A new diosaccin species, Paramphiascella aestuarii n. sp., was found in sediment samples from a shallow estuary in north-western Mexico. The morphological distinction between the new species and its congeners is discussed. The new species is morphologically close to P. calcarifer (Sewell, 1940) from the Maldivian Archipelago. However, P. calcarifer differs from the new species in the proximal and subdistal outer spiniform processes on the female second antennulary segment. Bayesian inference based upon molecular characterization (nuclear 18S ribosomal DNA and the mitochondrial cytochrome oxidase subunit I genes) places the new species within a large group of Diosaccinae Sars G.O., 1906, for which molecular sequences are available, composed of Typhlamphiascus typhlops (Sars G.O., 1906), Rhyncholagena cuspis Yeom and Lee, 2023, Robertgurneya jejuensis Yeom and Lee, 2022, and Amphiascoides Nicholls, 1941. The close relationship with Amphiascoides as its sister genus, only morphologically distinct in the structure of the second endopodal segment of the male second swimming leg, is also confirmed.
