AbstractA new infaunal pinnotherid crab, Pinnixulala heardin. sp., is described from muddy substrates of the middle to inner continental shelf of the northern Gulf of Mexico. Molecular genetic analysis supports its separation from P. retinens (Rathbun, 1918), the only other western Atlantic member of the genus and a species most commonly found living as a symbiont of large polychaete worms. Both western Atlantic species are clearly separated from several described and undescribed populations of congeners occurring along shorelines of the eastern Pacific. We herein include a rediagnosis of P. retinens to facilitate morphological comparisons with P. heardin. sp. Unlike P. retinens, which is more common in shallow inshore waters and reaches larger sizes, the new species lacks development of a strong distally directed spine on the fourth pereopod ischium and has chelae that are at least as heavily developed as the second and third pereopods. In addition, the mature male first gonopods in P. heardin. sp. are tapered over their terminal one-third and directed distally, rather than being curved in their terminal one-third with the tips directed mesially, as they are in P. retinens. The new species is invariably collected from samples including infaunal worms and other invertebrates, but the specific hosts remain to be determined