A new species of <i>Biacanthus</i> (Copepoda: Cyclopoida: Taeniacanthidae) parasitic on <i>Pseudorhombus ocellifer</i> (Pleuronectiformes: Paralichthyidae) from Japan, including a phylogenetic analysis of Bomolochidae and Taen

Morphological and molecular analyses identifed Biacanthus debera n. sp. (Cyclopoida: Taeniacanthidae) as the second member of the previously monotypic genus Biacanthus which was originally proposed to accommodate Anchistrotos pleuronichthydis (type species by original designation). Biacanthus debera n. sp. is associated with the Longnosed-fivespot Flounder Pseudorhombus ocellifer (Paralichthyidae) and can be differentiated from B. pleuronichthydis by two key features: the tip of the terminal segment of the maxilliped showing a stronger curvature (approximately 90° vs 120°) and the presence of transverse rows of minute spinules on the ventral surface of the anal somite that are absent in the type species. Although recent phylogenetic studies have challenged the monophyly of the Bomolochidae and Taeniacanthidae, the present analysis based on 18S rDNA, 28S rDNA, and cytochrome c oxidase I (cox1) sequence data of a broad sampling of members from both families obtained in Japanese coastal waters revealed different phylogenetic relationships. Our analysis supported the early divergence of Pseudotaeniacanthus (here proposed as the type genus of Pseudotaeniacanthidae n. fam.) in apposition of two distinct monophyletic clades, representing the Taeniacanthidae and Bomolochidae, respectively. A comprehensive list of parasites known to infect P. ocellifer is provided.