Differs from Strongylocentrotus in having a much thinner and more fragile test, and in having the ambulacral zones narrowing adorally so as to be much narrower than the interambulacra at the peristome edge. Jensen (1974) discusses this genus.
Mitochondrial DNA data (Biermann et al. 2003: fig. 2), however, show that Allocentrotus fragilis is nested within a monophyletic clade formed by Hemicentrotus pulcherrimus, Strongylocentrotus intermedius, S. purpuratus, S. pallidus, and S. droebachiensis. Recognition of Allocentrotus would thus render Strongylocentrotus paraphylletic. Pearse & Mooi (2007: p. 917) thus consider Allocentrotus as junior synonym of Strongylocentrotus.
Pearse, J. S. & Mooi, R. 2007. Echinoidea. Pp. 914-922 in J. Carlton (ed.) The Light and Smith Manual, Intertidal Invertebrates from Central California to Oregon. 4th Edition. University of California Press, Berkeley.
Biermann, C. H., Kessing, B. D. & Palumbi, S. R. 2003. Phylogeny and development of marine model species: strongylocentrotid sea urchins. Evolution and Development 5, 360-371.
Jensen, M. 1974. The Strongylocentrotidae (Echinoidea), a morphological and systematic study. Sarsia 57, 113-148.