Micaria pulicaria, % WSC, 2019

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 : 206-207

4.3 | The evolution of morphological crypsis in Micaria pulicaria

The spider fauna of central Europe is well known. In such faunas, the detection of hidden diversity is a rare phenomenon. Moreover, M. pulicaria is a common species. According to the Atlas of the European Arachnids %https://atlas.arages.de/), the species is placed 59 out of 1,050 in descending order of grid frequencies of spider species in Germany. Thus, it is rather surprising that two widespread species have been mistaken for such a long time. If one accepts the definition of cryptic species as two or more distinct species that were earlier classified as one % Bickford et al., 2007), then M. micans and pulicaria are cryptic species. Processes underlying the evolution of morphological crypsis have gained vast attention in recent years % Struck et al., 2018; Wagner et al., 2018). Which factors could have driven crypsis in Micaria spiders?

The long-lasting taxonomic confusion of the two Micaria species certainly relates to the high levels of intraspecific variation as compared to low interspecific dissimilarity. On the one hand, the organization of both, the male palp and the female epigyne features, is simple as compared to other spiders, and few diagnostically useful characters exist. On the other hand, we found exaggerated variation within species— particularly in vulva structures—that is almost unrivalled in spiders %but see Crews, 2009). Spiders generally show no to very little intraspecific variation in genitalic characters % Kraus, 2002). However, spider genitalia are usually distinct even among closely related species and thus reliable indicators of species limits %Huber, Rheims, & Brescovit, 2005). The reasons for higher intraspecific variation in spider genitalia in some taxonomic groups than in others are still not clear % Eberhard & Huber, 2010). We hypothesize that the high levels of standing variation within species as well as overall morphological stasis among the two Micaria species could be triggered by their myrmecomorphy. As frequency and distribution of model species changes, the mimics have to adapt, thus intraspecific polymorphism is an important characteristic of Batesian mimicry % Joron & Mallet, 1998). On the other hand, morphological conversion in the speciation process is constrained by selection towards maintenance of mimetic similarity with the model species. Hence, myrmecophily is listed as one source for the evolution of cryptic species in Bickford et al. %2007).

Presumably, the exceptional high levels of variation within the two Micaria species also promoted character displacement in male genitalic traits. Reproductive character displacement is the selective process by which reproductive traits diverge in order to minimize the risks of hybridization; it results in a geographic pattern in which species are more dissimilar where they occur together than in allopatry % Pfennig & Pfennig, 2009). Exactly such a pattern has been observed in M. micans / pulicaria %Figure 5a): Palearctic M. pulicaria %OTU-2) and M. micans %OTU-1), which occur sympatrically in vast areas of Eurasia, are separated in morphospace, while the Nearctic OTU-3 takes an intermediate position. Although our morphometric analysis includes nongenitalic traits, the characters of the male palp contribute by far the highest loadings to shape PC1. Moreover, species may also diverge in traits that are not directly involved in reproduction owing to correlated evolution with those traits actually targeted by character displacement % Pfennig & Pfennig, 2005). At the moment, we can only speculate whether character displacement in M. micans / pulicaria results from postspeciation divergence, for example reinforcement of specific differences that evolved in allopatry, for example through range contraction during Pleistocene glaciations, or if character displacement itself initiated speciation. During reproductive character displacement, female preferences on male traits may become so divergent that females in sympatry fail to recognize allopatric males as acceptable mates %or vice versa), ultimately resulting in reproductive isolation %Hoskin, Higgie, McDonald, & Moritz, 2005). Effective character displacement would also explain the absence of signals for introgression between M. pulicaria and micans . Introgression through occasional hybridization needs certainly to be considered in morphologically and ecologically similar congeners that occur in sympatry. Recent genomic studies have shown that it may be more prevalent in spiders than previously assumed %Ivanov, Lee, & Mutanen, 2018; Leduc-Robert & Maddison, 2018). However, we observed no single case of disagreement between morphology and mtDNA %i.e. no signs of mitonuclear discordance) in the studied Micaria species.