Meiofaunal cryptic species challenge species delimitation: the case of the Monocelis lineata (Platyhelminthes: Proseriata) species complex

Given the pending biodiversity crisis, species delimitation is a
critically important task in conservation biology, but its efficacy
based on single lines of evidence has been questioned as it may
not accurately reflect species limits and relationships. Hence,
the use of multiple lines of evidence has been portrayed as a
means to overcome identification issues arising from gene/
species tree discordance, morphological convergence or recent
adaptive radiations. Here, the integrative taxonomic approach
has been used to address the study of the Monocelis lineata
species complex. The taxonomic resolution of the complex is
challenging, as the species lacks sclerotised copulatory structures,
which as a rule of thumb aid identification in Proseriata.
Eighteen populations, which encompass most of the geographic
range of the complex, were studied using morphology, karyology,
crossbreeding experiments and molecular analysis. These
different markers provided evidence of four (karyology) to
eight (morphology) discrete entities, whereas crossings showed
various degrees of intersterility among the tested populations.
Molecular species delimitation revealed a different number of
candidate species, spanning from five (ABGD and K/θ) to 11
(GMYC). Such incongruences reflect the multifaceted evolutionary
history of M. lineata s.l. and hamper the full taxonomic
resolution of the complex. However, two candidate species were
consistently validated by all of the markers and are described as
new species: Monocelis algicola nov. sp. and M. exquisita nov.
sp. The latter species appear to have a restricted distribution,
and the possibility that meiofaunal taxa may be of conservation
concern is discussed.