Integrative taxonomy reveals a cryptic species of the nudibranch genusPolycera(Polyceridae) in European waters

Abstract

This work aimed to test whether the colour variability featured by the European nudibranch Polycera quadrilineata is consistent with the concept of a single polychromatic species or may hide multiple lineages. Samples from across the geographic range of P. quadrilineata together with representatives from worldwide species with a focus on Atlantic diversity, were gathered and studied using an integrative taxonomic approach. Morpho-anatomical characters were investigated by light and scanning electron microscopy. Bayesian molecular phylogenetics using MrBayes, the Automatic Barcode Gap Discovery species delimitation method, and haplotype network analysis using the PopArt software were employed to help delimit species using the mitochondrial gene cytochrome c oxidase subunit I (COI). The results supported the existence of a second species, here described and named Polycera norvegica sp. nov., only known from Norway where it is sympatric with P. quadrilineata. The COI uncorrected p-genetic distance between the two species was estimated at 9.6–12.4%. Polycera norvegica sp. nov. differs by exhibiting a black dotted or patchy dotted pattern occasionally with more or less defined orange/brown patches, but never black continuous or dashed stripes as in P. quadrilineata. The two species share a common colouration with a whitish base and yellow/orange tubercles. Anatomically, P. norvegica sp. nov. has a weaker labial cuticle, a smaller radula with fewer rows, and only four marginal teeth, a reproductive system with a single lobed bursa copulatrix, shorter reproductive ducts, and a penis armed with two kinds of spines: needle-like and hook-shaped penile spines.

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

“…Remarkably, in this study we also recovered two closely related species P. quadrilineata and P. capitata as sister species (Fig. 3 ), in contrast to a previous study 14 , because taxon selection was previously not exhaustive. True P. faeroensis is a more rarely encountered species than P. quadrilineata and P. capitata (at least in the relatively shallow water environments) and verified molecular data which were aligned with the fine-scale morphological data are presented in this study for the first time (Figs.…”

“…An undescribed species (“ Polycera sp. A”) which was included to a molecular phylogenetic study without any morphological data 14 is basal to the clade of the European Polycera (Fig. 3 ) but occurs exclusively in South Africa and is out of scope in the present study.…”

“…Using molecular data, Driessen et al 21 showed two lineages among European “ P. quadrilineata ” for the first time. Recently one of these lineages has been formally described as Polycera norvegica 14 . The description proposed the species to occur exclusively in Norway.…”

Molecular and morphological analyses reveal pseudocryptic diversity inMicromelo undatus(Bruguière, 1792) (Gastropoda: Heterobranchia: Aplustridae)

“…The advent of molecular systematics has led to the discovery of previously unrecognized pseudocryptic diversity (Carmona et al, 2014;Golestani et al, 2019;Lindsay & Vald es, 2016;Pola et al, 2012;Sørensen et al, 2020). Early identifications of Micromelo have been based solely upon morphological observations.…”

“…Cryptic species refers to two or more genetically distinct lineages that are morphologically indistinguishable (Saez et al, 2003). Pseudocrypsis refers to taxa previously thought to be one species but that in fact show molecular differences and when looked upon closely also subtle morpho-anatomical differences (Knowlton et al, 1993;Korshunova et al, 2019;Sørensen et al, 2020). The colour differences between specimens of M. undatus may indicate the presence of multiple cryptic or pseudocryptic species, but without genetic analysis, it is unclear whether this variability is interspecific or intraspecific.…”