Discovery and description of elaborate larval cusk-eels and the relationships among Acanthonus, Tauredophidium, and Xyelacyba (Teleostei: Ophidiidae)

The larvae of marine fishes often differ substantially in appearance from their adult forms. While sometimes visually striking, these larvae can be difficult to iden­tify. Cusk-eels of the predominantly deep-sea family Ophidiidae have epipelagic larval stages that are re­markably diverse in appearance and some of these larvae, such as the bony-eared assfish (Acanthonus ar­matus), are widely popular subjects for blackwater photographers. Re­cently, a larva was photographed and collected off the east coast of Florida that resembled a bony-eared assfish but differed in pectoral-fin morphology. Based on counts and DNA data, we identified this larva as the gargoyle cusk (Xyelacyba my­ersi), a species for which the larval stage has yet to be described. With an improved understanding of cusk-eel larval morphology, we found and identified a single larval specimen of the spiny blind brotulid (Taure­dophidium hextii) collected from the eastern Indian Ocean in 1977, another species for which the larval stage has yet to be described. We describe these novel larvae, compare them to a newly caught larval Acan­thonus from Hawaii, and highlight morphological similarities and differ­ences among them. Previous works have suggested a close relationship among the monotypic genera Acan­thonus, Tauredophidium, and Xy­elacyba, and we found 4 larval and 6 adult shared morphological traits, including an opercular-spine locking mechanism, that support these taxa forming a clade. Considering these findings and that more than a third of ophidiid genera are monotypic, we modify the classification of these 3 taxa so they are classified in the same genus, Acanthonus, to highlight their relatedness. Our study demonstrates the scientific importance of images and specimens collected by blackwa­ter divers for species identification and further highlights their signifi­cance in understanding evolutionary relationships.