The Echinoid Directory

Allocentrotus Mortensen, 1942, p. 232

Diagnostic Features
  • Test circular in outline.
  • Apical disc hemicyclic; posterior ocular plates exsert.
  • Ambulacra polygeminate, with 5 pore-pairs to a plate arranged into strongly oblique arcs; arcs separated by a row of secondary tubercles.
  • All ambulacral plates with single primary tubercle and smaller secondary tubercles.
  • Interambulacral plates with single primary tubercle and smaller secondary tubercles forming distinct rows on the oral surface but poorly developed adapically. Aboral plates appear finely granular.
  • No phyllodes developed adorally; ambulacral zones narrower than interambulacral zones around the peristome.
  • Spines relatively fine and short; less than 0.25 the diameter of the test.
  • Globiferous pedicellariae with muscular neck, tubular blade and no lateral teeth.
Recent, West coast of North America.
Name gender masculine
Strongylocentrotus fragilis Jackson, 1912, by original designation.
Species Included
Only the type species
Classification and/or Status

Camarodonta; Echinoida; Strongylocentrotidae.

Junior subjective synonym of Strongylocentrotus.


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.