Redescription of Hemitrygon akajei with description of the cryptic stingray species Hemitrygon ariakensis sp. nov. from the Northwest Pacific (Myliobatoidei: Dasyatidae)

Abstract We redescribe Hemitrygon akajei (Bürger in Müller and Henle 1841) based on the lectotype and newly collected specimens, and describe Hemitrygon ariakensis sp. nov. from western Japan. Owing to their considerable morphological similarity and sympatric distribution, these two species have been subject to longstanding taxonomic confusion, with one remaining unrecognized as a cryptic species for over 160 years prior to the availability of molecular evidence. The red stingray H . akajei was originally described from six syntypes, including juveniles that likely represented multiple species, including the cryptic one. Species identification in Hemitrygon is complicated not only by interspecific morphological resemblance but also by marked ontogenetic changes between juveniles and adults. Herein, we describe the external morphology of both species across developmental stages to aid in their distinction. Hemitrygon akajei and H . ariakensis sp. nov. can be distinguished by the following combination of characters: arrangement of denticles on dorsal surface of disc and posterior part of tail; shape, size, and arrangement of thorns on dorsal tail anterior to caudal spine; presence or absence of transverse groove on ventral surface of disc; coloration of ventral tail fold; shape of outer marginal color band on ventral disc; morphology of clasper and prepelvic processes of pelvic girdle; and number of prespine separate centra. Stingrays are important fishery resources and key mesopredators in Japanese coastal ecosystems, influencing populations of other fishery species. The ability to distinguish these two species will facilitate more accurate ecological research, inform species-specific conservation strategies, and promote effective biodiversity management. Such taxonomic resolution is essential for understanding coastal ecosystem dynamics.