Abstract
BackgroundMolecular methods of species delineation are rapidly developing and widely considered as fast and efficient means to discover species and face the 'taxonomic impediment’ in times of biodiversity crisis. So far, however, this form of DNA taxonomy frequently remains incomplete, lacking the final step of formal species description, thus enhancing rather than reducing impediments in taxonomy. DNA sequence information contributes valuable diagnostic characters and –at least for cryptic species – could even serve as the backbone of a taxonomic description. To this end solutions for a number of practical problems must be found, including a way in which molecular data can be presented to fulfill the formal requirements every description must meet. Multi-gene barcoding and a combined molecular species delineation approach recently revealed a radiation of at least 12 more or less cryptic species in the marine meiofaunal slug genus Pontohedyle (Acochlidia, Heterobranchia). All identified candidate species are well delimited by a consensus across different methods based on mitochondrial and nuclear markers.ResultsThe detailed microanatomical redescription of Pontohedyle verrucosa provided in the present paper does not reveal reliable characters for diagnosing even the two major clades identified within the genus on molecular data. We thus characterize three previously valid Pontohedyle species based on four genetic markers (mitochondrial cytochrome c oxidase subunit I, 16S rRNA, nuclear 28S and 18S rRNA) and formally describe nine cryptic new species (P. kepii sp. nov., P. joni sp. nov., P. neridae sp. nov., P. liliae sp. nov., P. wiggi sp. nov., P. wenzli sp. nov., P. peteryalli sp. nov., P. martynovi sp. nov., P. yurihookeri sp. nov.) applying molecular taxonomy, based on diagnostic nucleotides in DNA sequences of the four markers. Due to the minute size of the animals, entire specimens were used for extraction, consequently the holotype is a voucher of extracted DNA ('DNA-type’). We used the Character Attribute Organization System (CAOS) to determine diagnostic nucleotides, explore the dependence on input data and data processing, and aim for maximum traceability in our diagnoses for future research. Challenges, pitfalls and necessary considerations for applied DNA taxonomy are critically evaluated.ConclusionsTo describe cryptic species traditional lines of evidence in taxonomy need to be modified. DNA sequence information, for example, could even serve as the backbone of a taxonomic description. The present contribution demonstrates that few adaptations are needed to integrate into traditional taxonomy novel diagnoses based on molecular data. The taxonomic community is encouraged to join the discussion and develop a quality standard for molecular taxonomy, ideally in the form of an automated final step in molecular species delineation procedures.
Highlights
Molecular methods of species delineation are rapidly developing and widely considered as fast and efficient means to discover species and face the ‘taxonomic impediment’ in times of biodiversity crisis
Evaluation of putative morphological characters The diversity within Pontohedyle revealed by molecular data cannot be distinguished externally: the body shows the typical subdivision into the anterior head-foot complex and the posterior visceral hump
Monaxone rodlet-like spicules distributed all over the body and frequently found in an accumulation between the oral tentacles are characteristic for Pontohedyle. These spicules can be confirmed for P. wenzli sp. nov., for P. yurihookeri sp. nov., P. milaschewitchii (Kowalevsky, 1901) and P. brasilensis (Rankin, 1979), and, in contrast to the original description [53], in P. verrucosa
Summary
Molecular methods of species delineation are rapidly developing and widely considered as fast and efficient means to discover species and face the ‘taxonomic impediment’ in times of biodiversity crisis. This form of DNA taxonomy frequently remains incomplete, lacking the final step of formal species description, enhancing rather than reducing impediments in taxonomy. DNA sequence information contributes valuable diagnostic characters and –at least for cryptic species – could even serve as the backbone of a taxonomic description To this end solutions for a number of practical problems must be found, including a way in which molecular data can be presented to fulfill the formal requirements every description must meet. In cases of pseudo-cryptic species (among which morphological differences can be detected upon re-examining lineages separated e.g. on molecular data) or of fully cryptic species (that morphology fails to delimit), the traditional lines of evidence have to be modified by using, e.g., molecular information to break out of the ‘taxonomic circle’ [6,7]
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