Morphological changes during ontogeny of the male first and second gonopods of Mithraculus forceps A. Milne-Edwards, 1875 (Brachyura: Majoidea: Mithracidae)

In brachyuran crabs, the complexity of the copulatory system provides important information about the systematics of the group. Here we used scanning electron microscopy and light microscopy to provide information about the morphological changes during the gonopods ontogeny in the spider crab Mithraculus forceps. For this, a series of specimens were analysed and grouped in size classes of cephalothorax width (CW). The smaller males (first and second size classes) presented the first gonopod without a formed ejaculatory aperture and with an indistinct apical plate. The ejaculatory canal aperture is fully formed in size class 5 with 8.8 to 9.9 mm (CW). In adult males, the apical plate is divided in two lobes: a rounded mesial lobe and a distinct tapering lateral lobe with a large ejaculatory canal aperture. Throughout growth, there was an increment in number and/or size of the secondary cuticle structures, such as setae, spines, and sensilla. Morphological changes in the second gonopod are not abrupt during the ontogenetic development, with the girdle in the apical region and the median process becoming more prominent and tapered. Although to clearly determine the sexual maturity, information on the gonadal development (physiological sexual maturation) is needed, we observed that the gonopods morphology could provide important information to determine the sexual maturation. In our analyses, the morphological maturity occurred in line with the physiological maturity, both diagnosed in the size class 5.