Narrowly defined taxa on a global scale: The phylogeny and taxonomy of the genera and (Nudibranchia, Trinchesiidae) favours fine-scale taxonomic differentiation and dissolution of the "lumpers & splitters" dilemma.

By applying morphological and molecular data on two genera of the nudibranch molluscs it is shown that the tension between taxonomic practice and evolutionary processes persists. A review of the related genera and is used to demonstrate that the fine-scale taxonomic differentiation is an important tool in the integration of morphological and molecular data. This is highlighted by the hidden species problem and provides strong argument that the genus must be kept as a maximally narrowly-defined entity. Otherwise, we are forced to compare a highly disparate species under the putatively lumped name "". We demonstrate this in the present study by applying a suite of delimitation methods and describing a new species of from the Baltic Sea. The new species possesses fine-scale morphological distinguishing features, which were not investigated before. The true, narrowly defined genus represents a peculiar taxon with a clearly expressed paedomorphic characters and predominantly brackish-water habitats. The phylogenetically related genus , of which three new species are described here, clearly demonstrates different features. A lumping decision to call many morphologically and evolutionary different taxa as "" will downgrade the taxonomic and phylogenetic resolution of the entire family Trinchesiidae to just a single genus. The dissolution of the dilemma of "lumpers & splitters", which still significantly affects taxonomy, will further help to make systematics a true evolutionary discipline.