Recent studies revealed a high diversity of reproductive structures in heterotreme brachyurans, while those of Thoracotremata seem rather uniform. Yet, there still is a huge lack of data in this group as only few species have been studied with respect to their reproductive system. The phylogenetic position of Percnidae is ambiguous. Recent molecular studies place it within polyphyletic grapsoids. We herein study the reproductive morphology of Percnon gibbesi using histology, scanning electron microscopy, micro-computed tomography and 3D-reconstructions to test whether this species shows the characteristic thoracotreme pattern. Our results reveal that the male copulatory system conforms to other thoracotremes. It is composed of two pairs of pleopods (gonopods) and likewise paired penes. The first gonopod is relatively long. It possesses a bent terminal process with a distal opening of the ejaculatory canal, a character also present in other grapsoids. The second gonopod is short and terminates in an apical girdle. The female reproductive system reveals a combination of characters, so far unknown for thoracotremes. The paired oviducts do not lead into the seminal receptacles, but run into separate cuticular ducts joined with the vaginae. Accessory sperm storage organs, the bursae, are also connected to the vaginae. Bursae have previously only been described in heterotreme crabs. The data presented in this study reveals a higher diversity of thoracotreme reproductive systems than anticipated.

The eubrachyuran Hymenosomatoidea is widely distributed in tropical and subtropical regions ranging from marine to freshwater habitats. Even though the biology of this taxon has been studied to some extent, its phylogenetic relationships are not resolved. Based on different morphological characters, some authors suggested a close affinity of hymenosomatid crabs to heterotremes. However, many of these characters are ambiguous, and the few molecular studies did not provide convincing solutions either. To address this issue, we studied the reproductive system of the hymenosomatid freshwater species Limnopilos naiyanetri Chuang and Ng, 1991 using histology and scanning electron microscopy. The females show the characteristic organization of the paired eubrachyuran reproductive system. Additionally, a bursa (an accessory sperm storing cuticle cavity) is present. The male copulatory system is characterized by paired long first and short second gonopods, and a pair of sternal gonopores equipped with a penis. Both, the female and male reproductive organs reveal a number of similarities to thoracotreme crabs. The seminal receptacle is lined by a very thin cuticle and by a mono-layered glandular epithelium. The male gonopods and the sternal genital opening also resemble the thoracotreme condition. Thus, our results indicate that Hymenosomatidae are most likely part of the Thoracotremata.

The reproductive system of spider crabs (Majoidea) has raised considerable interest due to the complexity of female sperm storage organs. In several majoid species, the seminal receptacle has been described as being divided into a dorsal storage chamber and a ventral fertilization chamber separated by a muscular velum. The velum is supposed to control the amount of sperm used for fertilization and to play an important role in sperm competition. Here, we present a study on the reproductive systems of the two majoid species, Mithraculus sculptus (Lamarck, 1818) and Stenorhynchus seticornis (Herbst, 1788) using various morphological techniques such as μCT scans and 3D-reconstructions, complemented by paraffin histology. The male gonopods of the herein investigated species are similar in their general morphology and in the presence and distribution of setae. The tubular first gonopod holding the ejaculatory canal is much longer than the short and stout second gonopod, which is supposed to function as a piston in the transport of sperm into the female ducts. The female reproductive system of M. sculptus and S. seticornis conforms to that of other Eubrachyura in possessing paired ovaries, oviducts, seminal receptacles, vaginae, and vulvae. Based on our 3D-reconstructions we demonstrate that there is no division of the seminal receptacle into two chambers separated by a velum. In contrast to this, we observed a spatially restricted invagination of the seminal receptacle. A comparison of our data with those of previous studies, allows for the conclusion that the invagination of the seminal receptacle may have been misinterpreted and mistaken for a velum by other authors. Thus, the division of the seminal receptacle into two chambers separated by a velum is a character which needs to be re-evaluated.

No abstract