Towards systematics-based guidelines for stable taxonomy: lessons from sea slugs

Abstract Taxonomy is a cumulative science that translates evolutionary knowledge into a stable, communicable, and functional system of names. Yet its progress is often undermined not by new data, but by how evidence is interpreted and acted upon. We argue that taxonomic decisions should meet five foundational thresholds: (i) a classification reflecting the tree of life, ensuring replicated monophyly and rank symmetry; (ii) diagnosability, linking molecular and morphological traits to recognizable features; (iii) compatibility with topotypical material, minimizing misinterpretation and the need for further taxonomic remediation actions; (iv) collateral-impact assessment, preventing changes that induce downstream paraphyly and instability; and (v) transparent communication, fostering a civil, evidence-focused scientific debate. Together, these criteria frame taxonomy as a reproducible and accountable scientific enterprise, in which individual taxonomic decisions contribute coherently to an evolving classification. Using heterobranch sea slugs as a case study, we illustrate how departures from these standards generate instability without corresponding gains in explanatory power and demonstrate that the same evidence-based principles apply across the taxonomic hierarchy, from higher-level clades to families, genera, and species. Applying these principles supports retaining historically entrenched, monophyletic names, such as Nudibranchia s.l., when redefining them offers no phylogenetic or practical advantage. We encourage shared, transparent standards across the taxonomic community.