Biogeography and phylogeny of two deep-sea amphipod species: Hirondellea gigas and Hirondellea dubia (Crustacea: malacostraca: Amphipoda)

Enhancing exploration of deep-sea ecosystems is essential for understanding the evolutionary processes shaping these poorly understood environments. This study investigates two deep-sea amphipod species, Hirondellea gigas and Hirondellea dubia, known for their vast geographic distributions and remarkable depth ranges. Specimens were included from 26 abyssal and hadal features, spanning the globe from water depths of 2500 to 10929 m. Using DNA sequence data, we present the most comprehensive phylogenetic analysis to date, test for cryptic species, and explore biogeographic patterns. Prior to this study, H. gigas was known from several hadal features in the North Pacific Ocean yet here we show multiple range extensions into other deep-sea features, including within the Southern Hemisphere. This study also extends the range of H. dubia, a predominantly Southern Hemisphere species, into the Northern Hemisphere, revealing that they are sympatric in multiple features spanning both hemispheres. Genetic structuring, particularly in 16S and COI datasets, suggests geographic isolation may influence population divergence and highlights important differences in evolutionary processes among loci. These results support that both species exhibit wide-scale distributions without significant cryptic speciation, though local geographic factors may still influence population structuring.