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

Cessa Rauch,Manuel António E Malaquias,Marta Pola,Cecilie Gotaas Sørensen

doi:10.1017/s0025315420000612

Abstract

AbstractThis 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.

Highlights

The concept of ‘cryptic species’ has been debated and its definition remains a topic of controversy (Struck et al, 2018; Korshunova et al, 2019)
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
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: needlelike and hook-shaped penile spines

Summary

Introduction

The concept of ‘cryptic species’ has been debated and its definition remains a topic of controversy (Struck et al, 2018; Korshunova et al, 2019). In behavioural ecology the concept applies to species with colourations and shapes that blend with the environment (Todd, 1981, 1983; Claridge et al, 2005; Bickford et al, 2007; Korshunova et al, 2019). Several derivatives of the concept have been proposed such as ‘true cryptic species’ (when a priori no morphological differences are recognized regardless of the distribution and ecology of species; Horsáková et al, 2019; Korshunova et al, 2019), ‘pseudo-cryptic species’ and ‘quasi-cryptic species’ (when subtle morphological difference can be recognized; Horsáková et al, 2019; Korshunova et al, 2019), ‘semi-cryptic species’ (morphological differences are very difficult to define; Vondrák et al, 2009; Korshunova et al, 2019), or ‘false cryptic species’ (morphological differences are obvious, but for some reason were missed or not highlighted in previous studies; Korshunova et al, 2019). IP address: 54.197.18.102, on 02 Nov 2021 at 11:19:05, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.

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