Résumé
Cryptic biodiversity poses challenges to the accurate description and assessment of global biodiversity and ecosystem health. In this work, the morphology and DNA sequence of Ancistrocheirus lesueurii, the sole accepted member of the family Ancistrocheiridae, was studied. The primary literature and, where possible, type specimens of all synonyms of Ancistrocheirus were reviewed and re-examined, respectively. Large morphological differences were found among the 46 examined individuals. Ancistrocheirus alessandrinii was resurrected, its morphology was redescribed based on eight mature specimens, it was molecularly characterized, and a neotype was proposed to ensure taxonomic stability. The morphology of two specimens of Ancistrocheirus sp. A did not match any previously known Ancistrocheirus species and could not be linked to a taxonomic name. The remaining 34 individuals exhibited morphological variation, yet the limited number of adult individuals and deteriorated state of these specimens precluded further morphospecies’ classification. Nonetheless, the observed morphological diversity among those individuals suggests the presence of additional undescribed species within this complex. Molecular analyses revealed at least six cryptic Ancistrocheirus species. Future molecular and morphological research might solve this cryptic species complex. Unexpectedly, a specimen previously identified as A. lesueurii could not be assigned to any oegopsid family. Consequently, Mobydickia poseidonii fam. nov., gen. nov., et sp. nov. was described from this single specimen sampled from a sperm whale stomach from Antarctic or sub-Antarctic waters.