No discovery without exploration: Deep-sea sampling across the Pacific reveals three new genera of Typhlotanaidae (Crustacea: Tanaidacea)

• By combining morphological and genetic analyses (histone H3 marker), researchers revealed deep genetic divergences among species that appear morphologically similar. • The findings show that traditional classifications based solely on morphology were obscured by convergent evolution, where unrelated lineages evolved similar traits. • Closely related yet geographically distant species suggest potential trans-Pacific connections among deep-sea populations. This study examines the “slender-bodied” representatives of the family Typhlotanaidae (Crustacea: Tanaidacea), characterized by straight pereonite margins and scale-like ornamentation on the chelipeds, collected from multiple deep-sea Pacific localities, including the Aleutian Trench, the Sea of Okhotsk, the Clarion-Clipperton Fracture Zone, and eastern Australia. Integrative analyses combining detailed morphology and molecular data (histone H3 marker) were used to reassess the systematics of these taxa. Morphological comparisons of the family’s type species Typhlotanais aequiremis and related slender-bodied forms ( Typhlotanais plebejus , species of Starkus , Baratheonus , and the “spinicauda” group) revealed a pattern of extensive homoplasy, particularly in cheliped ornamentation and pereonite morphology. These convergent traits have historically obscured true phylogenetic relationships within the group. Genetic results, although available for a limited number of specimens, provided strong support for deep genetic divergence among several morphologically similar lineages. The molecular phylogeny recovered a well-supported monophyletic group, with new species forming a distinct, independent clade consistent with genus-level differentiation. By integrating both data sources, this study establishes three new genera and four new deep-sea species. These taxa collectively highlight the cryptic diversity and complex evolutionary history of Typhlotanaidae. The discovery of closely related yet geographically distant species indicates possible trans-Pacific connections among deep-sea populations. An updated identification key and a review of the morphological diversity of slender-bodied Typhlotanaidae are also provided, contributing to the ongoing redefinition of generic boundaries within this morphologically challenging family. LSIDurn:lsid:zoobank.org:pub:037D9EC3-1945–4082-971B-EB4FBB5ABE21.