Sensitivity of genital and somatic traits of scorpions to developmental instability caused by increasing urbanization: A 20-year experiment

Abstract

Urbanization causes the loss of large amounts of habitat and produces significant changes in environmental conditions with consequences in the individual's behavior, morphology, and physiology in natural populations. The urbanization can impact in the individual's development instability (DI) that may be measured with the fluctuating asymmetry (FA) level making it an environmental quality bioindicator. In addition, the FA of each trait should be interpreted considering the trait functional importance, morphogenesis and history of selection pressures. In this paper, we evaluated the impact of two decades of increasing urbanization levels over a population of the pampean scorpion Bothriurus bonariensis. We analyze the FA level in somatic and genital traits in adult individuals of both sexes, collected every ~ 10 years (1997, 2008, 2018). Interestingly, in this study the somatic traits (pedipalp chelae) would be under directional selection and are shaped throughout the individual's growth. In contrast, the genital traits (male's hemispermatophore) are involved in the sperm transfer process and could be under stabilizing selection. Also, hemispermatophores are affected for less time by environmental stressors since they are formed once the individual has reached sexual maturity. Our results showed a significant increase in the FA levels in somatic traits of males and females, according to the increase of urbanization along years. In contrast, genital traits showed no changes in the level of FA in females, and only a few male genital traits presented an increase in the FA level. This work shows evidence that the measured scorpion traits (under different selective pressures and morphogenesis) might present differences in their susceptibility to environmental stress. We discuss these results in a context of DI of individuals caused by urbanization. We support the idea that the level of FA can be useful as an index to evaluate the effects of environmental stress on invertebrate populations. Furthermore, our paper presents the scorpions as a good model that could be used as an indicator group for studying the impact of the anthropogenic disturbance factors on natural ecosystems.