Across trench and ridge: description of five new species of the Haploniscus belyaevi Birstein, 1963 species complex (Isopoda, Haploniscidae) from the Kuril-Kamchatka Trench region

Integrative taxonomy provides a valuable approach to discover and unravel even morphologically almost indiscernible species, such as those forming “cryptic” species complexes. The six members of the recently discovered Haploniscus belyaevi species complex (short: belyaevi -complex) from the abysso-hadal Kuril-Kamchatka Trench (KKT) region in the Northwest Pacific Ocean are taxonomically described herein. The eponymous Haploniscus belyaevi is redescribed alongside new descriptions of the five closely related species Haploniscus apaticus sp. nov. , H. erebus sp. nov. , H. hades sp. nov. , H. kerberos sp. nov. , and H. nyx sp. nov. The morphological differences between these species are most eminent in the rostral and pleotelson morphology of the adult males. Alongside light-microscopical drawings, CLSM scans, and 16S and COI barcodes, these species descriptions are complemented by the first genomic data of deep-sea haploniscid isopods. Geometric morphometrics was applied to quantify interspecific and intraspecific morphological differences of the pleotelson considering the pronounced sexual dimorphism. The distributional range of the belyaevi -complex covers a large geographical area, ranging across the greater KKT region and extending beyond large-scale geomorphological barriers such as the KKT and the Kuril Island Ridge, turning these into promising species to study differentiation processes in the deep sea.