Species delineation in the Capitella species complex (Annelida: Capitellidae): geographic and genetic variation in the northern Gulf of Mexico

Capitella capitata was traditionally used as a biological indicator species due to its ubiquitous distribution and high densities in disturbed and polluted marine and estuarine sediments. Based on allozyme and developmental studies, it is now clear that C. capitata is a species complex consisting of multiple distinct lineages worldwide, including the recently described C. teleta, a model species for spiralian development. The coast of the northern Gulf of Mexico, with its numerous bays and estuaries and frequently occurring natural and anthropogenic disturbances, provides an appropriate region for such studies. We sequenced a fragment of the mitochondrial cytochrome c oxidase subunit I gene for individuals of C. cf. capitata and C. cf. aciculata (distinguished by acicular spines on the first two chaetigers) collected from Texas and Florida coasts and analyzed them in conjunction with data available in GenBank. Our results indicate the presence of a Gulf of Mexico clade that is distinct from populations in Canada and the Indo‐Pacific. Populations in the northern Gulf of Mexico are structured geographically, with support for Texas and Florida clades, and there do not seem to be clear boundaries between C. cf. capitata and C. cf. aciculata. This is corroborated by the fact that multiple specimens were morphologically intermediate between the two species. In future studies, we aim to clarify whether the intermediate morphologies represent ontogenetic stages, neutral morphological variation, phenotypic plasticity, or sexual dimorphism in a single species or whether several lineages with incomplete reproductive barriers are present.