Acta Zoologica. 2018;1–11. wileyonlinelibrary.com/journal/azo | 1 © 2018 The Royal Swedish Academy of Sciences 1 | INTRODUCTION The order Decapoda includes two taxonomic groups, Axiidea and Gebiidea, which comprise the burrowing shrimps, in- cluding Callichirus major. The two infraorders were pre- viously included in the paraphyletic taxon Thalassinidea (Poore et al., 2014). According to Dworschak, Felder, and Tudge (2012), these infraorders exhibit morphological and ecological convergence in their evolutionary history, present- ing burrowing habits and underground galleries building. Several studies are known on the biology of C. major, high- lighting those reporting on ecological distribution and repro- ductive biology (Botter‐Carvalho, Santos, & Carvalho, 2002, 2007 ; Peiró, Wehrtmann, & Mantelatto, 2014; Rodrigues & Shimizu, 1997; Souza & Borzone, 1996; Souza, Borzone, & Brey, 1998). Knowledge about reproduction provides data on the sexual pattern of a species. Instances of intersexuality and hermaphroditism are commonly reported in the literature, particularly for the order Decapoda, describing species that do not fit in the gonochoristic pattern of reproduction (Bauer, 2000; Braga, López Greco, Santos, & Fransozo, 2009; Ferreira & Guzmán, 2013; Vazquez & López‐Greco, 2007). According to Vazquez and López‐Greco (2007), intersexu- ality is marked by the presence of male and female sexual characteristics, being primary or secondary in a single indi- vidual. Intersexuality in Decapoda has been reported in the family Parastacidae, for the burrowing freshwater crayfish Received: 4 February 2018 | Revised: 5 August 2018 | Accepted: 9 August 2018 DOI: 10.1111/azo.12272 ORIGINAL ARTICLE Morphological study for understanding the sexual pattern in ghost shrimp Callichirus major (Crustacea: Axiidea) Tugstênio L. Souza 1 | Adriane A. Braga 1 | Laura S. López Greco 2 | Giovana Bertini 3 | Erika T. Nunes 1 1 Department of Biology – Center of Exact, Natural and Health Sciences, Federal University of Espírito Santo, Alegre, Espírito Santo, Brazil 2 Departamento de Biodiversidad y Biología Experimental, Laboratorio de Biología de la Reproducción y el Crecimiento de Crustáceos Decápodos, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires,, Buenos Aires, Argentina 3 LABCRUST – Laboratory of Biology and Crustacean Cultivation, São Paulo State University – UNESP, Registro, São Paulo, Brazil Correspondence Tugstênio L. Souza, Department of Biology – Center of Exact, Natural and Health Sciences, Federal University of Espírito Santo, Alegre, Espírito Santo, Brazil. Email: tugstenio94@hotmail.com Abstract Callichirus major is a species of ghost shrimp that has burrowing habits, building underground galleries in the intertidal region of beaches. This study aimed to analyse some morphological parameters to verify its type of sexual pattern. A sample of 79 individuals, collected at the beach of Corujão, Espírito Santo, Brazil, was used. Data on the external morphology, presence of gonopores, carapace length (CL) and cheliped lengths (Lch) were obtained. In addition, the gonads were processed for histological analysis. The animals were divided into three categories: A, individuals with male characteristics; B, those presenting exclusively female characteristics; C, animals with characteristics of intersex, which presented supernumerary gonopores. Positive allometries were found for Lch versus CL only for category A, characteriz- ing the group’s marked heterochely. Histologically, it was possible to observe the presence of oocytes in category A and one case of male tissue in an intersex indi- vidual (category C). However, no genital ducts of both sexes were found in that indi- vidual. Callichirus major presents controversial sexual characters, which makes it difficult to understand the reproductive parameters and sexual pattern. However, maybe that species can develop a sexual system of hermaphroditism. KEYWORDS burrowing shrimp, Callichirus, hermaphroditism, intersexuality, sexual pattern