Nudibranchs: taxonomic revolution

In 2025, Tatiana Korshunova, Karin Fletcher, and Alexander Martynov published a groundbreaking study titled “The endless forms are the most differentiated—how taxonomic pseudo-optimization masked natural diversity and evolution: the nudibranch case”. This article marks a turning point in the taxonomy of nudibranchs, especially aeolidaceans, by questioning recent “lumping” practices (excessive synonymization and merging) that, according to the authors, concealed true evolutionary diversity.

The study emphasizes the importance of recognizing fine-scale diversity, reinstating lineages, and describing new families, genera, and species. It redefines the order Nudibranchia, eliminating the suborder Doridina and reinstating the order Doridida.

What is the article about?

The authors review how, in the study of nudibranchs (striking marine mollusks also known as “sea slugs”, which lack an external shell), taxonomy has sometimes been applied in a way that seeks to “optimize” -that is, simplify- at the cost of ignoring real differences. This can result in overlooking natural diversity, evolutionary history, and important relationships among species.

Main findings

Revision of the order Nudibranchia

Within the order Nudibranchia, the authors restrict what is considered part of it and reinstate certain orders/suborders that had been previously removed or merged in earlier classifications.

Five suborders within Nudibranchia are reinstated, essentially corresponding to groups from the former suborder Cladobranchia, now obsolete:

• Arminacea (restricted, reinstated)
• Tritoniacea (reinstated)
• Dendronotacea (restricted, reinstated)
• Janolacea (reinstated)
• Aeolidacea (revised, reinstated)

The order Nudibranchia is restricted, and the suborder Doridina is removed.

Peltodoris atromaculata by Miquel Pontes

Reinstatement of the order Doridida

The suborder Doridina, which included all dorid nudibranchs, is removed from the order Nudibranchia, meaning this group is no longer considered part of the “nudibranchs”, it now forms its own independent group, the order Doridida (they remain as “dorids”, but are classified as a separate entity).

Revision of the group Aeolidacea

The suborder Aeolidacea is reinstated as a valid group.

The superfamily system within Aeolidacea is restructured, recognizing the following 11 superfamilies (genera in parentheses):

• Notaeolidioidea Eliot, 1910 (reinstated)
  • Notaeolidiidae Eliot, 1910 (Notaeolidia)
• Samloidea Korshunova et al., 2017
  • Samlidae Korshunova et al., 2017 (Samla, Luisella)
• Apataoidea Korshunova et al., 2017
  • Apataidae Korshunova et al., 2017 (Apata)
• Unidentioidea Millen & Hermosillo, 2012
  • Unidentiidae Millen & Hermosillo, 2012 (Unidentia, Phetia)
• Fionoidea Gray, 1857 (restricted)
  • Fionidae Gray, 1857 (Fiona, Tergiposacca, Zatteria)
  • Cuthonellidae Miller, 1977 (Cuthonella, Fiocuthona, Margina, …)
  • Itaxidae reinstated
  • Boreanidae reinstated
  • Murmanidae reinstated
• Cumanotoidea Odhner, 1907 (expanded)
  • Cumanotidae Odhner, 1907 (Cumanotus)
  • Pseudovermidae Salvini-Plawen, 1978 (paedomorphic, included here) (Pseudovermis)
• Chudoidea superfam. nov.
  • Chudidae fam. nov. (Chudo)
• Aeolidioidea Gray, 1827 (restricted)
  • Aeolidiidae Gray, 1827 (Aeolidia, Aeolidiella, Baeolidia, Spurilla, etc.)
  • Myrrhinidae
  • Facelinidae
  • Favorinidae
  • Glaucidae
  • Babakinidae Roller, 1973 (Babakina)
  • Pleurolidiidae Burn, 1963 (Pleurolidia, Protaeolidiella)
• Flabellinopsoidea Korshunova et al., 2017
  • Flabellinopsidae Korshunova et al., 2017 (Flabellinopsis, Baenopsis)
  • Notaeolidiidae (relationship clarified)
• Flabellinoidea Bergh, 1889 (reinstated)
  • Coryphellidae Bergh, 1889 (with 11 genera, 9 reinstated + 2 new ones, incl. Coryphella, Fjordia, Gulenia, Himatina, Itaxia, Microchlamylla, Orienthella, Borealea, Corrupta, Portorchardia)
  • Flabellinidae Bergh, 1889 (redefined, not polyphyletic) (Flabellina, Edmundsella, Paraflabellina, Calmella, Carronella, Coryphellina)
  • Paracoryphellidae reinstated (Paracoryphella, Ziminella, Chlamylla, Polaria)
  • Hantazuidae fam. nov. (Hantazuia)
• Embletonioidea Alder & Hancock, 1845 (included as a separate superfamily)
  • Embletoniidae Alder & Hancock, 1845 (Embletonia, Embletoniella)

Criticism of the “excessive synonymization” practice

One of the central points of the article is that it shows how, in some previous studies, other authors had “lumped” many different species or genera under a single name, using low-resolution phylogenetic analyses, ignoring relevant morphological characters, and assuming preconceived taxonomic ideas. This leads to what the authors call pseudo-optimization (“lumping”): “optimizing” taxonomy but losing real information about diversity. The paper demonstrates that such practices can obscure important evolutionary pathways, such as the emergence of distinct morphological adaptations, ecological differences, and more.

