Discovery of new species of mesoparasitic pennellid (Copepoda: Siphonostomatoida) from the endemic mesopelagic lightfish <i>Vinciguerria mabahiss</i> in the Red Sea

A new species of the genus Cardiodectes Wilson, 1917 (Siphonostomatoida: Pennellidae), Cardiodectes tofaili n. sp., is described based on 13 adult females from ten specimens of the endemic lightfish Vinciguerria mabahiss (Stomiiformes: Phosichthyidae). These hosts were inadvertently captured by a remotely operated vehicle at depths of 454–645 m in the pelagic waters of the Saudi Arabian Red Sea. The new species is placed under the “ rubosus ” group, characterized by possession of a trunk without a discrete abdomen. It is distinguished from its 12 congeners within this group by having a short neck region with a distinct fourth pedigerous somite, and a trunk that is ca. 5 times longer than wide. Phylogenetic analysis based on concatenated 18S + 28S rDNA sequences supports the distinctiveness of the new species. This species is endemic to the Red Sea, representing the first recorded mesoparasite from the mesopelagic environment of the region. This discovery highlights the unique biodiversity of the Red Sea and underscores the importance of exploring mesopelagic ecosystems.