Morphological and molecular investigation of <italic>Scolelepis agilis</italic> (Verrill, 1873) (Annelida: Spionidae) from beaches of the Atlantic coast of North America.

Polychaete worms of the genus <italic>Scolelepis</italic> are one of the most abundant members of beach ecosystems worldwide. Along the Atlantic coast of North America, specimens of <italic>Scolelepis</italic> inhabiting high-energy beaches have long been erroneously identified as the European species <italic>Scolelepis squamata</italic> (O.F. Müller, 1806). In this study, we examined specimens of <italic>Scolelepis</italic> collected from Massachusetts to Virginia, finding they match <italic>Scolelepis agilis</italic> (Verrill, 1873), originally described from beaches in New Jersey. <italic>Scolelepis agilis</italic> can be most easily distinguished from <italic>S. squamata</italic> based on differences in palp ciliary pattern, morphology of neuropodial lamellae, and the quantity and start of both notopodial and neuropodial hooded hooks. Live specimens of <italic>S. agilis</italic> collected from New York were also examined to provide data on their reproduction and morphometric analyses. In addition, molecular data (18S, 28S rRNA, cytochrome c oxidase I and 16S rRNA) showed that specimens of <italic>S. agilis</italic> from the north to mid-Atlantic coast formed a monophyletic group (<italic>p</italic>-distance ≤ 0.004 for MA, RI, NY and VA populations) and were distinct from <italic>S. squamata</italic> (<italic>p</italic>-distance ≥ 0.196 (0.03 SE)). The molecular data also suggest the presence of several misidentified or undescribed species of <italic>Scolelepis</italic>, highlighting the need for additional taxonomic work on the genus from these localities as well as others (e.g. South Africa) where the worms have been incorrectly identified as <italic>S. squamata</italic>. A key to species of <italic>Scolelepis</italic> from the east coast of the United States is provided and a lectotype and paralectotypes for <italic>S. agilis</italic> are herein designated. [ABSTRACT FROM AUTHOR]