Why does this matter?

These reinterpretations, increasingly common in sea slug taxonomy, are necessary to better understand actual biodiversity: if fine morphological differences are ignored or species are grouped incorrectly, we fail to see the full richness of life and how groups have truly diversified (evolved) over time.

The approach proposed here considers not only genetic and morphological traits but also that different lineages may have different geographical distributions (with corresponding environmental adaptations, vulnerabilities, etc.). Ignoring geographic factors could mean overlooking species that are, in fact, distinct.

Furthermore, clarifying relationships between species helps study how adaptations arose, how physical traits (shape, color, etc.) diversified, and provides a better understanding of speciation processes within Evolutionary Biology.

As usual in modern taxonomy, the authors combine molecular data (DNA, phylogeny) with detailed morphological analyses. However, they acknowledge that geographic sampling may be incomplete, meaning some species or morphological characteristics might remain unstudied.

Changing taxonomy (restructuring families and genera) implies that many previous classifications become obsolete or require revision, which has many practical implications (literature, databases, field identification).

Key studies cited

• Gosliner TM, Griffiths RJ. Description and revision of some South African aeolidacean Nudibranchia (Mollusca, Gastropoda). Annals of the South African Museum 1981;84:105–50.
• Gosliner TM, Valdes A, Behrens DW. *Nudibranch & Sea Slug Identification. Indo-Pacific*, 2nd edn. Jacksonville: New World Publications Inc., 2018.
• Korshunova TA, Martynov AV, Bakken T et al. *Polyphyly of the traditional family Flabellinidae affects a major group of Nudibranchia: aeolidacean taxonomic reassessment with descriptions of several new families, genera, and species* (Mollusca, Gastropoda). *Zookeys* 2017a;717:1–139. https://doi.org/10.3897/zookeys.717.21885
• Martynov AV, Korshunova TA. *Opisthobranch Molluscs of the Seas of Russia. A Colour Guide to their Taxonomy and Biology.* Moscow: Fiton Press, 2011.
• Martynov AV, Korshunova TA. *A new deep-sea genus of the family Polyceridae (Nudibranchia) possesses a gill cavity…* *Journal of Molluscan Studies* 2015;81:365–79. https://doi.org/10.1093/mollus/eyv003
• Korshunova, T., Fletcher, K., & Martynov, A. (2025). *The endless forms are the most differentiated…* *Zoological Journal of the Linnean Society*, 204(4), 1–93. https://doi.org/10.1093/zoolinnean/zlaf057
• Korshunova TA, Martynov AV, Picton BE. *Ontogeny as an important part of integrative taxonomy…* *Zootaxa* 2017c;4324:1–22. https://doi.org/10.11646/zootaxa.4324.1.1
• Korshunova TA, Lundin K, Malmberg K et al. *First true brackish water nudibranch mollusc provides new insights for phylogeny and biogeography and reveals paedomorphosis-driven evolution.* *PLoS One* 2018;13:e0192177. https://doi.org/10.1371/journal.pone.0192177
• Korshunova TA, Picton B, Furfaro G et al. *Multilevel fine-scale diversity challenges the “cryptic species” concept.* *Scientific Reports* 2019a;9:1–23. https://doi.org/10.1038/s41598-019-42297-5
• Ekimova I, Valdes A, Malaquias M et al. *High-level taxonomic splitting in allopatric taxa causes confusion downstream: a revision of the nudibranch family Coryphellidae.* *Zoological Journal of the Linnean Society* 2022;196:215–49. https://doi.org/10.1093/zoolinnean/zlab109
• Goodheart JA, Bleidißel S, Schillo D et al. *Comparative morphology and evolution of the cnidosac in Cladobranchia (Gastropoda: Heterobranchia: Nudibranchia).* *Frontiers in Zoology* 2018;15:43. https://doi.org/10.1186/s12983-018-0289-2
• Karmeinski D, Meusemann K, Goodheart JA et al. *Transcriptomics provides a robust framework for the relationships of the major clades of cladobranch sea slugs (Mollusca, Gastropoda, Heterobranchia)…* *BMC Ecology and Evolution* 2021;21:226. https://doi.org/10.1186/s12862-021-01944-0

Cite this article as: