Paradorippe granulata - A crab with external fertilization and a novel type of sperm storage organ challenges prevalent ideas on the evolution of reproduction in Eubrachyura (Crustacea: Brachyura: Dorippidae)

Two fundamentally different sperm storage organs occur in Brachyura. The probably paraphyletic podotremes show intersegmental spermathecae, which are distant from oviducts and coxal gonopores. Hence, fertilization is external. In contrast to this, the seminal receptacles of Eubrachyura are directly connected with the ovaries. Thus, at least initial fertilization is internal. This pattern has been interpreted as an apomorphy of Eubrachyura. To test this hypothesis, we studied the morphology of the reproductive organs of Paradorippe granulata, a representative of the putatively early diverging eubrachyuran lineage Dorippoidea. Applying histology, 3D-reconstructions and micro-computed-tomography we revealed a novel type of sperm storage organ. Female P.granulata lack the characteristic eubrachyuran seminal receptacle. Instead sperm is stored in four cuticle-lined bursae, two on each side of the paired oviducts. The elaborate bulbous male gonopod with several terminal processes is adapted to transferring sperm into the female twin bursae. Since oviducts and twin bursae are not directly connected, spermatozoa and oocytes mix when gametes pass through the sternal vulva. Thus, fertilization in P.granulata is external. Our finding of a eubrachyuran crab that lacks seminal receptacles and exhibits external fertilization calls prevailing concepts on the evolution of sperm storage in Eubrachyura into question.

Phylogenetics of the brachyuran crabs (Crustacea: Decapoda): The status of Podotremata based on small subunit nuclear ribosomal RNA

The true crabs, the Brachyura, are generally divided into two major groups: Eubrachyura or 'advanced' crabs, and Podotremata or 'primitive' crabs. The status of Podotremata is one of the most controversial issues in brachyuran systematics. The podotreme crabs, best recognised by the possession of gonopores on the coxae of the pereopods, have variously been regarded as mono-, para- or polyphyletic, or even as non-brachyuran. For the first time, the phylogenetic positions of the podotreme crabs were studied by cladistic analysis of small subunit nuclear ribosomal RNA sequences. Eight of 10 podotreme families were represented along with representatives of 17 eubrachyuran families. Under both maximum parsimony and Bayesian Inference, Podotremata was found to be significantly paraphyletic, comprising three major clades: Dromiacea, Raninoida, and Cyclodorippoida. The most 'basal' is Dromiacea, followed by Raninoida and Cylodorippoida. Notably, Cyclodorippoida was identified as the sister group of the Eubrachyura. Previous hypotheses that the dromiid crab, Hypoconcha, is an anomuran were unsupported, though Dromiidae as presently composed could be paraphyletic. Topologies constrained for podotreme monophyly were found to be significantly worse (P < 0.04) than unconstrained topologies under Templeton and S-H tests. The clear pattern of podotreme paraphyly and robustness of topologies recovered indicates that Podotremata as a formal concept is untenable. Relationships among the eubrachyurans were generally equivocal, though results indicate the majoids or dorippoids were the least derived of the Eubrachyura. A new high level classification of the Brachyura is proposed.