A new species of giant-eyed bathypelagic eusirid amphipod (Crustacea: Amphipoda: Eusiridae) from the western North Pacific

Deep-sea visual environments show extreme gradients in light intensity, driving divergence in eye morphology. In the order Amphipoda, such specializations are documented in pelagic hyperiideans, whereas deep-sea Eusiridae remain poorly known. During a bathypelagic survey in Japan, we collected a eusirid amphipod with conspicuously enlarged eyes. Integrative taxonomy, using morphology and four DNA barcoding regions, shows that it represents a new species of the monotypic genus Harcledo Barnard, Citation1964, described here as Harcledo toyoshioae sp. nov. It is characterized by markedly enlarged eyes that cover most of the lateral head and meet across the dorsal midline, and by a distinctive maxilla 1 armature. Phylogenetic analyses place H. toyoshioae sp. nov. within Eusiridae but leave its relationships to H. curvidactyla unresolved, owing to the lack of sequence data. Comparisons with historical records suggest rapid diversification of eye size within this clade and indicate that it is unlikely to be governed by depth or pelagic–benthic mode alone. We propose that the enlarged eyes are plausibly associated with a pelagic lifestyle in dim mid-water habitats, while noting that this interpretation remains tentative. Our findings highlight visual specializations in deep-sea eusirids and the value of combining new collections, historical literature, and molecular data when reassessing deep-sea taxa.