Nudibranch taxonomy: when is it justified to change a name?

The taxonomy of nudibranchs has undergone a profound transformation over the last few decades. The incorporation of molecular data, phylogenomics, and increased taxon sampling has made it possible to uncover evolutionary relationships that were impossible to recognize using traditional morphology. In many cases, these advances have required classifications that had remained unchanged for decades to be revised, with modifications to the names of families, genera, and even species.

These changes are a natural consequence of scientific progress. But there is a less obvious question: should every new phylogenetic hypothesis necessarily result in a name change?

This is the question addressed by an international study specifically focused on how to achieve greater taxonomic stability while incorporating the available systematic evidence. The study, led by Juan Moles of the University of Barcelona and involving numerous specialists in heterobranchs, proposes a series of criteria intended to ensure that taxonomic changes are not only scientifically robust, but also reproducible, proportionate, and durable. The article was published in Zoological Journal of the Linnean Society in September 2026.

Taxonomy is not simply about assigning names

One of the fundamental starting points of the study is to consider taxonomy as more than a system for assigning names to organisms. Scientific names constitute a common infrastructure linking publications, zoological collections, databases, legislation, conservation, education, and citizen science. Consequently, a classification must be scientifically rigorous, but also sufficiently stable and reproducible to be used by all these communities.

This is particularly important in groups such as nudibranchs. In recent years, new molecular techniques have revealed numerous cases of cryptic diversity, as well as groups traditionally regarded as natural units that were in fact polyphyletic or paraphyletic. Correcting these situations is, of course, necessary. The problem arises when the interpretation of new data leads to successive reorganizations before enough independent evidence exists to regard the new classification as stable.

For this reason, the authors propose five principles that should act as a kind of filter before major taxonomic changes are made.

Five criteria for a more stable taxonomy

The first criterion is that classification should reflect evolutionary history, with monophyly providing its foundation. The authors, however, make an important qualification: whenever possible, a phylogenetic hypothesis should be supported by independent lines of evidence that consistently recover the same group. Replication using different datasets or analytical approaches reduces the risk of turning a particular result into a premature nomenclatural change.

The second criterion is diagnosability. A formally recognized taxon should be identifiable through characters that other researchers can verify and reproduce. These characters do not necessarily have to be external: they may be anatomical, ecological, molecular, or even behavioural. What matters is that they are properly documented and linked to specimens, images, sequences, and other reference data.

The third principle is particularly relevant in zoological taxonomy: comparison with topotypical material. When revising a species, genus, or group associated with historical names, interpretation should not depend exclusively on sequences deposited in databases. Whenever possible, material from the type locality, or demonstrably equivalent reference material, should be examined directly. This reduces the risk of perpetuating misidentifications and building new classifications on an incorrect taxonomic foundation.

The fourth principle is the assessment of collateral effects. A change may apparently solve a problem at one level of the classification while generating other problems at higher or lower levels. The authors therefore propose explicitly comparing the topology before and after a change and assessing whether the modification introduces new cases of paraphyly, polyphyly, or nomenclatural instability.

Finally, there is transparency. The data, methods, specimens, images, and diagnoses supporting a taxonomic modification should be available and properly documented. Taxonomic discussion should focus on evidence and methodology, rather than on the people proposing a particular classification.

There is an important consequence to all this: the fact that a clade is monophyletic does not automatically mean that it should receive a new formal name or a new taxonomic rank. The study emphasizes that the usefulness of a name should also be assessed according to its ability to improve communication and understanding of diversity.

Nudibranchia: when stability does not conflict with phylogeny

In a recent reinterpretation, Korshunova et al. (2025) proposed restricting Nudibranchia to Cladobranchia, thereby returning to a more restricted historical concept and leaving Doridina outside the group. The problem, according to the analysis by Moles and colleagues, is that this modification does not respond to Nudibranchia being a paraphyletic group in need of correction.

On the contrary, Cladobranchia and Doridina are well-supported monophyletic lineages and together form a natural group, a result recovered through different morphological, molecular, and phylogenomic approaches (Wägele & Willan, 2000; Zapata et al., 2014; Brenzinger et al., 2021; Krug et al., 2022).

The discussion therefore does not really concern which group is monophyletic, but rather what rank and scope the name Nudibranchia should have.

This is where the question of stability arises. Nudibranchia is a name that has been used continuously for more than two centuries. Restricting it to Cladobranchia would exclude Doridina from a term that has accumulated enormous scientific, educational, and public meaning, without providing an equivalent phylogenetic benefit. The study therefore considers that, in this case, retaining Nudibranchia in its broad sense (sensu lato) preserves nomenclatural continuity without sacrificing evolutionary accuracy.

This also makes it possible to distinguish between two situations that are sometimes conflated. It is not the same to retain a traditional group that we know to be paraphyletic as it is to retain a traditional name that still corresponds to a monophyletic group. The abandonment of historically used concepts such as Opisthobranchia or Pulmonata was justified precisely because evolutionary analyses demonstrated that these groups did not represent natural lineages. The case of Nudibranchia is different.

Flabellinidae: when change is necessary

The case of Flabellinidae demonstrates precisely the opposite. Molecular analyses showed that the family as traditionally understood was deeply polyphyletic (Cella et al., 2016; Korshunova et al., 2017). Subsequent studies also provided independent evidence supporting this structure.

Here, there was a genuine phylogenetic problem that needed to be corrected. The subsequent challenge was to determine how this new evolutionary hypothesis should be translated into a stable classification.

The history of Flabellinidae shows that a phylogeny can correctly reveal that a traditional group is not natural, but that the taxonomic solution is not automatically determined by the tree. It is necessary to decide which lineages deserve formal recognition, which rank is appropriate for each of them, and, above all, whether they can be consistently diagnosed.

The successive proposals that followed have demonstrated precisely the difficulties involved: some entities proved difficult to distinguish using stable characters, certain interpretations depended on alternative topologies, and some subsequent modifications had to be revised again.

The lesson is not that the revision of Flabellinidae was wrong. It is that demonstrating a phylogenetic structure and deciding how to translate it into a formal classification are two different steps.

Dendrodoris: better to wait than to split too soon

Galià-Camps et al. (2024) analysed complete mitochondrial genomes and nuclear genes and found three deeply divergent lineages within Dendrodoris, also associated with different external morphotypes: tuberculate animals, slender forms, and smooth-bodied forms. This divergence could be interpreted as an argument for recognizing several genera.

However, there were reasons for caution. An independent analysis by Maniei and Wägele (2024) showed that mitochondrial and nuclear signals were not completely congruent: some mitochondrial markers produced a paraphyletic arrangement, whereas the nuclear 18S marker supported the monophyly of Dendrodoris. In addition, the initial sampling was still limited.

Rather than resurrecting Doridopsis for the tuberculate species, creating another genus for the slender forms, and restricting Dendrodoris to the smooth-bodied species, Galià-Camps et al. (2024) chose to retain a single inclusive genus while acknowledging the existence of the different lineages.

This decision became particularly interesting when a subsequent study, using broader sampling, found the tuberculate species Dendrodoris warta within the lineage of smooth-bodied species. Thus, the dorsal tubercles, which initially appeared to be a potentially useful character for separating groups, proved not to be sufficiently stable to define a genus (Galià-Camps et al., 2025).

This case neatly summarizes one of the study’s central ideas: recognizing an evolutionary lineage does not require immediately turning it into a new genus. A hypothesis can remain a hypothesis, continue to be investigated, and eventually lead to a formal modification when the evidence is sufficiently robust.

What happens when morphology cannot distinguish the species?

In some cases, genetics is precisely what makes it possible to discover diversity that morphology cannot recognize. The genus Pontohedyle provides a good example: molecular analyses and DNA barcoding revealed several cryptic lineages that could not be reliably distinguished using traditional morphology (Jörger et al., 2012; Jörger & Schrödl, 2013).

A molecular character can therefore be perfectly valid as a taxonomic diagnostic. But it must be reproducible and linked to appropriately identified reference material. A sequence in a database is not, by itself, a guarantee that the specimen was correctly identified.

This is particularly important in nudibranchs, where numerous species are externally very similar and historical misidentifications can subsequently propagate through collections, publications, and databases.

Changing less does not mean doing less science

Taxonomic stability does not mean preventing change. A scientific classification must change when evidence demonstrates that the previous classification was incorrect. What is being proposed is something different: the magnitude of a change should be proportional to the strength of the evidence and to the consequences that change will have for the rest of the classification.

A new phylogeny may modify our interpretation of evolutionary relationships without requiring all the names associated with those relationships to be changed immediately. Likewise, a new hypothesis may justify a major change when there is convergent evidence, reproducible diagnoses, appropriate reference material, and an assessment of its consequences.

In this sense, a stable taxonomy is not a conservative taxonomy. It is a taxonomy in which every change has to earn its place.

For nudibranchs, this means accepting that molecular phylogenetics and phylogenomics will continue to modify our understanding of their evolution. Cryptic species, new lineages, and unexpected relationships will undoubtedly continue to emerge. But the goal should not be to turn every new result into a new name. Rather, it should be to build a classification that progressively incorporates accumulated knowledge while preserving those names that remain scientifically valid and useful.

Ultimately, the best taxonomy is not necessarily the one that changes the most, but the one that ensures its changes are robust, reproducible, and durable.

References

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