Tritoniidae Lamarck 1809

Silva, Felipe De Vasconcelos, Pola, Marta & Cervera, Juan Lucas, 2023, A stomach plate to divide them all: a phylogenetic reassessment of the family Tritoniidae (Nudibranchia: Cladobranchia), Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) 199 (2), pp. 445-476 : 456-459

Family Tritoniidae Lamarck 1809

Duvaucelidae Iredale and O’Donoghue 1923, Bull. Zool., 20: 272.

Aranucidae Odhner 1936 , Mém. Mus. R. his. Nat. Belg, 2(3): 1090, pl.1.

Marianinidae Odhner 1968 , Ark. Zool., 20(13): 253.

Diagnosis: Rhinophore palmate retractable and sheathed, club smooth surrounded by a distal circle of vertically branched processes. One row of dorsolateral, arborescent secondary gills.

Morphology: Body elongated and slightly quadrangular. Notum smooth or tuberculate. Oral veil entire or bilobed, with simple or branched digitiform velar processes. Outermost processes are ventrally grooved tentacles. Rhinophore sheath tubular, with margins smooth, undulated or with one or more processes. Rhinophore retractile and palmate formed by a smooth club surrounded by a distal circle of vertical branched processes. Secondary gills project dorsolateral. Radular formula n.1.1.1.n or n.1.n. Rachidian tooth uni- or tricuspid. Masticatory borders of the jaws denticulate, differentiated or smooth. True stomach plates absent or present. ODG complex holohepatic, with right and leħ lobe fused, or cladohepatic, with right and leħ lobe separated. Penis unarmed.

Remarks: The traditional family Tritoniidae (sensu Odhner 1963) has been recovered as paraphyletic in most phylogenetic studies with broad taxa sampling focusing on the family ( Huleư et al. 2015, Korshunova and Martynov 2020, Rossi et al. 2021). Notably, the family was recovered with different topologies, depending on the composition of the superfamily Proctonotoidea in our phylogenetic analyses ( Fig. 1 ). A poorly supported relationship between Proctonotoidea and Tritoniidae has previously only been suggested in a phylogeny based on transcriptomes ( Goodheart et al. 2015). The only comprehensive phylogenetic study based on Sanger sequencing with members of the Proctonotoidea , Tritoniella and Marionia , recovered Tritoniidae as paraphyletic without a close relationship between Tritoniella and Marionia with other tritoniids ( Rossi et al. 2021). Phylogenetic studies that included Tritoniella and Marionia , but not members of the Proctonotoidea , recovered a relationship between Tritoniella and other plateless tritoniids ( Valdés et al. 2018, Korshunova and Martynov 2020, Moles et al. 2021). This paưern was confirmed in our phylogenetic analyses: the presence of Proctonotoidea renders Tritoniidae paraphyletic, with no relationships recovered between Tritoniella , Tochuina and the marioniids with other members of the family ( Fig. 1A, B ). Moreover, this phenomenon is probably not due to any particular sequence included, as two distinct compositions of Proctonotoidea produced the same effect ( Fig. 1A, B ). Nevertheless, three-gene analyses are notoriously poor at providing robust family-level phylogenies because they suffer from problems with homology and variable region alignments ( Hallas et al. 2017), and the possible relationship between Proctonotoidea and Tritoniidae is not supported by recent transcriptome-based studies ( Goodheart et al. 2017).

Based on the chosen phylogenetic hypothesis ( Fig. 2 ) and morphological analysis, the traditional family Tritoniidae is a non-monophyletic group primarily divided into plateless (clade A + B) and plate-bearing species (clade C). Thus, we propose the division of tritoniids into two new monophyletic subfamilies, Tritoniinae and Marioniinae subfam. nov., to reflect these new findings, while preserving the rank of Tritoniidae within Cladobranchia based on two well-established synapomorphies: (1) a retractable and sheathed palmate rhinophore consisting of a smooth club surrounded by a distal circle of vertically branched processes, and (2) one row of dorsolateral, arborescent secondary gills (Ev. Marcus 1983). This reassessment is also consistent with the current conflicting molecular evidence for the monophyly of Tritoniidae and its sister-group, which could be Arminidae , Dendronotidae or some proctonotoidean family. Henceforth, the term ‘tritoniids’ will be used in this study to refer to all species of Tritoniidae , whilst ‘plate less tritoniids’ will refer only to species recovered in the Tritoniinae and ‘plate-bearing tritoniids’ or ‘marioniids’ will refer only to species recovered in the Marioniinae . It is not possible to assign Tritoniidae to the superfamily Tritonioidea based on our results.

Figures

Figure 1. Bayesian phylogenetic trees based on the concatenated molecular data (COI + 16S + H3) of Tritoniidae (sensu Odhner 1963) with three distinct outgroup compositions: A, Proctonotoidea including Curnon granulosa (Curnonidae) and Dirona picta (Dironidae); B, Proctonotoidea including Antiopella barbarensis (Janolidae) and Antiopella capensis (Janolidae); C, no Proctonotoidea were included. Outgroup and clades are depicted in grey and Tritoniidae genera are independently coloured. PP are shown right/top and BS leħ/boưom. Unsupported branches are not labelled. Green circles indicate maximum support by both analyses.

Figure 2. Bayesian phylogenetic tree based on the concatenated molecular data (COI + 16S + H3) of Tritoniidae (sensu Odhner 1963). Outgroup taxa are depicted in grey.Tritoniidae genera are independently coloured, and branches with several replicates for each species hypothesis were collapsed for clarity.Names in bold are species which sequences were generated for this study with the number of sequenced specimens showed in brackets. PP are shown right/top and BS leħ/boưom.Unsupported branches are not labelled. Green circles indicate maximum support by both analyses. Solid vertical bars indicate specimens grouped as species entities based on COI dataset. Empty vertical bars indicate the specimens grouped as species based on the 16S dataset.