Apristurus iterum, a new shark discovered based on egg case morphology and sequence data obtained from a formalin-fixed specimen.

Apristurus iterum, a new species of deepwater catshark, is described from northeastern Australia. The new species is diagnosed through examination of its unique egg case morphology based on a single whole gravid female specimen and 10 egg cases deposited in different Australian ichthyological collections. This discovery prompted a more detailed investigation of Apristurus specimens collected off Queensland, northeastern Australia. The voucher of the gravid female with two egg cases, previously identified as 'Apristurus', collected off southeast Queensland, was designated as the holotype and used to describe the new species. The egg cases of A. iterum n. sp. differ from other Australian congeners by possessing distinct, weakly T-shaped, longitudinal ridges on the dorsal and ventral surfaces. A recently developed molecular approach was successfully applied to extract viable genetic material from the formalin-fixed holotype specimen. The extracted DNA underwent high-volume next-generation sequencing, and the resulting data were processed using a targeted mitochondrial genome alignment approach to generate a short, 160-nucleotide fragment of the cytochrome c oxidase subunit I (COI) gene, which was used for maximum likelihood phylogenetic analysis. The phylogenetic relationships and egg case morphology strongly support this species as being most closely related to Apristurus ovicorrugatus from northwestern Australia and Apristurus nakayai from New Caledonia and Papua New Guinea, with these three species representing a new subgroup within the Apristurus brunneus group.