Integrative taxonomy of the temporary fish parasitic isopod Gnathia gunayalaensis n. sp. (Isopoda: Gnathiidae) from Panama, with supplementary data on Gnathia magdalenensis Müller, 1988

Abstract Gnathiid isopods are common ectoparasites of fishes whose unusual life history has created challenges for characterising their biodiversity and biogeography. Based on morphological and molecular data, we describe a new species, Gnathia gunayalaensis n. sp., from Panama in the Southwestern Caribbean ecoregion of the Tropical Atlantic realm. The new species can be identified by a distinctly pointed superior, inferior and produced frontal margin, and by falciform mandibles with a finely serrated inner margin of the blade. In addition, supplementary morphological characters, as well as new scanning electron microscopy (SEM) and molecular data, are provided for Gnathia magdalenensis Müller, 1988, from the same locality. Newly collected specimens conform to the original description by Müller and subsequent information, notably in possessing a frontal margin with 3 processes (projections), with the medio-frontal process slightly shorter than the superior (submedial) pair, and an internal lobe on the mandible. The SEM observations revealed an additional character: a secondary process on top of the medio-frontal process at the base of the setae. Both species have distinct pigmentation patterns. Molecular analysis confirms that these species are distinct from any previously sequenced gnathiids and expands the available molecular dataset for this family. By establishing robust morphological and molecular reference data for the adult males of these species, this study provides a basis for linking and identifying their parasitic juvenile stages in future investigations of host associations, parasite ecology and biogeographical patterns.