How many sources of evidence are needed to delimit a taxon? Disentangling the identity of two Coccoloba species (Polygonaceae)

Species are fundamental biological units and can be treated as testable scientific hypotheses. Historically, species in the genus Coccoloba have been published based only on classical morphology. One of these species, Coccoloba laevis, contains many specimens with strong morphological differences. We here test the hypothesis that these morphological variations are consistent with the circumscription of C. laevis as a single taxonomic entity. In addition to classical morphology, we used linear morphometry, vascular anatomy, and potential geographic distribution to delineate a distinctive morphotype of C. laevis from the remainder of C. laevis s.s. Statistical analysis showed significant differences for all ten morphometric variables. Petiole and midrib anatomy also pointed to differences in both shape and arrangement of vascular bundles between the distinctive morphotype and the remainder of C. laevis s.s. The potential distribution model yielded a wide distribution for C. laevis s.s. in the coastal zone of northeastern Brazil. The distinctive morphotype, however, had a contiguous and restricted distribution within this range. Based on our results, we here reestablish Coccoloba candolleana at the species level, which has been previously treated as a synonym of C. laevis. Both species differ morphologically mainly by habit, and the shape and size of ochrea, leaf, petiole, inflorescence, pedicel, and fruit. Because of its restricted distribution in a coastal ecosystem associated with strong anthropogenic disturbance, we consider C. candolleana as endangered.