Hidden diversity in Antarctica: Molecular and morphological evidence of two different species within one of the most conspicuous ascidian species

Abstract

The Southern Ocean is one of the most isolated marine ecosystems, characterized by high levels of endemism, diversity, and biomass. Ascidians are among the dominant groups in Antarctic benthic assemblages; thus, recording the evolutionary patterns of this group is crucial to improve our current understanding of the assembly of this polar ocean. We studied the genetic variation within Cnemidocarpa verrucosa sensu lato, one of the most widely distributed abundant and studied ascidian species in Antarctica. Using a mitochondrial and a nuclear gene (COI and 18S), the phylogeography of fifteen populations distributed along the West Antarctic Peninsula and Burdwood Bank/MPA Namuncurá (South American shelf) was characterized, where the distribution of the genetic distance suggested the existence of, at least, two species within nominal C. verrucosa . When reevaluating morphological traits to distinguish between genetically defined species, the presence of a basal disk in one of the genotypes could be a diagnostic morphological trait to differentiate the species. These results are surprising due to the large research that has been carried out with the conspicuous C. verrucosa with no differentiation between species. Furthermore, it provides important tools to distinguish species in the field and laboratory. But also, these results give new insights into patterns of differentiation between closely related species that are distributed in sympatry, where the permeability of species boundaries still needs to be well understood.

Detecting the invisible through DNA metabarcoding: The role of gelatinous taxa in the diet of two demersal Antarctic key stone fish species (Notothenioidei)

“…Even though the mitochondrial COI gene is the most commonly used marker in metazoan (meta)barcoding studies, providing high resolution taxon discrimination, COI was often less successfully amplified than other markers (e. g. Pappalardo et al., 2021; van der Reis et al., 2018). This is related to the fact that the high degeneracy of the DNA regions in the protein‐coding COI gene limit universal amplification in eukaryotes (Deagle et al., 2014); and this is particularly true for tunicates, where the high mutation rate in the primer binding sites can affect the amplification success (Goodall‐Copestake, 2014, 2017; Ruiz et al., 2020). On the other hand, the nuclear 18S marker is frequently used to target a broad spectrum of eukaryotic phyla because it has highly conserved regions (so‐called stem regions of the ribosomal RNA gene), yet it suffers often from a lower discriminatory power compared to mitochondrial markers (Clarke et al., 2017; Wangensteen et al., 2018).…”

The multilevel organismal diversity approach deciphers difficult to distinguish nudibranch species complex

“…However, despite that, there are continuous recent attempts to putatively distinguish the terms “cryptic” and “non-cryptic” species, commonly with the presentation of some morphological distinguishing characters that, by definition, mean that the newly separated species cannot be considered as “cryptic” 13 – 15 . Hidden diversity therefore is an important problem, especially in the context of the polymorphism 16 , 17 , which can appear as a parallel-like pattern of characters in two or more closely related species 18 , 19 . Because within already fine-scale delineated species, it is still possible to uncover more diversity 12 , the commonest European nudibranchs Polycera are investigated in this study in order to further test the reliability of the currently widely discussed “cryptic species” concept .…”