Myrella Odhner1963

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 : 466

Myrella Odhner1963 ( Fig. 3G )

Microlophus Mabille and Rochebrune 1889 , non- Microlophus Duméril and Bibron 1837

Type species: Myrella challengeriana (Bergh 1884) comb. nov. (= Myrella poirieri Mabille and Rochebrune 1889 ), by original designation.

Diagnosis: Tricuspid rachidian tooth with minor folds. Masticatory borders denticulate with conical denticles. Penis thin, oħen conical at the distal end and flagelliform towards proximal end.

Morphology: Body broad, length up to 80 mm ( Fig. 3G ). Oral veil broad, bilobed or entire. Five to 20 simple or branched velar processes. Notal border distinct. Six to 40 pairs of fully arborescent gills. Rachidian tooth tricuspid, denticulate (minor folds) with rectangular base. First lateral tooth differentiated. Masticatory borders of jaws denticulate with medium-sized (up to 50 µm) denticles with conical bases and sharp cusps. Ratio jaw/body length: 0.21–0.4. Cuticular folds absent. Gonopore at end of one-third of body length, furthest from rhinophores. Anus at middle of body length; nephroproct above anus. Ampulla cylindrical and coiled. Penis thin and flagelliform or conical at the distal end and flagelliform towards proximal end. Capsule and mucous gland distinct.

Species composition: Myrella antarctica (Pfeffer in Martens and Pfeffer 1886) comb. nov., Myrella australis (Bergh 1898) comb. nov., Myrella challengeriana (Bergh 1884) comb. nov., Myrella dantarti (Ballesteros and Avila 2006) comb. nov. and Myrella vorax (Odhner 1926) comb. nov.

Remarks: Based on our phylogenetic analyses, the clade formed by the Antarctic ‘ Tritonia ’ challengeriana and the sub-Antarctic ‘ Tritonia ’ dantarti corresponds to a distinct genus, molecularly and morphologically different from Tritonia s. s. This group, first depicted in Korshunova and Martynov (2020) and further focused in Moles et al. (2021), corresponds to the resurrected Myrella Odhner 1963 , based on the type species My. challengeriana . The species included in this group are characterized by a unique combination of features within Tritoniinae : moderate size (up to 80 mm); tricuspid rachidian tooth with minor folds; denticulate masticatory borders with conical denticles; penis thin and flagelliform or conical at the distal end and digitiform towards the proximal end; and a distinct capsule and mucous gland ( Wägele 1995, Ballesteros and Ávila 2006, Rossi et al. 2021). In addition, they are distinguished by their unique Antarctic, sub-Antarctic and Magellanic distribution.

Externally, Myrella and Tritonia are similar: both genera include large species with several arborescent gills and a bilobed oral veil with several processes. Some of the internal morpho-anatomical characters found in Myrella are also found individually in a few species of the heterogeneous Tritonia , but their combination is unique to Myrella within the Tritoniidae . For example, Tritonia tetraquetra also has a denticulate rachidian tooth like Myrella , but it is larger than Myrella species. (up to 300 mm) and its denticulate masticatory border is characterized by rod-shaped denticles instead of conical denticles ( Korshunova and Martynov 2020). The penis is also an important feature in distinguishing the two genera: Tritonia is characterized by a large penis that is elongate or conical and has distal characters in a few species, while Myrella is characterized by a thin penis that is completely flagelliform ( Rossi et al. 2021: 564) or conical at the distal end and digitiform towards the proximal end ( Wägele 1995: 37), with no distal features. Furthermore, most studies described Myrella species. with a conspicuous capsule and mucous gland ( Wägele 1995: 37; Ballesteros and Ávila 2006: 134). Although further morphological studies are needed to beưer understand this clade, our phylogenetic analyses, species delimitation analyses and interspecific uncorrected p -distances strongly support the resurrection of the cohesive, well-supported clade Myrella .

Currently, My. challengeriana, My. vorax and My. dantarti are the three valid species of Myrella found in the South Atlantic Ocean. However, recent studies have confirmed that the species diversity of Myrella is richer than presented in our phylogenetic analyses, including a complex of unnamed species previously identified as My. challengeriana ( Moles et al. 2021) . Although the original description of My. antarctica and My. australis fits our proposed diagnosis, these species are included with caution due to their uncertain taxonomic validity ( Schrödl 2003, Moles et al. 2021).

Figures

Figure 3. Representative species of genera of Tritoniidae (with exception of Paratritonia). A, Tritonia hombergii (MNCN15.05/93379) (Portaferry, Ireland); B, Candiella plebeia (MNCN15.05/93393) (Portaferry, Ireland); C, Tritoniopsis elegans (MB28-0045061) (Zavora, Mozambique); D, Tritonidoxa capensis (MNCN15.05/93399) (Scoưburg, South Africa); E, Tritoniella belli (Antarctica); F, Marianina rosea (French Polynesia); G, Myrella challengeriana comb.nov.(Bahía Camarones, Argentina); H, Tochuina gigantea (Agnew Pass, USA); I, Tritonicula khaleesi (MZUSP113165) (Fortaleza, Brazil); J, Marionia cucullata (Golfo Nuevo, Argentina); (K) Marioniopsis levis ‘1’ (MNHNC/ MB28-004895) (Nacala, Mozambique). Photo A and B, courtesy of B. Picton; photo C and K courtesy of Y.Tibiriça; photo D courtesy of V. Fraser; photo E courtesy of M. Caballer; photo F courtesy of G. Rouse; photo G and J, courtesy of N. Baưini; photo H courtesy of S. Thiebaud; photo I by F. Silva.