Shape variation of the prawn Macrobrachium jelskii (Palaemonidae: Decapoda) in the Neotropical semiarid drainages: an intra- and inter-basin investigation Sa ´ vio A. S. N. Moraes A,D , Carlos E. R. D. Alencar A,B , Sergio M. Q. Lima C and Fu ´ lvio A. M. Freire A A Laborato ´ rio de Biologia, Ecologia e Evoluc ¸a ˜ o de Crusta ´ ceos (LABEEC), Universidade Federal do Rio Grande do Norte, Avenida Senador Salgado Filho, 59064-741, Natal, Brazil. B Departamento de Cie ˆncias Biolo ´ gicas e da Sau ´ de, Universidade Regional do Cariri (URCA), Campus Pimenta, 63105-010, Crato, Brazil. C Laborato ´ rio de Ictiologia Sistema ´ tica e Evolutiva (LISE), Universidade Federal do Rio Grande do Norte, Avenida Senador Salgado Filho, 59064-741 Natal, Brazil. D Corresponding author. Email: savioarcanjo@hotmail.com Abstract. The present study aims to show the intra- and inter-basin body-shape variations of the freshwater prawn Macrobrachium jelskii (Miers, 1877) in Brazilian Neotropical semiarid basins, quantifying relations to address if its occurrence is old and natural or recent and artificial (anthropic). We used a geometric-morphometric tool to explore patterns of shape variation of the cephalothorax, abdomen and cheliped regarding eco-evolutionary traces (sexual dimorphism, hydrodynamism and geographical isolation) among the drainages. The sexual shape dimorphism was observed in cephalothorax and abdomen of females, which presented more horizontally stretched than in males, possibly favouring gonadal development, vitellogenesis and egg attachment. No apparent hydrodynamic trend was observed in the morpho-space. Moreover, no consensus was shown in the morpho-space about the geographic isolation in semiarid basins regarding the three body-structure planes. However, the abdominal plane indicates a low population/basin structure, which can be an indirect reflection of variations of vitellogenesis in eggs modulated by environmental factors. Furthermore, morphometric variations show low phenotypic plasticity, which is supported by the literature, indicating a scenario of non- natural distribution. Additional keywords: Agar river prawn, Caatinga’s temporary rivers, geometric morphometrics, hydrodynamics, phenotypic plasticity, sexual shape dimorphism. Received 16 July 2019, accepted 29 March 2020, published online 29 May 2020 Introduction Within any species, the nuances in the relationship between ecology and evolution are best understood through an exami- nation of the processes that influence diversity, community structure, and the complexity of ecosystems (dynamics and interactions; Palkovacs and Post 2009). This integrated and theoretical–biological approach, which links the species’ func- tionality within its environment and the plasticity of its mor- phological characteristics, allows us to infer the presence of robust adaptive radiations (Grant 1986; Schluter 2001). Some examples of these ‘eco-evolutionary’ indicators on a short time scale are intra-population variations of sexual dimorphism or phenotypic adaptations to environmental conditions. And over the long time scale, inter-population variations reflect vicariant dispersal processes and population structure (Hendry 2013). These ‘eco-evolutionary indicators’ can be visualised from the external morphological variation and co-variation by abstraction and qualification of the shape (Adams et al. 2004). Crustaceans have rigid exoskeletons, and frequently possess obvious spines, tubercles and sculpted regions, which should make them great candidates for shape analyses through geo- metric morphometrics (Rufino et al. 2006). These morphometric variations and co-variations are appar- ent in sexual dimorphism (SD), environmental factors, and the populations structure of animal’s species. Sexual dimorphism is a result of specific selective pressures on both sexes (Darwin 1871; Slatkin 1984; Shine 1989; Andersson 1994). In males of decapods, SD is usually known by large-sized body and robust chelipeds. Nevertheless, this can correlate with many factors as sexual behaviours, agonistic encounters and mating systems (Mariappan et al. 2000; Duffy and Thiel 2007; Bauer 2010) and population aspects such as sex ratio, population establishment or intraspecific divergences between lineages (prezygotic isolation mechanisms; Panhuis et al. 2001; Ledesma et al. 2010). CSIRO PUBLISHING Marine and Freshwater Research https://doi.org/10.1071/MF19201 Journal compilation Ó CSIRO 2020 www.publish.csiro.au/journals/mfr