Fionoidea Gray, 1857

Superfamily Fionoidea Gray, 1857 View in CoL , restricted

( Figs 1, 2; Tables 1–3)

‘Tribe Acleioprocta’ part. Odhner 1939: 50–3.

Diagnosis: Aeolidacean superfamily predominantly with uniserial radula, and so far includes only a single known triserial family, Eubranchidae . Notal edge commonly completely reduced, very rarely present. Cerata commonly placed in regular, simple rows, more rarely in branched rows. Anus commonly acleioproctic, rarely in cleioproctic position, and only a unique example of occasional pleuroproctic position. Anterior foot corners commonly absent, more rarely present. Commonly no elaborate oral glands, but in some taxa oral glands more distinct. Masticatory edges of jaws commonly bear a single row of simple or compound, sharpened or tubercle-like denticles, rarely several rows. Central teeth cusp commonly not compressed, rarely compressed by adjacent lateral denticles. Lateral teeth, if present, usually smooth. Commonly single distal receptaculum, sometimes both proximal and distal seminal receptaculums. Clasping organ in female part of reproductive system absent. Vas deferens usually short to moderate, prostate indistinct to distinct. Special supplementary gland in the male part of reproductive system present in all known families, except for the stem-bearing name Fionidae . Other accessory glands absent. Massive external permanent penial collar absent. Penis internal, of various patterns, but commonly not considerably widened; in several families usually with well-defined hollow stylet, or stylet system is considerably modified, in other families penis unarmed.

Families included: Abronicidae Korshunova et al., 2017 , Eubranchidae Odhner, 1934 , Murmaniidae Korshunova et al., 2017 , Xenocratenidae Martynov et al., 2020 , Cuthonellidae Miller, 1971 , Calmidae Iredale and O’Donoghue, 1923 , Cuthonidae Odhner, 1934 , Tergipedidae Bergh, 1889 , Fionidae Gray, 1857 , and Trinchesiidae Nordsieck, 1972 .

Remarks: The taxonomic volume of the superfamily Fionoidea is restricted here [with the removal of the obviously flabellinoidean Coryphellidae and Paracoryphellidae (WoRMS 2024) , as well as any other non-related families; see Synopsis above and below and Fig. 1], to the core monophyletic predominantly uniserial (triserial only in one family, Eubranchidae ) families with a distinct supplementary gland present in the majority of families, except the genus-name bearing Fionidae proper.

The superfamily Fionoidea ( Figs 1, 2) currently is among one of the most finely-differentiated major aeolidacean super-groups in which fine-scale morphological and molecular data are most consistently conjoined, although more families potentially need to be separated (see: Korshunova et al. 2017a –c, 2019a, 2021, 2022, Martynov et al. 2019, 2020). However, there are continuous attempts to dismissively lump all Fionoidea superfamily diversity into a single ‘family Fionidae’ (e.g. Kim et al. 2024), despite substantial morphological and molecular arguments. To do so immediately implies that we must lump all of the nearly 30 families of the suborder Aeolidacea into a single family, Aeolidiidae Gray, 1827 / Glaucidae Gray, 1827 , because almost all major characters that are known for the superfamily Fionoidea can potentially be found in other superfamilies (see Synopsis above and below). Particularly, an acleioproctic anus, one of the most common characteristic features of Fionoidea in many taxa, has been reported for some taxa within the superfamilies Apataoidea , Cumanotoidea , Aeolidioidea , and Flabellinoidea . At the same time, within Fionoidea there are instances of a definitely cleioproctic anus, among the Murmaniidae , Cuthonellidae , and Cuthonidae families, which is in turn common for the superfamily Aeolidioidea . Along with common cleioproctic anus, the latter superfamily may in turn rarely include a pleuroproctic anus, which is very common within superfamily Flabellinoidea . The regular ceratal rows, also characteristics for Fionoidea , are also present in Apataoidea and some of Aeolidioidea . As previously confirmed ( Korshunova et al. 2017a), and in the present study ( Fig. 2), the uniserial radula, one of the key characters within Aeolidacea , originated based on the reduction of the triserial radula, possibly no less than four (and may be more) times independently in the course of the evolution of the suborder Aeolidacea ( Fig. 1; Tables 1, 2). Therefore, every apparently exclusively uniserial superfamily of the suborder Aeolidacea may actually or potentially include triserial families as part of its ancestral diversity, which is well-highlighted in the phyloperiodic tables [ Tables 1, 2; see details of building of Phyloperiodic Tables in Martynov and Korshunova (2022) and, Korshunova and Martynov (2024)]. Remarkably, in the present study we report the crucial discovery of the uniserial family Hantazuidae fam. nov., which is robustly placed within the otherwise triserial superfamily Flabellinopsoidea (see Synopsis below; Figs 1, 2; Tables 1, 2), thus uncovering a confirmed example of the independent reduction of the triserial radula into a uniserial one ( Fig. 1).

