Tritonia Cuvier 1798
Creators: name: Silva, Felipe De Vasconcelos
name: Pola, Marta
name: Cervera, Juan Lucas
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Gastropoda
Nudibranchia
Tritoniidae
Tritonia

description: Tritonia Cuvier 1798
(Figs 3A, 4A, 5A, 6A)
Sphaerostoma MacGillivray 1843, Hist. Moll, 24: 335. Lateribranchiaea Stearns 1873, Proc. Calif. Acad. Nat. Sci, 5: 77. Candellista Iredale and O’Donoghue 1923, Proc. Malacol. Soc. Lond, 15(5): 197.
Type species: Tritonia hombergii Cuvier 1803, by subsequent designation.
Diagnosis: Oral veil broad and bilobed with several simple or branched velar processes. Gills fully arborescent with at least seven major pairs. Penis large, elongated or conical.
Morphology: Body broad, length up to 300 mm (Fig. 3A). Oral veil broad and bilobed. Seven to 40 simple or branched velar processes. Notal border may be distinct. Fully arborescent gills with at least seven large pairs and smaller ones in between. Rachidian tooth uni- or tricuspid (Fig. 4A), smooth or denticulate (with accessory denticles or folds) with rectangular base. First lateral tooth differentiated. Large number of lateral teeth (up to 200 per half row). Ratio jaw/body length: 0.27–0.40. Masticatory borders of jaws denticulate, with medium-sized (up to 100 µm) denticles with conical bases and sharp cusps; large-sized (up to 300 µm) denticles with conical bases and blunt cusps; or rod-shaped denticles. Cuticular folds may be present. ODG complex holohepatic (Fig. 5A). Gonopore at end of one-third of body length, furthest from rhinophores. Anus at middle of the body length; nephroproct above anus. Bursa copulatrix oħen pyriform. Penis large, elongate or conical; distal characters may be present. Ampulla long, cylindrical and coiled (Fig. 6A).
Species composition: Tritonia callogorgiae Chimienti et al. 2020, Tritonia exsulans, Tritonia festiva, Tritonia griegi Odhner 1922, Tritonia hirondelle Ortea and Moro 2020, Tritonia hombergii, Tritonia incerta Bergh 1904, Tritonia indecora Bergh 1907, Tritonia iocasica Zhang and Zhang 2021, Tritonia odhneri, Tritonia primorjensis Minichev 1971, Tritonia psoloides and Tritonia tetraquetra.
Remarks: The recent reassessment of Tritonia by Korshunova and Martynov (2020) resulted in several species being transferred to the genera Duvaucelia and Tritonicula. Yet, Tritonia remains the most heterogeneous genus of plateless tritoniids, with a majority of the species exhibiting characters not found in the type species Tritonia hombergii, as well as high values of minimum COI interspecific p -distances (Table 3). For example, the Pacific Tritonia festiva and Tritonia exsulans, recovered as sister-species in our phylogenetic analyses (Fig. 2), have a similar penial shape (boưle-shaped penis with distal tip in Tritonia festiva and boưle-shaped penis with distal ring in Tritonia exsulans) and cuticular folds in the stomach (MacFarland 1966). These characters are also found in the Pacific Tritonia odhneri (Schrödl 2003), which was recovered clustered with Tritonia festiva and Tritonia exsulans in a recent molecular study (Moles et al. 2021). However, such characters are not reported for Atlantic Tritonia hombergii. Furthermore, in recent molecular phylogenies (Korshunova and Martynov 2020, Moles et al. 2021; present study), the type species Tritonia hombergii was recovered as sister-species to the Pacific Tritonia festiva, Tritonia exsulans and Tritonia tetraquetra group (Fig. 2). This molecular evidence is consistent with morphological data that further distinguish the Pacific Tritonia species from Tritonia hombergii.
Although our molecular data support the monophyly of Tritonia, it is difficult to establish synapomorphies for such a heterogeneous group. Korshunova and Martynov (2020) included in Tritonia mainly large tritoniids with numerous processes on the bilobed oral veil and a large number of lateral teeth, which also included the species assigned to Myrella in this study. The two genera are externally similar, with numerous oral processes and arborescent gills that intersperse in size, but can be distinguished by a unique combination of morphological characters, particularly the penial morphology of Tritonia, which is significantly larger than that of Myrella species. Thus, we have characterized Tritonia by a unique combination of features: a broad and bilobed oral veil with several simple or branched velar processes; fully arborescent gills with at least seven large pairs and smaller ones in between; and a large, elongate or conical penis. Due to the high morphological variance, the radula of Tritonia is not a good diagnostic character. The masticatory border of the jaw is also the most morphologically variable in Tritoniidae, with species having medium-sized denticles with conical bases and sharp cusps, e.g. Tritonia exsulans Korshunova and Martynov 2020 and Tritonia festiva Stearns 1873; large denticles with conical bases and blunt cusps, e.g. Tritonia griegi Bouchet 1977; or rod-shaped denticles, e.g. Tritonia hombergii (present study) and Tritonia iocasica Zhang and Zhang 2021.
The recently described Tritonia iocasica clearly belongs to this genus based on its molecular and morphological data (Zhang and Zhang 2021). The species Tritonia callogorgiae, Tritonia griegi, Tritonia hirondelle, Tritonia incerta, Tritonia indecora and Tritonia primorjensis for which no molecular data are available, are included based on their original description and recent morphological evidence, which fit our proposed diagnosis (Bergh 1904, 1907; Odhner 1922, Chimienti et al. 2020, Korshunova and Martynov 2020, Ortea and Moro 2020). The remaining genetically unexplored species currently assigned to Tritonia are not included in the genus in this study due to inconsistencies with our proposed diagnosis. For example, Tritonia episcopalis has an entire oral veil and accessory denticles on the first lateral tooth (Bouchet 1977), which are characteristic of Tritonicula. The transfer of Antarctic and sub-Antarctic Tritonia species. to Myrella proposed in this study helps to alleviate the taxonomic issue of Tritonia. However, new collection efforts are needed to investigate potentially hidden generic lineages within the genus and to define clear generic boundaries.
description: Published as part of 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), pp. 445-476 in Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) (Zool. J. Linn. Soc.) 199 (2) on pages 462-463, DOI: 10.1093/zoolinnean/zlad013, http://zenodo.org/record/8432389