One or many? Molecular versus morphological diversity in the aplacophoran Chaetoderma nitidulum Loven, 1844 (Mollusca: Caudofoveata)

Chaetoderma nitidulum Loven, 1844 is a common species of caudofoveate (chaetodermomorph) with a wide distribution range in the northeastern Atlantic Ocean. It was the first species of aplacophoran mollusc to be described, but its species identity has been debated. Here, we investigate the molecular and morphological diversity of specimens from a large geographical area and size range, which had been preliminarily identified, based on morphology, as C. nitidulum. Analyses of molecular data revealed two distinct genetic sister lineages in the Eastern Atlantic and one clade sister to these in the Western Atlantic. Morphological analyses showed significant variation that does not reflect the genetic linages. In addition, molecular phylo-genetics combined with comparative morphological analyses showed that radula characters used to distinguish the genera Chaetoderma and Falcidens within Chaetodermatidae do not represent apomorphies, but are a result of ontogenetic changes in C. nitidulum. The emerging phylogenetic patterns were surprising, bringing us one step closer to understanding chaetodermatid ontogeny and evolution, but they also emphasize that the systematic relationships of chaetodermatid caudofoveates are still far from understood.