Cuthonidae Odhner, 1934

Family Cuthonidae Odhner, 1934 View in CoL

( Figs 1, 2; Table 4)

Odhner 1934: 278.

Family Cuthonidae View in CoL restricted: Korshunova et al. 2017c: 9, 14, 17, 18.

Korshunova et al. 2017a: 73–8, supplementary materials.

Diagnosis: Body wide. Notal edge completely reduced. Ceratal rows branched to simple, numerous non-elevated cerata per row. Rhinophores smooth. Anus acleioproctic or cleioproctic. Masticatory edges of jaws commonly bear single row of compound, sharpened denticles. Radula formula 0.1.0. Central teeth with strong cusp not compressed by adjacent lateral denticles. Proximal and distal receptaculum seminis. Vas deferens moderately short, prostate indistinct. Supplementary gland present, inserts into penis. Massive external permanent penial collar absent. Penis internal, narrow, unarmed.

Genera included: Cuthona Alder and Hancock, 1855 [restricted only to three valid species, type C. nana (Alder and Hancock, 1842) , C. divae Er. Marcus, 1961 , and C. hermitophila Martynov et al., 2015 ] and Bohuslania, Korshunova et al., 2018 .

Remarks: About 45 years ago, almost all Fionoidean diversity (at that time listed under the family named Tergipedidae ) was dismissively lumped into the seemingly single genus Cuthona [see an illustrated history of lumping events within the superfamily Fionoidea in Korshunova et al. (2021)] and that evolutionary and morphologically unsubstantiated action has lasted a considerable time, despite explicitly presented evidence of significant morphological and taxonomic heterogeneity within the so-called ‘Cuthona’ for a long time ( Martynov 1992a, 2006a, Martynov and Korshunova 2011). That lumping generated taxonomic confusion because under the putatively ‘same’ genus-level such evidently different family-level taxa, such as Cuthonidae and Trinchesiidae , were concealed. However, when molecular phylogenetic data came into use, it became so obvious (although it had previously repeatedly been predicted) that ‘Cuthona’ in no way could encompasses the uncovered molecular phylogenetic diversity ( Fig. 2) corresponding to many families, and not merely a single genus ‘Cuthona’ ( Korshunova et al. 2017c, 2018, 2021, 2022). It is also worth noting that an absolute majority of the fine-scale differentiated Fionoidea families—including Eubranchidae , Tergipedidae , Calmidae , Cuthonellidae , and Cuthonidae , which are well supported by molecular phylogenetic data—were proposed a long time ago (e.g. Bergh 1889, Iredale and O’Donoghue 1923, Odhner 1934, Miller 1971, 1977).

The family Cuthonidae is fundamentally restricted to only the genus Cuthona proper, with the inclusion of several valid, fine-scale diagnosed species only from the North Atlantic and North Pacific. Species such as Cuthona nana (Alder and Hancock, 1842) , Cuthona divae Er. Marcus, 1961 and Cuthona hermitophila Martynov et al., 2015 , and not hundreds of highly heterogenous species from all over the world, which in reality belong to at least seven separate families ( Figs 1, 2). Any other species of ‘Cuthona’ that currently may be listed under this genus in WoRMS (2024) do not belong to Cuthona proper, and many of them belong to the family Trinchesiidae (see below). The true genus Cuthona , with only three currently recognized species, possesses a very characteristic semi-lunar head with oral tentacles placed toward the middle part of head, has branched ceratal rows, a commonly cleioproctic anal position, an unarmed penis and a supplementary gland inserted into the penis, and whose distribution is restricted to the temperate and cold waters of the Northern Hemisphere. So far, the only other confirmed genus within the family Cuthonidae , the genus Bohuslania Korshunova et al., 2018 , is evidently a paedomorphic taxon with a reduced head and ceratal row structure, which inhabits a predominantly brackish environment in a fjord at the Swedish and Norwegian border ( Korshunova et al. 2018).

The lesson that needs to be learned both from the superfamily Fionoidea case in general, and the family Cuthonidae in this strongly-restricted sense in particular, is that the more differentiated the taxonomy of a group is, the more it reflects actual and always very complex evolutionary (phylogenetic) pathways. Even at the level of pre-molecular work, some of the current families and genera are partly poly- or paraphyletic. This is no reason to blame morphology-based taxonomy for previous attempts to make taxonomy as finely differentiated as possible, especially when some morphology-based studies predicted today’s fine-scale differentiation, which has been polished and improved by molecular phylogenetic methods (Synopsis; Figs 1, 2). In this respect, if further Fionoidea diversity is uncovered, especially in the deep sea, that potential diversity does not precisely ‘fit’ into currently recognized families of the superfamily Fionoidea — either morphologically or molecularly, or both—this must not imply that family borders should be stretched or that morphological and molecular phylogenetic diversity ( Figs 1, 2) should be lumped into an unmanageable, highly heterogenous putative ‘family Fionidae’ or genus ‘Cuthona’ sensu latissimo. Instead, more genus and family levels should be further separated following the incredibly complex at every possible level and ‘endlessly beautiful’ (see: Darwin 1859, Carrol 2005) organismal evolutionary way.