Secondary sexual characteristics as tool for sexing individuals of Synalpheus Spence Bate, 1888, a Caridean genus with males lacking appendices masculinae

Sexual dimorphism includes external morphological differences between sexes. In caridean shrimps, the established characteristics for sexing individuals are the presence or absence of an appendix masculina in males and the position of the male/female gonopores. Shrimps of the genus Synalpheus are an exception because males lack appendices masculinae on the second pleopods. Synalpheus males can be distinguished from females by the position of the gonopores, which are located on the coxae of the fifth and the third pereopod, respectively. However, in very small species such as Synalpheus, the gonopore position is difficult to observe under light stereomicroscopy and is only clearly visible with scanning electron microscopy (SEM), a lengthy, expensive process requiring specialized instrumentation. The present study describes practical tools for analyzing sexual dimorphism in Synalpheus, using S. apioceros, S. fritzmuelleri, and S. brevicarpus as models. We used a light stereomicroscope to describe all pleopods, appendices internae, and pleura; SEM for morphological description of gonopores and endopodal cincinnuli; and characteristics of the pleopods and appendices internae. Morphological differences were found in several characters. The first pleonal pleuron of the male has a posteroventral, somewhat hook-like projection. It is rounded in females and ovigerous females. We proposed and tested morphological measures to describe the relative position of the appendix interna (AI) on the endopods in females and males. The sex indicated by the relative AI position was confirmed by SEM observation of the gonopore position, a definitive indicator of sex.