Engelmann, Proceedings of the American Academy of Arts and Sciences 3: 301, 1856/1857
Opuntia arenaria is described as a variety of O. polyacantha in the Flora of North America. We regard it as a stand-alone species. Britton and Rose reported that the species grows in pure sand. Some reports indicate the plants have rhizomes up to 5-ft long growing under the surface of the sand. Cladodes arise from the rhizomes to 8- to 12-inches tall. Cladodes are 2 to 3-inches long by 1- or 2-inches wide. Spines are found on most areoles of the small pads; longest spines may be 1-inch long. O. arenaria habitat is being destroyed by development, dumping, and off-road driving, etc. See the original citation. Opuntia arenaria is diploid.
New Mexico Rare Plants reports:
This species is closely akin to other dry-fruited species of Opuntia, but due to its low chromosome number and morphological stability, it is usually considered to be a distinct and well-defined species. Opuntia polyacantha (incl. O. erinacea) may be separated by its larger size (joints usually well over 3 cm in diameter) and firmly attached joints, as well as the fact that many individuals have pink, red, or magenta pigmentation in their flowers. Opuntia polyacantha is tetraploid (2n = 44), whereas O. arenaria is diploid (2n = 22). Opuntia polyacantha and O. arenaria are not known to occur sympatrically.