Appearances can be deceptive: different diversification patterns within a group of Mediterranean earthworms (Oligochaeta, Hormogastridae)

Abstract

Many recent studies on invertebrates have shown how morphology not always captures the true diversity of taxa, with cryptic speciation often being discussed in this context. Here, we show how diversification patterns can be very different in two clades of closely related earthworms in the genus Hormogaster stressing the risk of using nonspecific substitution rate values across taxa. On the one hand, the Hormogaster elisae species complex, endemic to the central Iberian Peninsula, shows morphological stasis. On the other hand, a clade of Hormogaster from the NE Iberian Peninsula shows an enormous morphological variability, with 15 described morphospecies. The H. elisae complex, however, evolves faster genetically, and this could be explained by the harsher environmental conditions to which it is confined-as detected in this study, that is, sandier and slightly poorer soils with lower pH values than those of the other species in the family. These extreme conditions could be at the same time limiting morphological evolution and thus be responsible for the observed morphological stasis in this clade. Contrarily, Hormogaster species from the NE Iberian Peninsula, although still inhabiting harsher milieu than other earthworm groups, have had the opportunity to evolve into a greater morphological disparity. An attempt to delimit species within this group following the recently proposed general mixed Yule-coalescent method showed a higher number of entities than expected under the morphospecies concept, most probably due to the low vagility of these animals, which considerably limits gene flow between distant conspecific populations, but also because of the decoupling between morphological and genetic evolution in the H. elisae complex.

Underground evolution: New roots for the old tree of lumbricid earthworms

“…Unfortunately, as far as we know, no Lumbricidae fossils have been found. Additionally, molecular rates of substitution may vary across taxa (Novo et al, 2012). Nonetheless, the molecular time estimates appear to be supported by additional evidence from at least one taxon.…”

Cryptic speciation and limited hybridization within Lumbricus earthworms (Clitellata: Lumbricidae)

“…Cryptic lineages have also been found in the Mediterranean family Hormogastridae (e.g., Novo et al, 2012;Novo et al, 2010), in morphospecies of Megascolecidae from New Zealand (Buckley et al, 2011) and Taiwan (Chang et al, 2008), and in Glossoscolecidae from Brazil (Siqueira et al, 2013). This list of where cryptic lineages are found within Crassiclitellata should not been seen as complete, and the distribution of cryptic lineages probably reflects research intensity more than anything else.…”

Fauna Europaea: Annelida - Terrestrial Oligochaeta (Enchytraeidae and Megadrili), Aphanoneura and Polychaeta

“…The high diversity of species of Hormogaster in the southern Pyrenées, ventured by Qiu and Bouché (1998a) on the basis of not always clearcut morphological evidence, was dismissed as " H. pretiosa species complex" in the previous version of FaEu database, with the taxonomic comment: "Qiu and Bouché (1998a) recognized as many as 11 separate new species in material from Southern Pyrenées ( H. riojana , H. ireguana , H. eserana , H. huescana , H. lleidana , H. multilamella , H. arenicola , H. catalaunensis , H. sylvestris , H. najaformis , H. castillana )". However, DNA sequence data from multiple markers (Novo et al 2011, Novo et al 2012b) confirm the genetic diversification and taxonomic validity for most of those species. The discovery of H. joseantonioi Fernández-Marchán et al 2014 in Aragón, and its placement as sister taxon of H. elisae , further supports the anticipation that several representatives of this family await discovery in unexplored regions of suitable habitat.…”

“…Novo et al 2012a). Molecular studies (Cobolli Sbordoni et al 1992, Novo et al 2009, Novo et al 2010, Novo et al 2011, Novo et al 2012b, Novo et al 2014) have also pointed out the occurrence of highly divergent cryptic lineages within certain morphospecies (e.g. H. pretiosa , H. elisae ), despite the low geographical distance between populations, and a much lower genetic differentiation within and between other morphospecies separated by a long geographical distance.…”