New Mitochondrial Sequences Provide Insights into Disputed Marine Mussel Taxonomy

Marine mussels occupy diverse coastal and deep-sea habitats, support fisheries, and serve as indicators of pollution because they accumulate contaminants. Researchers have increasingly used molecular evidence to reconstruct their evolutionary history, supplementing traditional classifications based on shell morphology.

However, environmental conditions and developmental stages can alter shell appearance, while unrelated lineages can evolve similar features. These complications make identification difficult and contribute to conflicting classifications. Previous genetic studies have also produced inconsistent evolutionary trees, partly because some groups remain poorly sampled. Resolving these disagreements requires comparisons across multiple sources of genetic evidence, supported by careful examination of specimens and existing sequence records.

A study (DOI: 10.48130/jzser-0026-0004) published in Journal of Zoological Systematics and Evolutionary Research on 30 July 2026 by Zhen Zhang's team, Chinese Academy of Sciences, reveals two principal mussel lineages and provides integrated molecular and morphological evidence for revising disputed taxonomic placements while highlighting relationships that remain unresolved.

The researchers examined specimens collected along the Chinese coast between 2016 and 2024, obtaining complete mitochondrial genomes and five mitochondrial and nuclear gene fragments from nine species representing four subfamilies. Five species received their first complete mitochondrial genome sequences. High-throughput sequencing, genome assembly, and annotation established each genome's composition and gene arrangement.

The team then combined the new sequences with public data, reconstructing mitochondrial evolutionary trees for 67 Mytiloidea species using maximum-likelihood and Bayesian approaches. A separate analysis of the five gene fragments provided an independent comparison. Additional tests examined gene rearrangements, genetic distances, and species boundaries, while three fossil calibration points anchored estimates of evolutionary divergence.

The newly sequenced mitochondrial genomes ranged from 14,714 to 19,454 base pairs and showed a consistent bias toward adenine and thymine. Gene order and transfer RNA copy numbers varied substantially, demonstrating that mitochondrial architecture supplies useful evolutionary information alongside DNA sequences. The analyses recovered two major lineages corresponding to Mytilidae and Modiolidae. Combined evidence supported Mytilinae and Crenellinae as groups descended from their respective common ancestors. The placement of Perna within Crenellinae was reinforced by its evolutionary relationships and protein-coding gene arrangement.

Molecular and shell evidence also supported retaining T. hirsuta in Trichomya and placing Dentimodiolus striatulus and D. subsulcatus within Mytilinae. Species-boundary analyses linked an unidentified Brachidontes specimen to a public sequence labeled B. mutabilis, while distinguishing both from verified B. mutabilis specimens, suggesting a database misidentification. Its distinctive shell features indicated a potentially undescribed species, although further specimens and morphological documentation are needed.

Fossil-calibrated estimates placed the separation of the two major lineages approximately 427 million years ago, during the Silurian, with many subsequent divergences occurring in the Mesozoic. However, mitochondrial and gene-fragment trees disagreed over Brachidontinae relationships, showing that broader sampling remains essential before a stable classification of this subfamily can be established.

“By bringing complementary evidence together, the study advances understanding of mussel evolution while showing where uncertainty persists.” Zhen Zhang, Chinese Academy of Sciences

Its expanded genetic resources can support future taxonomic comparisons and help researchers scrutinize questionable sequence identifications. Broader species sampling, additional molecular markers, and detailed morphological studies will be needed to resolve remaining disagreements and evaluate the potentially undescribed species.