Micaria pulicaria

Abstract, Christoph Muster Peter Michalik Correspondence, 2020, Cryptic diversity in ant-mimic Micaria spiders (Araneae, Gnaphosidae) and a tribute to early naturalists, Zoologica Scripta 49 (2), pp. 197-209 : 203-205

4.1 | How many species in Micaria pulicaria ?

Taxonomists faced problems in species delimitation of the M. pulicaria species complex for almost 200 years. Using an in-depth integrative taxonomy approach, we propose the species status for two out of eight names that are currently in synonymy of M. pulicaria % WSC, 2019 ). We applied data from morphology, mitochondrial DNA and ecology %Table 1), and species were accepted only if they differed in all three disciplines. This is based on rationales provided by Schlick-Steiner et al. %2010), who demonstrated that three disciplines are required to lower the average error rate in species delimitation below 5%. This may be considered a conservative approach. Nowadays, many species are described primarily based on molecular delineation % Atherton & Jondelius, 2018; Zhang et al., 2018), and Barcode Index Numbers %BINs) are frequently used as a species proxy % Hebert et al., 2016). In our study, reliance exclusively on molecular delineation methods would result in splitting M. pulicaria into seven species, of which only two were morphologically distinguishable. We do not consider morphological differentiation essential for species distinction, as morphological crypsis among species is not uncommon % Bickford et al., 2007). However, we assume that morphological variation reflects genome-wide divergence better than single gene trees. This effect is shown in the lower failure rate of morphology as compared to mitochondrial DNA and nuclear DNA in a literature survey on species delimitation % Schlick-Steiner et al., 2010). More importantly, molecular delimitation methods detect lineages, which must not necessarily correspond to species. It is well known that popular delineation methods that were designed for single-locus molecular data, such as GMYC, PTP and BIN, tend to overestimate species diversity %Carstens, Pelletier, Reid, & Satler, 2013; Hawlitschek, Scherz, Ruthensteiner, Crottini, & Glaw, 2018; Luo, Ling, Ho, & Zhu, 2018; Miralles & Vences, 2013). In a recent simulation study, Sukumaran and Knowles %2017) have convincingly shown that even the multispecies coalescent model, which is increasingly used with genomic data, consistently overestimated the number of true species due to misidentification of population structure for species entities. Since overinflation of species numbers may have serious consequences, for example in global biodiversity estimates or for conservation strategies %Larsen, Miller, Rhodes, & Wiens, 2017; Robuchon et al., 2019) the integration of genetic and nongenetic sources of data %morphological, ecological and ethological information) is generally recommended. In our Micaria example, OTU-2 populates vast areas of the Holarctic region with its complex Pleistocene climatic history. Therefore, lineage sorting is not expected to be complete and we predict a profound genetic structure that should not be mistaken for putative species unless supported by other lines of evidence.

While we are confident that the relatively deep divergence within Palearctic M. pulicaria %OTU-2) is attributable to phylogeographic structure instead of speciation, the situation is more challenging with respect to species status of OTU-3. This clade is exclusively Nearctic and overlaps with OTU-2 only in a small area near the Pacific coast. The oldest available name for North-American specimens is Micaria montana Emerton, 1890 with the type locality Mt. Washington situated in New Hampshire. Since only OTU-3 occurs in this region, this would be the valid name for the widespread North-American clade. The morphometric analysis %Figure 5a, b) confirms that the holotype of M. montana as well as type material of M. gentilis Banks, 1896 and M. perfecta Banks, 1896 correspond to OTU-3. The crucial question is whether OTU-3 should be considered conspecific with OTU-2 or not. Our data suggest diagnostic substitutions for each clade in the COI gene. However, we do not know whether these characters would remain specific if the sampling was more comprehensive than in our study % Bergsten et al., 2012). On the other hand, we did not detect significant differences in morphological characters nor in ecology between OTU-2 and OTU-3. For the sake of taxonomic stability, we therefore advocate to keep M. montana %and also M. gentilis and M. perfecta ) in the synonymy of M. pulicaria , as proposed by Hackman %1954) and Platnick and Shadab %1988) until contradicting evidence is provided. Phylogenomic data may once decide upon this matter.