When morphology does not match phylogeny: The puzzling case of two sibling nudibranchs (Gastropoda)

Abstract

Species delimitation is particularly challenging in Mediterranean nudibranchs because morphological distinction between closely related species is subtle and several cryptic species have been discovered. In this study, we apply an integrative approach to investigate taxonomic distinction of Flabellina cavolini and F. gaditana and assess their genetic and morphologic distinction. We combined a comprehensive phylogeographic survey from across their distribution range and compared genetic variation at mitochondrial and nuclear markers with the variation in colour pattern of cerata that is used to discriminate these two species. Phylogenetic, phylogeographic and species delimitation analyses consistently identify two evolutionary and taxonomic units that do not match with the morphospecies, demonstrating that Flabellina cavolini and F. gaditana are indeed two cryptic, closely related, species and that colour pattern of cerata is not a valid diagnostic character. This study emphasizes how the comparison of range‐wide patterns of phylogenetic and morphological variation, with the inclusion of samples from the type localities, is essential for species delimitation of Mediterranean nudibranchs. Moreover, this study demonstrates the occurrence of Flabellina gaditana in the Mediterranean Sea, allowing to extend the known distribution range of the species, that was previously restricted to the eastern Atlantic Ocean, as well as increasing our inventory of Mediterranean nudibranch diversity.

Variation and Diagnostic Power of the Internal Transcribed Spacer 2 in Mediterranean and Atlantic Eolid Nudibranchs (Mollusca, Gastropoda)

“…Mediterranean marine biodiversity is still underestimated and new cryptic species continue to be identified, described, and added to our inventory of marine fauna (Calvo et al, 2009;Uriz et al, 2017;González-Castellano et al, 2020;Furfaro et al, 2021). Our knowledge on diversity of groups such as nudibranchs is recently increasing as demonstrated by the number of studies published in the last years (Cella et al, 2016;Furfaro and Trainito, 2017;Korshunova et al, 2017Korshunova et al, , 2019Furfaro et al, 2018;Chimienti et al, 2020;Furfaro and Mariottini, 2020).…”

Looking at the Nudibranch Family Myrrhinidae (Gastropoda, Heterobranchia) from a Mitochondrial ‘2D Folding Structure’ Point of View

“…Integrative taxonomy applied to marine Heterobranchia is proving to be a fruitful field of study. In fact, in the last few years, many papers have been published, which included information derived from several biological points of view, including morphological, molecular, chemical, ecological and behavioural characters [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. Regarding the molecular approach, there is a continuous search in the genome for new informative DNA coding and non-coding regions, consequentially resulting in molecules such as proteins and RNA [ 7 , 8 , 9 ].…”

“…In fact, in the last few years, many papers have been published, which included information derived from several biological points of view, including morphological, molecular, chemical, ecological and behavioural characters [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. Regarding the molecular approach, there is a continuous search in the genome for new informative DNA coding and non-coding regions, consequentially resulting in molecules such as proteins and RNA [ 7 , 8 , 9 ]. Mitochondrial DNA (mtDNA) is currently, in the greatest number of cases, the most powerful genome being exploited to shed light on lower taxonomic levels, such as species and genera, and it remains the most used DNA molecule to delimit species and investigate cryptic diversity and recent speciation events [ 7 , 10 , 11 , 12 ].…”

“…To avoid these pitfalls, nuclear markers are usually added to the molecular analyses. This approach has been applied to marine Heterobranchia (Mollusca, Gastropoda) whose evolutionary history, when studied at family level, is reconstructed using mainly three molecular markers, the two mitochondrial genes, part of Cytochrome oxidase subunit I (COI) and part of the ribosomal subunit 16S (16S), as well as the nuclear gene histone 3 (H3) which is well known to be poorly or not quite informative at lower taxonomic levels [ 7 , 17 , 18 ]. By means of these markers, the systematics of several heterobranchs families has been clarified and erroneous outcomes derived from previous morphological studies have been resolved and corrected [ 2 , 3 , 5 ].…”

“…Regarding the molecular approach, there is a continuous search in the genome for new informative DNA coding and non-coding regions, consequentially resulting in molecules such as proteins and RNA [7][8][9]. Mitochondrial DNA (mtDNA) is currently, in the greatest number of cases, the most powerful genome being exploited to shed light on lower taxonomic levels, Life 2021, 11, 583 2 of 16 such as species and genera, and it remains the most used DNA molecule to delimit species and investigate cryptic diversity and recent speciation events [7,[10][11][12]. In fact, animal mitochondrial DNA is characterised by a high copy number, largely maternal inheritance, lack of recombination, and displays a higher mutation rate than the nuclear DNA.…”