On the Heterotreme - Thoracotremedistinction in the Eubrachyurade Saint Laurent , 1980 ( Decapoda , Brachyura )

Eubrachyurans are currently divided into two subsections based on the anatomical position of the seminal duct termini. Heterotremes have the seminal ducts passing through the pereiopod 5 coxae, emerging externally in the form of “penes” to contact the basal segments of the gonopods. Thoracotrematous crabs, on the other hand, have the “male openings” located on sternite 8 and the ejaculatory ducts never pass through the pereiopod 5 coxae. Some brachyurologistsmaintain in the literature that the heterotreme-thoracotreme distinction re ects the extremes of two evolutionary grades. According to this speculation, “advanced” thoracotrematous crabs probably arose from intermediate goneplacid forms which, in turn, originated out of the Heterotremata in response to selection pressures for improved locomotion. The cladistic hypothesis of De Saint Laurent (1980), in contrast, positions Heterotremata and Thoracotremata as sister groups. Indeed, the heterotremethoracotreme difference considered alone does support two discrete types as no truly intermediate seminal duct condition bridges the subsections. Nor is there any evidence to suggest that the heterotrematous state is the anatomical precursor of sternal openings for the seminal ducts. The fact that basal members of the Pinnotheroidea De Haan are thoracotremes whereas more derived forms have an heterotrematousmorphology, refutes De Saint Laurent’s hypothesis of a deep phylogenetic cleavage between the two subsections. In addition to this information, the 850C species of freshwater crabs (many of which are semi-terrestrial)share an array of somatic apomorphieswith thoracotremes and yet belong to the Heterotremata.And the cladistic hypothesis of De Saint Laurent (1980) is also falsiŽ ed by previous parsimony analyses which place a monophyletic taxon Thoracotremata within a paraphyletic group Heterotremata. It is therefore possible that Heterotremata is synonomous with Eubrachyura, with Thoracotremata remaining as a distinct eubrachyuran subclade. One trenchant conclusion is that the functional explanation for the heterotreme-thoracotreme distinction lacks empirical support.