater. 3, 357–372. Population density, growth and Our results, consistent with the ones reproduction of arapaima in an Amazonian 1 3. Torres, F.G., Troncoso, O.P., Nakamatsu, J., river-flood plain. Ecol. Freshwat. 19, presented by Huskey et al. and Huby Grande, C.J., and Gomez, C.M. (2008). 455–465. et al.,8 reinforce our belief that the ara- Characterization of the nanocomposite laminate structure occurring in fish scales from 7. Velasco-Hogan, A., and Meyers, M.A. (2020). paima scales are an effective defense Arapaima gigas. Mater. Sci. Eng. C 28, 1276– Bite force and mechanics of piranha against predation by, among others, 1283. (Serrasalmidae). piranhas. 4. Wikipedia. Fish scale. https://en.wikipedia. 8. Huby, A., Lowie, A., Herrel, A., Vigouroux, R., org/wiki/Fish_scale. Fre´ de´ rich, B., Raick, X., Kurchevski, G., Godinho, A.L., and Parmentier, E. (2019). 1. Huskey, S.H., Westneat, M.W., and Grubich, 5. Yang, W., Gludovatz, B., Zimmermann, E.A., Functional diversity in biters: the evolutionary J.R. (2020). The temporal, behavioral, and Bale, H.A., Ritchie, R.O., and Meyers, M.A. morphology of the oral jaw system in pacus, biomechanical improbability of piranhas (2013). Structure and fracture resistance of piranhas and relatives (Teleostei: driving the evolution of Arapaima gigas scales. alligator gar (Atractosteus spatula) armored Serrasalmidae). Biol. J. Linn. Soc. Lond. 127, Matter 3, this issue, 1976–1978. fish scales. Acta Biomater. 9, 5876–5889. 722–741.

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