Fine-scale species delimitation: speciation in process and periodic patterns in nudibranch diversity

Using the nudibranch genus <i>Amphorina</i> as a model, ongoing speciation is demonstrated, as well as how periodic-like patterns in colouration can be included in an integrated method of fine-scale species delimitation. By combining several methods, including BPP analysis and the study of molecular, morphological, and ecological data from a large number of specimens within a broad geographic range from northern Europe to the Mediterranean, five species are recognised within the genus <i>Amphorina</i>, reviewed here for the first time. Two new species from the southwestern coast of Sweden are described, <i>A. viriola</i> <b>sp. nov.</b> and <i>A. andra</i> <b>sp. nov.</b> Evidence is provided of a recent speciation process between the two closely related, yet separate, species which inhabit the same geographic localities but demonstrate strict water depth differentiation, with one species inhabiting the shallow brackish top layer above the halocline and the other species inhabiting the underlying saltier water. The results presented here are of relevance for currently debated issues such as conservation in relation to speciation, fine species delimitation, and integration of molecular, morphological and ecological information in biodiversity studies. The periodic approach to biological taxonomy has considerable practical potential for various organismal groups.