The triserial families are so far firmly confirmed for a majority of the aeolidacean superfamilies, except Unidentioidea and Aeolidioidea ( Fig. 2; Tables 1–3). In this respect, the new family Chudidae fam. nov., described in the present study, may potentially present a ‘missing evolutionary link’ of the yet unknown, but definitely existent, triserial ancestor of the superfamily Aeolidioidea . But because Chudidae fam. nov. are also sister to the superfamilies Cumanotoidea , Aeolidioidea , Flabellinopsoidea , and Flabellinoidea , we prefer to establish the superfamily Chudoidea superfam. nov. (see Synopsis below; Figs 1, 2; Tables 1, 2) for the new family Chudidae fam. nov. In many respects, the triserial superfamily Chudoidea superfam. nov. demonstrates affinities to both the predominantly triserial superfamilies Cumanotoidea , Flabellinoidea , Flabellinopsoidea , and to the exclusively uniserial Aeolidioidea , and its discovery uncovers this evolutionary highly important taxon, which has partly preserved its ancestral features, including a triserial radula.

Remarkably, the superfamily Fionoidea robustly includes precisely the triserial family Eubranchidae ( Korshunova et al. 2017a, 2020b, 2021, present study; Figs 1, 2), which unequivocally suggests that potentially a triserial ancestral family can be discovered (see Phyloperiodic Table, 1) in any superfamily, even in a completely uniserial modern superfamily of the suborder Aeolidacea . Therefore, in spite of the fact that all previously provided arguments are more than enough evidence for the unavoidable necessity of the family-level fine-scale differentiation among the superfamily Fionoidea , as well as in any other aeolidacean superfamilies (present Synopsis, Fig. 2), the strong bias and ad hoc preconceptions for lumping of all finely-differentiated families of the superfamily Fionoidea into putatively a ‘single family’ still persists (see details and discussion in: Korshunova et al. 2021, 2022). Instead, and simultaneously, equally strong preconceptions persist for the ‘necessity’ of several families and truly numerous genera only within the superfamily Aeolidioidea (see details and references in Synopsis below; Figs 1, 2), but for some unsubstantiated reasons are not allowed within the superfamily Fionoidea or the flabellinoidean family Coryphellidae (see Synopsis below, and Discussion; Figs 1, 2). Therefore, we propose herein, that if, for example, one lumps the distinctly triserial family Eubranchidae into the ‘single family Fionidae’ instead of placing it within the fine-scale family system of the superfamily Fionoidea ( Fig. 2), one should also, without any hesitation, lump all of the nearly 30 families of the whole suborder Aeolidacea into just a single family (see also details in Discussion). Triserial families, again, are present in the absolute majority of aeolidacean superfamilies over the whole molecular phylogenetic tree of the suborder Aeolidacea ( Fig. 2), and all supposed ‘intermediate’ states can be potentially ‘designated’ between the majority of existent families. Thus, the preconception against truly evolutionary morphological and molecular arguments, and the persistent denying of fine-scale family level differentiation within the superfamily Fionoidea , clearly show an adherence to an undefined ‘taxonomic-policy’.

The even more remarkable lumping case of the family Fionidae proper (a family fundamentally based mainly on the characters of the stem-genus Fiona , and not from the majority of other Fionoidea families) is connected not only with those who deny fine-scale taxonomic differentiation, but also with the still commonly employed cladistic principle of searching for ‘apomorphies/synapomorphies’. Because when pan-lumping the morphologically unmanageable assemblage of the putative ‘ Fionidae sensu latissimo ’ was suggested, an ‘apomorphy-based diagnosis’ was proposed, which unambiguously included the ‘presence of a “penial” gland’. Ironically in that case, however, the very genus Fiona Alder and Hancock, 1853 does not possesses any ‘penial’ gland in the reproductive system. (see details in: Korshunova et al. 2017c, 2021, 2022).

