Molecular Phylogeny of Western Atlantic Representatives of the Genus Hexapanopeus (Decapoda: Brachyura: Panopeidae) BRENT P. THOMA 1 , CHRISTOPH D. SCHUBART 2 & DARRYL L. FELDER 1 1 University of Louisiana at Lafayette, Department of Biology and Laboratory for Crustacean Research, PO Box 42451, Lafayette, Louisiana 70504-2451, U.S.A. 2 Universittit Regensburg, Biologie 1,93040 Regensburg, Germany ABSTRACT Species of the brachyuran crab genus Hexapanopeus Rathbun, 1898, are common benthic inhabi- tants in coastal and nearshore waters of the Americas. Despite the frequency with which they are encountered, they are taxonomically problematic and commonly misidentified by non-experts. Lit- tle previous work has been undertaken to explain relationships among the 13 nominal species of Hexapanopeus or their relationship to other phenotypically similar genera of the family Panopei- dae. In the present study we examine partial sequences for 16S and 12S mitochondrial rDNA for 71 individuals representing 46 species of Panopeidae and related families of the Brachyura. Phylo- genies inferred from both of these datasets are largely congruent and show, with one exception, the included genera and species of the Panopeidae to represent a monophyletic grouping. Within this group, Hexapanopeus is polyphyletic, being distributed among several separate major clades and clearly warranting taxonomic subdivision. INTRODUCTION As part of ongoing studies of the superfamily Xanthoidea sensu Martin & Davis (2001), we have undertaken a reexamination of phylogenetic relationships among genera assigned to the family Panopeidae Ortmann, 1893, on molecular and morphological bases. Early in the course of our morphological studies, we saw reason to conclude that the genus Hexapanopeus Rathbun, 1898, as currently defined, was polyphyletic. Differences in the characters of the carapace, chelipeds, and male first pleopod (gonopod) served to obscure what, if any, relationship existed among the species in the genus. The present study serves as the first step towards restricting species composition of the genus Hexapanopeus s.s. (sensu stricto) and defining its phylogenetic relationships. Presently, the genus Hexapanopeus consists of 13 species distributed on both coasts of the Amer- icas; six species are known from the western Atlantic ranging from Massachusetts to Uruguay, while seven more range in the eastern Pacific from Mexico to Ecuador (Table 1). Representatives of Hexa- panopeus are commonly encountered in environmental studies and inhabit a variety of nearshore environments ranging from sand-shell bottoms to rubble and surface fouling accumulations, where they often reside amongst sponges and ascidians (Rathbun 1930; Felder 1973; Williams 1984; Sankarankutty & Manning 1997). Even so, available illustrations and morphological descriptions are of limited detail and quality for many species, and little can be deduced from present literature to clarify their phylogenetic relationships. Herein, we provide evidence for polyphyly in the genus Hexapanopeus on the basis of two mitochondrial genes (16S rDNA and 12S rDNA). We also examine relationships among species