Two chthamalids live on the west coast.  Chthamalus dalli inhabits the high intertidal zone from Alaska to San Diego, California and C. fissus lives in the same high zone from Bodega Bay, California to Baja California, Mexico.  In the region of greatest overlap, from San Franciso to San Luis Obispo, both species are abundant, and identification of adults can be difficult.  Authors of a study at Stanfield University, California using specimens collected at Pacific Grove, California state that the 2 species can be identified in the field with 90% accuracy.

In appearance, C. dalli is characterised by an oval orifical opening, a smoother shell, and more even coloration of the test plates than C. fissus (see photographs on Left). Note that the orifice of C. fissus is more variably shaped and its test plates more corrugated than C. dalli, and that there is a dark band running around the lower portions of the plates in C. fissus. The authors note, however, that about 4% of C. dalli have brown bases and 2% possess corrugated shells, while about 6% of C. fissus have even coloration and 31% possess relatively smooth shells.  The larval forms are no help, because they are virtually identical in the 2 species (see drawings above Right).

For more certain differentiation, microscopical study of the setae of the second cirri of the 2 species is required. The setae of C. dalli are finely bipectinate, while those of C. fissus are coarsely bipectinate and have a pair of basal guards (see arrow in photos on Right).  Finally, electrophoretic separation of MDH (malate dehydrogenase) allozymes provides a “100%-confident” separation of the 2 species.  Miller et al. 1989 J Crust Biol 9: 242. Photographs courtesy C. Patton and the authors.

NOTE  the difference in "oval-ness" of the orifice is not as easy to discern in these photographs as the authors imply

NOTE  however, stage I-IV larvae are about 10% bigger in C. dalli than in C. fissus

Genetic studies on the 2 chthamalid species confirm the extent of overlap of their distributions and indicate that, rather than a clinal decrease in abundance at the limits of the species’ distributions, their population abundances decline abruptly.  An analysis of molecular variance indicates no significant regional population genetic structure in C. dalli.  The authors suggest that more work needs to be done on the population biology of the 2 species in the overlap zone of their distributions. Wares & Castañeda 2005 J Mar Behav UK 85: 327.