That case, therefore, not only refutes the strong ‘pan-lumping bias’ in a particular nudibranch superfamily, Fionoidea , but also undermines apomorphy-based taxonomy, one of the most seemingly fundamental principles in contemporary taxonomy. Thus, there must now be a complete ‘full stop’ to any further attempts to dismissively lump the fine-scale Fionoidea (and other superfamilies) family diversity (Synopsis above and below; Figs 1, 2; Tables 1–4). However, a very important conclusion, both theoretical and practical, needs to be made that has relevance not only for the particular ‘nudibranch case’ made here, but without exception for all diversity-based studies of all living organisms. Because it is clear that any attempts to comprehend the simultaneously highly irregular and partly regular in some sense (see: Martynov and Korshunova 2022) evolutionary process, which has resulted in the appearance of astonishing diversity, in all its multilevel scale, from the smallest molecules to morphologically distinct individuals and their groups, cannot be truly understood using only such typological seemingly ‘strict’ diagnoses of classic anti-evolutionary taxonomy (still in operation), or ‘strictly defined’ apomorphies of even apparently ‘evolutionary’ phylogenetic taxonomy.

In opposition to the pan-lumping scenario, here we present proof of the profound mosaicism of diagnoses at almost every level, from the Aeolidacea suborder to each individual genus, including the fundamental absence of any ‘completely clear and distinct from other taxa’ diagnosis. But at the same time we highlight several obvious quasi-regular general trends caused by the common ancestral ontogenetic basis, such as, for example, distribution and proportions of the triseral and uniserial families within the superfamilies of the suborder Aeolidacea (see Synopsis; Figs 1, 2; Tables 1, 2), coupled with fundamentally blurred borders of all taxa even in the most ‘complete’ phylogenomic analyses, including ‘species’ and order levels, e.g., in Karmeinski et al. (2021), where the position of a single nudibranch family, Embletoniidae , has been ‘blurred in between’ the otherwise taxonomically distinct suborder Aeolidacea and ancestral non-aeolidaceans, even using applied transcriptomic analysis. Or in Coates et al. (2018), where blurred borders between ‘population’ and ‘species’ categories have been shown, and there is no doubt that an absence of clear borders between ‘species’ and ‘populations’ is characteristic for any group of organisms, including nudibranchs ( Korshunova and Martynov 2024). In other words, because the fundamentally dynamic nature of evolution in all its genetic and epigenetic totality is the principle underlying any true taxonomy, any taxon can never be made as clear and distinct as it has claimed to have been made over many centuries within the basically anti-evolutionary classic taxonomy, but we must still unavoidably use those methods while producing taxonomic systems. Likewise the relative and practical problems of ‘phylogenetic systematics’, which is also a rigidly proposed set of strict rules—such as the strict distinction of plesiomorphies and apomorphies ( Hennig 1966)—especially from the point of view that any organism represents not ‘phylogenetic lineages’ but an individual dynamic, interacting, evolving ontogeny, has been already pointed out (see e.g. Martynov and Korshunova 2022).

Therefore, we are facing current multiple crises, both in society and in science, particularly in biological systematics, where the old ‘structure of science’ has been largely undermined by the current ‘molecular phylogenetic era’. But new paradigms are only at the emerging stage, and we must remember that when we make a taxonomy/classification of any organism, we are dealing not with static preserved organisms in museums or even living organisms seeming to appear in relatively ‘static’ modern ecosystems, but with finely-differentiated organismal diversity that has been formed on the basis of very complex, constantly dynamic, multilevel, and clearly not strictly regular evolutionary processes. Therefore, any taxonomic arrangements or systems that pretend to be maximally aligned with those evolutionary processes, must be equally complex, not strictly regular or seeking an ‘absolute distinctness at the level of diagnoses’, and they must be unavoidably finely-differentiated regarding ‘subordinate taxa’, whose numbers must not be restricted by any ad hoc preconceptions such as ‘simplicity’ or ‘didactic usefulness’ but only by the endlessly evolutionary-based organismal forms.