Consolidated data on the phylogeny and evolution of the family Tritoniidae (Gastropoda: Nudibranchia) contribute to genera reassessment and clarify the taxonomic status of the neuroscience models Tritonia and Tochuina.

Tatiana Korshunova,Alexander Martynov,Michael Schubert

doi:10.1371/journal.pone.0242103

Tatiana Korshunova, Alexander Martynov + Show 1 more

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https://doi.org/10.1371/journal.pone.0242103

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Abstract

Nudibranch molluscs of the family Tritoniidae are widely used neuroscience model systems for understand the behavioural and genetic bases of learning and memory. However species identity and genus-level taxonomic assignment of the tritoniids remain contested. Herein we present a taxonomic review of the family Tritoniidae using integration of molecular phylogenetic analysis, morphological and biogeographical data. For the first time the identity of the model species Tritonia tetraquetra (Pallas, 1788) and Tritonia exsulans Bergh, 1894 is confirmed. T. tetraquetra distributes across the large geographic and bathymetric distances in the North-Eastern (NE) and North-Western (NW) Pacific. In turn, at NE Pacific coasts the separate species T. exsulans is commonly occured. Thus, it reveals a misidentification of T. tetraquetra and T. exsulans species in neuroscience applications. Presence of more hidden lineages within NW Pacific T. tetraquetra is suggested. The long lasting confusion over identity of the species from the genera Tritonia and Tochuina is resolved using molecular and morphological data. We also disprove a common indication about “edible T. tetraquetra” at the Kuril Islands. It is shown that Tochuina possesses specialized tritoniid features and also some characters of “arminacean nudibranchs”, such as Doridoxa and Heterodoris. Diagnoses for the families Doridoxidae and Heterodorididae are provided. Taxonomy of the genus Doridoxa is clarified and molecular data for the genus Heterodoris presented for the first time. A taxonomic synopsis for the family Tritoniidae is provided. A new genus among tritoniid taxa is proposed. Importance of the ontogeny-based taxonomy is highlighted. The cases when apomorphic characters considerably modified in a crown group due to the paedomorphosis are revealed. Tracing of the character evolution is presented for secondary gills–a key external feature of the family Tritoniidae and traditional dendronotacean nudibranchs.

Highlights

A remarkable nudibranch family Tritoniidae has an intricate phylogenetic position within Nudibranchia [1–5]
The HKY + I + G model was chosen for the 16S; the GTR + I + G model was chosen for c oxidase subunit I (COI), Histone 3 (H3), and for the combined dataset
All five Tritonia exsulans, five T. festiva, three T. tetraquetra and T. cf. psoloides clustered together in separate clade with high support (PP = 1, BS = 99), that branched to three clades

Summary

Introduction

A remarkable nudibranch family Tritoniidae has an intricate phylogenetic position within Nudibranchia [1–5]. The most commonly used for neuroscience purposes was a North Pacific species previously known under the name Tritonia diomedea Bergh, 1894 [14–16], which recently was showed to be a junior synonym of Tritonia tetraquetra (Pallas, 1788) [17, 18]. Though the latter name now is accepted [19–21] many questions have remained. Tritonia tetraquetra is frequently mentioned among few examples of apparent involvement of a nudibranch species in a culture of indigenous ethnic groups This species was originally described from the Kuril Islands and the genus name Tochuina was proposed to be derived from Ainu language [22, 23]. We for the first time summarize available morphological, molecular and biogeographic data across the NorthEastern (NE) and the North-Western (NW) Pacific “Tritonia tetraquetra” group of species and the genus Tochuina to clarify their taxonomic status and phylogenetic relationships (Figs 1–7)

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